Information Physics Homepage

Seminars: All

Thu 4/23/09, 3:30PM in Room 184 print event
Steven Rolston
TBA
TBA

Thu 4/9/09, 3:30PM in Room 184 print event
Michael Biercuk
Optimized Dynamical Decoupling in a Model Quantum Memory
Any quantum system, such as those used in quantum information, magnetic resonance, or the like, is subject to random phase errors that can dramatically impact the fidelity of a desired quantum operation or measurement. In the context of quantum information, quantum error correction techniques have been developed to correct these errors, but resource requirements are extraordinary. Realizing a physically tractable quantum information system will thus benefit significantly if qubit error rates are far below the so-called Fault-Tolerance error threshold, predicted to be of order 10^3 -10^6. The need to realize such low error rates motivates a search for alternate strategies to suppress errors in quantum systems. We present experimental measurements on a model quantum system (crystalline arrays of trapped atomic Be ions in a Penning trap) that demonstrate our ability to dramatically suppress qubit error rates by the application of optimized dynamical decoupling pulse sequences in a variety of experimentally relevant noise environments. We provide the first demonstration of an analytically derived pulse sequence developed by Uhrig, and find novel sequences through active, real-time experimental feedback. These new sequences are specially tailored to maximize error suppression without the need for a priori knowledge of the ambient noise environment. We compare these sequences against the Uhrig sequence, and the well established CPMG-style spin echo, demonstrating that our locally optimized pulse sequences outperform all others under test. Numerical simulations show that our locally optimized pulse sequences are capable of suppressing errors by orders of magnitude over other existing sequences. Our work includes the extension of a treatment to predict qubit decoherence under realistic conditions, including the use of finite-duration, square pi pulses, yielding strong agreement between experimental data and theory for arbitrary pulse sequences. These results demonstrate the robustness of qubit memory error suppression through dynamical decoupling techniques across a variety of qubit technologies.

Thu 4/9/09, 1:00PM in Room 184 print event
Michael Biercuk
TBA
TBA

Thu 4/2/09, 3:30PM in Room 184 print event
Kurt Jacobs
Creating Cat-States in a Nanoresonator using Continuous Measurement
I'll will discuss how a nanoresonator can be prepared in mesoscopic superposition states merely by monitoring a qubit coupled to the square of the resonator’s position. This works for thermal initial states, and does not require a third-order nonlinearity. The required coupling can be generated using a simple open-loop control protocol, obtained with optimal control theory. I will present simulations of the complete preparation process, including environmental noise. Our results indicate the power of open-loop control for state engineering and measurement in quantum nanosystems. The talk is based on PRL 102, 057208 (2009).

Thu 3/26/09, 3:30PM in Room 184 print event
Andrew Landahl
Fault-tolerant quantum computation with color codes
I will discuss the possibility of performing fault-tolerant quantum computation using a newly discovered family of quantum error-correcting codes known as color codes. These codes can be manipulated using only local quantum processing while protecting encoded information in global degrees of freedom. Such codes could be advantageous for quantum technologies in which long-distance quantum transport is unreliable. I will begin by comparing and contrasting color codes to a the related family of topological quantum codes known as surface codes. I will then present a new decoding algorithm for color codes based on integer programming. Using this algorithm, I will present simulation-based evidence that these codes have a capacity nearly saturating the quantum Gilbert-Varshamov bound for CSS codes. I will further show that this capacity is equivalent to the T=0 critical bond concentration at the order-disorder phase transition in the random 3-body Ising model in two dimensions. When faults in the encoding and decoding of these codes are present, I will show how to modify the integer program to be resilient to these faults. Using this modified decoding algorithm, I will present further simulation-based evidence that color codes have an accuracy threshold for fault-tolerant quantum memory that is competitive with the best-known accuracy thresholds, and without incurring large resource overheads. Finally, I will show how to perform universal quantum computation in these codes fault-tolerantly using techniques such as "code deformation," "transversal processing," and "magic states," each of which I will review in detail. Joint work with Jonas Anderson and Pat Rice.

Thu 3/12/09, 3:30PM in Room 184 print event
Hector Bombin
Interacting Anyonic Fermions in a Two-Body Color Code Model
We introduce a two-body quantum Hamiltonian model of spin-1/2 on a 2D spatial lattice with exact topological degeneracy in all coupling regimes. There exists a gapped phase in which the low-energy sector reproduces an effective color code model. High energy excitations fall into three families of anyonic fermions that turn out to be strongly interacting. The model exhibits a Z_2xZ_2 gauge symmetry and string-net integrals of motion, which are related to the existence of topological charges that are invisible to moving high-energy fermions.

Thu 3/5/09, 3:30PM in Room 184 print event
Trace Tessier
Complementarity-Inspired Measures of Classical and Separable Quantum Correlations
Only relatively recently has it been recognized that, in general, there can exist quantum mechanical correlations in mixed composite quantum systems that are distinct from entanglement. Quantum discord and other related measures attempt to quantify these correlations not by determining the degree of separability or nonseparability of the joint density operator, but by considering changes induced in the global state description of an uninformed observer resulting from local measurements made on one or more subsystems. Employing some recent results in the burgeoning field of measurement-induced disturbance, we propose new measures of (i) separable quantum correlations and (ii) classical correlations applicable to a mixed state of two qubits, and investigate their properties. These measures are inspired by, and intimately related to, the behavior of existing complementarity relations under the performance of local measurements with unknown results. The goal of this work is to give a complete accounting of the information content of this system via the combination of a full set of correlation measures, measures of individual subsystem properties, and the unavoidable tradeoffs between such quantities as captured by the complementarity relations.

Thu 2/26/09, 3:30PM in Room 184 print event
Brian Anderson
Watching the birth and death of a superfluid
One prominent element of many continuous phase transitions is the spontaneous formation of topological defects as a system passes rapidly through the critical point. Although the microscopic details of defect formation are generally exceedingly difficult to investigate, defect formation can be studied in such detail in the Bose-Einstein condensation phase transition of a dilute trapped gas. In this case, the "defects" are quantized vortices formed during the birth of the atomic superfluid, and their spontaneous formation has now been observed experimentally and in numerical simulations. This phase transition may also be studied in reverse by driving a condensate into a highly turbulent state. This talk will focus on our recent investigations into the dynamics of phase transitions and turbulence in the birth and death of Bose-Einstein condensates, and the potential roles of such studies in developing a deeper understanding of universal phase transition dynamics.

Thu 2/5/09, 3:30PM in Room 184 print event
David Bacon
The Symmetry Conjecture
Quantum computers can outperform their classical brethren at a variety of algorithmic tasks. Uncovering exactly when quantum computers can exponentially outperform classical computers is one of the central questions facing the theory of quantum algorithms today. In this talk I will argue that a key piece of this puzzle is the role played by symmetry in quantum algorithms. I will show how this point of view can be used to make progress in finding new quantum algorithms.

Thu 1/22/09, 3:30PM in Room 184 print event
Grant Biedermann
Gravity tests with cold atom interferometers
The remarkable success of light-pulse atom interferometer techniques has motivated competitive research in precision metrology. Gravimeters, gyroscopes and gradiometers based on these techniques are all at the forefront of their respective measurement classes. I will present progress toward a compact gravity gradiometer for precision gravitational tests. It is well known that these devices suffer from environmental perturbations. Spurious noise may enter through beam steering effects and laser frequency instability. In our device, we have overcome these obstacles to achieve a differential acceleration sensitivity of 4.2 × 10-9 g/√Hz over a 70 cm baseline. This corresponds to a phase noise of 3.1 mrad/√Hz inferred per interferometer which is the best performance achieved in such a system. I will describe our experimental results with this system. Finally, I will discuss current efforts to integrate cold atom techniques with atom chips for quantum sensing and information processing applications.

Thu 12/11/08, 3:30PM in Room 184 print event
Lorenza Viola
Open-loop quantum error control: From dynamical decoupling to dynamically corrected gates.
Achieving accurate control over quantum dynamics is a long-sought goal in a variety of quantum physics, chemistry, and engineering settings as well as in quantum information processing applications. Scalable quantum computation in realistic devices, in particular, requires that precise control can be implemented efficiently in the presence of decoherence and operational errors. In this talk, I will present open-loop dynamical control procedures for achieving robust storage and unitary gates on an open quantum system without encoding or measurement overhead. These results allow for a low-level error correction strategy solely based on Hamiltonian engineering and may prove instrumental to reduce implementation requirements for fault-tolerant quantum computing architectures. Illustrative examples will be discussed, including applications to controlling spin and/or charge degrees of freedom coherence in quantum dots.

Thu 12/4/08, 3:30PM in Room 184 print event
Konrad Lehnert
Microwave cavity optomechanics: Measuring and cooling the motion of nanomechanical oscillators with microwave "light"
Cavity optomechanics has recently blossomed into a vibrant field of research. In cavity optomechanical systems, an intracavity light field is tightly coupled to the motion of a mechanical oscillator through the radiation pressure force. These systems hold the promise of observing manifestly quantum behavior in a tangible mechanical oscillator. In addition to detecting the motion of the oscillator with nearly quantum-limited sensitivity, light can be used to cool or to amplify that motion. In this talk, I will describe our implementation of a cavity optomechanical system that uses microwave "light" instead of optical light. I'll show that the apparent disadvantage of using lower momentum microwave photons can be overcome by employing microfabricated, superconducting cavities integrated with nanomechanical oscillators. Because we operate these nano-electromechanical systems in a dilution refrigerator, the mechanical oscillators have less than 1000 thermal phonons. With optomechanical forces it should be possible to remove most of these remaining phonons and cool the oscillator close to its ground state. In our preliminary measurements, we have achieved both optomechanical cooling and amplification of mechanical motion. Furthermore, with these devices we have realized a displacement measurement operating within 20 times the standard quantum limit and a 0.6 aN/rtHz force sensor.

Thu 11/6/08, 3:30PM in Room 184 print event
Pradeep Kiran Sarvepalli
Sharing Classical Secrets with CSS Codes
In this talk I will present new schemes for sharing classical secrets using CSS codes. While every quantum secret sharing scheme is a quantum error correcting code, the converse is not true. Motivated by this we sought to find quantum codes which can be converted to secret sharing schemes. If we are interested in sharing classical secrets using quantum information, then we show that a pure [[n, 1, d]]q CSS code can be converted to a perfect secret sharing scheme. These secret sharing schemes are perfect in the sense the unauthorized parties do not learn anything about the secret. Gottesman had given conditions to test whether a given subset is an authorized or unauthorized set; they enable us to determine the access structure of quantum secret sharing schemes. For the secret sharing schemes proposed in this talk the access structure can be characterized in terms of minimal codewords of the classical code underlying the CSS code. This characterization of the access structure for quantum secret sharing schemes is thought to be new and potentially generalize to general quantum secret sharing schemes. Joint work with Andreas Klappenecker.

Thu 10/23/08, 3:30PM in Room 184 print event

Observation of two-atom Rydberg blockade and collective encoding of large quantum registers
We have recently demonstrated Rydberg blockade between single atoms separated by more than 10 microns. The Rydberg blockade mechanism whereby excitation of an atom to a Rydberg level prevents subsequent excitation of a nearby atom is an entangling operation that will enable a two-atom quantum gate. I will describe our recent blockade experiments and progress towards demonstration of a neutral atom CNOT gate. Rydberg blockade is also of interest for efficient creation of many atom entangled states. I will discuss a collective approach to encoding of quantum registers, and the prospects for experimental implementation in rare earth atoms with complex spectra.

Thu 10/9/08, 3:30PM in Room 184 print event
Ana Maria Rey
Alkaline-earth atom toolbox
Ultracold bosonic and fermionic atoms trapped in optical lattices are promising set-ups to ultimately implement Richard Feynman’s pioneering ideas of quantum simulation and quantum information. So far, mainly alkaline atoms (group I) have been used for these purposes due to our ability to laser cool and trap them relatively easily. However, recent developments in the trapping and cooling of alkaline-earth atoms (group II), driven mainly by the suitability of these atoms for clock technology, have opened a new avenue to exploit their richer but tractable electronic structure. In this talk I will discuss preliminary results showing that alkaline earth(-like) atoms can be used to engineer novel Hamiltonians beyond the generic one band Hubbard model, such as Haldane, ALKT and spin-orbit models. These have been regarded so far as idealized models developed in condensed matter physics to describe not-yet understood phenomena such as high-temperature superconductivity and itinerant ferromagnetism. Finally I will also discuss new methods that take advantage of the combined electronic and nuclear degrees of freedom in alkaline-earth(-like) atoms for quantum information science.

Thu 9/25/08, 3:30PM in Room 184 print event
Thomas Killian
Ultracold Strontium: Studies of Collisional Properties and Progress towards Quantum Degeneracy
Alkaline-earth atoms such as strontium and atoms with similar valence electronic structure differ significantly from alkali-metal atoms that are typically used in ultracold experiments. They have a closed-shell ground state structure, numerous isotopes including spinless bosons, and metastable triplet levels that lead to novel laser-cooling techniques and interactions. They present many new opportunities for the study and application of ultracold atoms, such as optical frequency standards, long-coherence-time interferometers, optical Feshbach resonances with little induced loss, and Bose and Fermi quantum degenerate gases and mixtures. I will describe recent experiments with two-photon spectroscopy of ground molecular levels that have accurately determined the s-wave scattering lengths for all strontium isotopes. This has guided our recent progress towards quantum degeneracy. We have also developed techniques for populating metastable levels in an optical dipole trap and studied collisional properties of these states. Finally, I will discuss prospects to use an optical Feshbach resonance in quantum degenerate strontium to stabilize solitons in 2-two dimensions and study quantum gases with disordered nonlinear interactions. This work is supported by the National Science Foundation, Welch Foundation, and the David and Lucille Packard Foundation. * In collaboration with Y. Natali Martinez de Escobar, Pascal Mickelson, and Mi Yan.

Thu 9/11/08, 3:30PM in Room 184 print event
Kris Helmerson
Vortices and persistent currents: Observations in topologically constrained Bose-Einstein condensates
Recent studies of both 1-D and 2-D weakly-interacting, atomic Bose-Einstein condensates have demonstrated a number of interesting phenomena including a Tonks-Girardeau gas and the Berezinskii-Kosterlitz-Thouless crossover, respectively. Clearly, reduced dimensionality or topological constraints enables the observation of new and interesting phenomena. I will describe recent experiments on the confinement of a sodium Bose-Einstein condensate in a toriodal-shaped trap and also in a flat optical trap to produce a ring-shaped and 2-D quantum degenerate gas, respectively. We have been able to observe a number of phenomena connected with the superfluid transition in such systems, such as persistent currents and quasi-condensate formation.

Thu 8/28/08, 3:30PM in Room 184 print event
Carlton M. Caves
Quantum-limited metrology: Dynamics vs. entanglement
Questions about quantum limits on measurement precision were once viewed from the perspective of how to reduce or avoid the effects of the quantum noise that is a consequence of the uncertainty principle. With the advent of quantum information science came a paradigm shift to proving rigorous bounds on measurement precision. These bounds have been interpreted as saying, first, that the best achievable sensitivity scales as 1/N, where N is the number of particles one has available for a measurement and, second, that the only way to achieve this Heisenberg-limited sensitivity is to use quantum entanglement. I will review these results and introduce a new perspective based on using nonlinear quantum dynamics to improve sensitivity. Using quadratic couplings of N particles to a parameter to be estimated, one can achieve sensitivities that scale as 1/N^2 if one uses entanglement, but even in the absence of any entanglement at any time during the measurement protocol, one can achieve a super-Heisenberg scaling of 1/N^{3/2}. Such sensitivity scalings might be achieved in Bose-Einstein condensates or in nanomechanical resonators.

Thu 8/21/08, 3:30PM in 1131 print event
Poul Jessen
Quantum Signatures of Chaos in Experiments with a Kicked Top
Theoretical studies have long indicated that quantum signatures of classical chaos may be found in behavior such as hypersensitivity to perturbation [1], or (for bipartite systems) the rate or degree of entanglement generation [2]. We will present results from a recent experiment in which we implement a Quantum Kicked Top [3] with the goal to study these and other aspects of quantum-classical correspondence. Our QKT consists of the F = 3 hyperfine spin in the electronic ground state of individual Cs atoms driven by a pulsed magnetic field and a rank-2 tensor AC Stark shift [4]. Among the advantages offered by this system is the ability to prepare arbitrary minimum-uncertainty spin states [5], the ability to precisely implement the desired QKT Hamiltonian, and the ability to accurately measure the entire spin density matrix and thus obtain kick by kick movies of the evolving quantum phase space distribution [6]. The spin dynamics seen in our experiment include dynamical tunneling between regular islands, rapid spreading of states through the chaotic sea, and surprisingly robust signatures of classical phase space boundaries. Our data show differences between regular and chaotic dynamics in the sensitivity to perturbations of the QKT Hamiltonian, and in the average electron-nuclear spin during the first 40 kicks. As expected, the distinction – though clear – is modest due to the small size of the atomic spin. Future versions of the experiment may circumvent this limitation by driving the electronic and nuclear spins independently, or by working with the collective spin of an ensemble of atoms. [1] A. Perez, Phys. Rev. A 30, 1610 (1984). [2] K. Fyruya, M. C. Nemes and G. Q. Pellegrino, Phys. Rev. Lett. 80, 5524 (1998). [3] F. Haake, M. Kus and R. Scharf, Z. Phys. B: Cond. Matt. 65, 381 (1987). [4] G. A. Smith et al., Phys. Rev. Lett. 93, 163602 (2004). [5] S. Chaudhury et al., Phys. Rev. Lett. 99, 163002 (2007). [6] G. A. Smith et al., Phys. Rev. Lett. 97, 180403 (2006).

Thu 5/8/08, 3:30PM in Room 184 print event
N. Bonesteel
Braiding and Entanglement in Non-Abelian Quantum Hall States
Fractional quantum Hall states exhibit a unique kind of quantum order known as topological order. Certain of these states, which have been experimentally observed, may have a sufficiently rich form of this order (i.e., they may be "Non-Abelian") to be used for so-called topological quantum computation, an intrinsically fault tolerant form of quantum computation which is carried out by braiding the world lines of quasiparticle excitations in 2+1 dimensional space time. In this talk I will review the properties of non-Abelian quantum Hall states and discuss some of the methods we have found for finding specific braiding patterns which can be used to carry out universal quantum computation using them. I will also discuss recent work on one-dimensional chains of "interacting" quasiparticles in non-Abelian states, focusing on their entanglement scaling at quantum critical points.

Thu 5/1/08, 3:30PM in Room 184 print event
Zhao Zhi
Multi-photon entanglement for quantum information processing
Multiparticle entangled states are at the basis for almost all the protocols for the distributed quantum information processing. Yet, the challenge is to be able to generate and manipulate particles well enough to carry out the various tasks. In this talk, I will report on the generation of bright sources of three-photon entanglement, four-photon entanglement, and the first five-photon entanglement. I will also discuss how to manipulate multi-photon entanglement for the variety of applications in quantum information science, such as entanglement concentration, quantum telecloning, open-destination teleportation and linear optics quantum computing.

Thu 4/24/08, 3:30PM in Room 184 print event
D. Dalvit
Recent progress in Casimir physics
This year is the 60th anniversary of Hendrik Casimir's seminal paper of the effect that carries his name. He predicted that electromagnetic quantum vacuum fluctuations are modified in the presence of material boundaries, and because of this two metallic, uncharged parallel plates in vacuum would attract each other when separated by sub-micron distances. In this talk I will review recent progress in theory and experiments in Casimir physics. I will also discuss some of my own work on the field, in particular how to use cold atoms to measure non-trivial quantum vacuum geometry effects in atom-surface interactions, and ideas (and difficulties) to obtain Casimir repulsion with engineered metamaterials. References: D.A.R. Dalvit, P.A. Maia Neto, A. Lambrecht, and S. Reynaud, Phys. Rev. Lett. 100, 040405 (2008) F.S.S. Rosa, D.A.R. Dalvit, and P.W. Milonni, Phys. Rev. Lett. (in press).

Thu 4/17/08, 3:30PM in Room 184 print event
B. Damski
Dynamics of quantum phase transitions in spin-1 Bose-Einstein condensates
I will discuss in this talk how dynamics of quantum phase transitions can be studied in ultracold atomic gases. The talk will start with a brief introduction to the fascinating physics of Bose-Einstein condensates: dilute, ultracold, samples composed of weakly interacting bosonic atoms. After the introduction, I will shortly explain the basics of quantum phase transitions, and the reasons why the cold atom studies can provide a significant boost to this field of research traditionally associated with condensed matter physics. Then, I will discuss dynamics of the quantum phase transition in a ferromagnetic Bose-Einstein condensate: a beautiful cold atom system currently studied in various experimental groups worldwide. In particular, I will explain why a symmetry-breaking quantum phase transition leads to formation of magnetic domains and spin vortices, what happens when the condensate undergoes a non-equilibrium evolution at the critical point, and how the condensate dynamics can be mapped onto dynamics of a classical mechanical system with anti-friction.

Thu 3/27/08, 3:30PM in Room 184 print event
P. Lett
Entangled images from 4-wave mixing in an atomic vapor
Squeezed light was first generated in 1985 using 4-wave mixing (4WM) in an atomic vapor. Since that time this approach has had limited success and most work on squeezed light has been performed using nonlinear crystals in cavities (optical parametric amplifiers and oscillators). In an attempt to generate narrowband correlated photons to use to interact with cold atoms, we have found an approach to 4WM in Rb vapor that produces strong intensity-difference squeezing. The technique is experimentally quite simple and does not require a cavity. This has allowed us to generate quantum-entangled images; multi-spatial-mode light with intensity-difference and phase-sum squeezing at levels that imply continuous-variable EPR entanglement between the beams. We can generate "arbitrary" images, including beams carrying orbital angular momentum, which we use to demonstrate the independence of the spatial modes. Future plans include entangling light with matter waves out-coupled from a BEC.

Thu 3/6/08, 3:30PM in Room 184 print event

What is time reversal good for?
Time reversal techniques (e.g. spin echo) are commonly used in many experimental setups to measure properties of systems. I will describe a general framework for time reversal and its relation to decoherence and quantum information. Then, I will discuss applications of time reversal for quantum simulations of critical systems. I will focus on spin chains simulated with magnetic nuclear resonance, and a proposal for time reversal in optical lattices in the Bose-Hubbard regime.

Sat 3/1/08, 3:30PM in Room 184 print event
Poul Jessen
TBA
TBA

Thu 2/28/08, 3:30PM in Room 184 print event
N. Khaneja
Nonlinear control in quantum information science
The talk describes some ideas from geometry that have found applications in problems of control of coupled spin dynamics. The role of Lie group decompositions and symmetric spaces in efficient design of quantum circuits is described. I will discuss the role of Lie algebras in understanding robust manipulation of quantum dynamics that can compensate for various errors and dispersions. Finally, we describe some methods for computing the reachable set of quantum dynamics in the presence of decoherence. We use these ideas to compute the maximum coherence or polarization that can be transferred between coupled spins in presence of relaxation. Application of these methods to design of multidimensioal experiments in NMR spectroscopy will be discussed.

Thu 2/21/08, 3:30PM in Room 184 print event
D. Jin
Ultracold heteronuclear molecules created near a Feshbach resonance
Feshbach resonances provide a mechanism for the efficient creation of weakly bound ultracold molecules. Heteronuclear Feshbach molecules could provide an excellent starting point for producing a dense gas of ground-state polar molecules by driving optical transitions to deeper bound molecule states. I will discuss experiments exploring this possibility using weakly bound KRb molecules near an interspecies Feshbach resonance.

Thu 2/7/08, 3:30PM in Room 184 print event
P. Aliferis
How to quantum compute against biased noise
In several promising systems for the implementation of quantum computation, noise is expected to be highly biased with dephasing being much stronger than relaxation in the computation basis. I will discuss a scheme of fault-tolerant quantum computation that is especially designed to work effectively in this setting [arXiv:0710.1301]. Along the way, I will review basic concepts such as recursive fault-tolerant simulations and level reduction.

Thu 1/31/08, 3:30PM in Room 184 print event
S. Aaronson
TBA
TBA

Thu 1/24/08, 3:30PM in Room 184 print event
A. Aspuru-Guzik
Quantum algorithms for the simulation of classical and quantum systems: protein folding and chemical reaction dynamics
Quantum computation could provide exact, polynomial-time simulation of chemical reactions. I will talk about the algorithm required for simulating the dynamics of chemical systems and how to efficiently obtain observables of chemical interest, such asstate-to-state transition probabilities and reaction rates. I will also discuss a quantum optimization algorithm for obtaining low-energy conformations of protein models. I discuss mappings between protein models and optimization variables, which are in turn mapped to a quantum system of coupled quantum bits.

Thu 12/6/07, 3:30PM in Room 184 print event
N. Khaneja
TBA
TBA

Thu 11/29/07, 3:30PM in Room 184 print event
J. Emerson
Efficient Characterization of Quantum Devices
I will describe a general symmetrisation method that allows for direct, efficient experimental characterisation of important features of the noise affecting a multi-body quantum system. Compared to existing methods this protocol yields an exponential to polynomial speed-up in both the number of experiments and the classical post-processing required to obtain such information. In the context of applications, this information enables tests of key assumptions underlying estimates of the fault-tolerant threshold as well as optimisation of error-correction strategy. Moreover, the estimated noise parameters are immediately relevant for optimizing experimental control methods. [Emerson et al, arXiv:0707.0685v1]. I will also describe an extension of the method that enables scalable experimental identification of correctable codes such as noiseless subsystems. [Silva et al,arXiv:0710.1900].

Thu 11/8/07, 3:30PM in Room 184 print event
D. Heinzen
Bosons, lattices, and quantum simulation
We describe experiments with bosons in optical lattices for quantum simulations of condensed matter phenomena. In one experiment, we have studied Raman photoassociation spectra of 87Rb bosons in an optical lattice. The collisional energy shifts of the atoms and molecules in the lattice are resolved due to the quantization of atom numbers in the lattice sites; this allows us to determine the lattice site occupancies. We also study the excitations of bosons in a lattice with Bragg spectroscopy. It is observed that the Bragg excitations are in surprisingly good agreement with Bogoliubov theory in the superfluid regime, and we also see evidence of excitation in the insulating phase. Finally, efforts to realize a homogenous optical lattice for applications to quantum simulation will be described.

Thu 11/1/07, 3:30PM in Room 184 print event
G. Boreman University of Central Florida
Infrared Antennas
We fabricate nanoscale electromagnetic structures resonant at infrared frequencies using direct-write electron-beam lithography. Planar lithographic antennas in the infrared portion of the spectrum have a number of unique characteristics. Incident radiation induces IR-frequency current waves to flow in the arms of the antenna, and places an IR-frequency sinusoidal voltage across the sensor element. The resonant frequency is determined by antenna dimensions rather than by sensor-material characteristics and the specific antenna design allows tailoring of the spectral bandwidth around the center frequency. For sensing applications, the angular response of the antenna is of critical importance. In the typical situation where the antenna is fabricated on a dielectric substrate, there are significant differences in the response of the antenna from the air side and from the substrate side, in terms of gain as well as angular pattern. We also explore the related issues of spatial response (effective collection area), polarization response, and impedance matching. Discussion is included of some aspects of the electron-beam lithographic processes required for fabrication, and on the procedures for performance characterization of the IR antennas. We also have ongoing investigations of IR frequency selective surface (FSS) technology. The resonant frequency and spectral bandwidth of the FSS are controlled by the geometry of the metallic structures in the periodic surface. We discuss potential applications of the FSS structures as spectral filters, polarization retarders, and phase retarders. In the design process for both IR antennas and IR FSS, we find that knowledge of the spectral variation of real and imaginary parts of the permittivity is crucial for accurate agreement between numerical models and measured results. This is particularly important for the thin metallic films comprising the antenna arms and the FSS structures, for which inadequate tabular data are available in the literature. IR-optical properties are measured as a function of frequency with an IR variable-angle spectroscopic ellipsometer, and then imported into full-wave electromagnetic models for design and analysis. Designs for specific infrared antennas and FSSs are developed by an integrated approach of modeling, fabrication, and testing.

Thu 10/25/07, 3:30PM in Room 184 print event
L. Orozco
Correlations with Spontaneous Emission in Cavity QED
Correlations and their associated conditional dynamics are different depending on the information gained from the detection of light coming from fluorescence or from transmission in cavity QED. We measure auto- and cross-correlations as a possible avenue for the study of the steady state entanglement in this system.

Thu 10/18/07, 3:30PM in Room 184 print event
D. Lidar
Quantum Fault Tolerance via Dynamical Decoupling
I will describe our recent work on quantum computing using concatenated dynamical decoupling, which provides fault tolerance against environmental and control noise in an open-loop setting, without the overhead of traditional concatenated error correcting codes. Concatenated dynamical decoupling is particularly useful against highly non-Markovian environments as encountered in solid-state spin-based quantum information processing. I will discuss applications to circuit-model and adiabatic quantum computing. These applications involve a synthesis of ideas from the theory of noiseless subsystems, stabilizer codes, and protection via energy gaps.

Thu 9/27/07, 3:30PM in Room 184 print event
G. Brennen
Quantum Simulators for Topological Order in Spin Lattices
Highly correlated many-body systems can exhibit emergent symmetries not present in the microscopic equations of motion. A manifestation of this is in spin lattice models with ground states that are topologically ordered, and which in 2D can have anyonic excitations. Degenerate ground states of such models are useful for protecting quantum information because they are indistinguishable by local operators and there is a finite gap to excited states. However, this very protection can make it difficult to manipulate the information. I will discuss some new ideas for implementing analogue and digital simulations of spin lattice models using atomic and molecular ensembles trapped in an optical lattice. Global operations are mediated by collective coupling to high-Q cavities. Application of these techniques include manipulations of subsystem codes and anyonic interferometry by weaving string operators.

Thu 9/13/07, 3:30PM in Room 184 print event
P. Schwidt
Micro-Fabricated Devices for Neutral Atoms
Historically, neutral atoms have been the quintessential proving ground for quantum mechanics. However, the intersection between atomic physics and Microsystems has generated a renewed interest in neutral atoms for practical devices such as magnetometers and inertial sensors as well as a medium for quantum information processing. Using the techniques of micromachining, researchers at Sandia and at the National Institute of Standards and Technology have pioneered the drastic miniaturization of vapor cell atomic clocks, magnetometers, and gyroscopes while little has been sacrificed in terms of performance. In particular, I will discuss miniaturized atomic magnetometers. In an effort at Sandia we are miniaturizing a “zero-field” magnetometer while maintaining sub-picotesla sensitivity using a novel tuned cavity approach. In another effort, we are developing improved “atom chips” to cool and trap ultra-cold neutral atoms within tens of microns the chip’s surface. In addition, high-finesse, open-access optical cavities will be integrated onto the atom chip to allow strong atom-photon coupling. Such devices are essential for quantum computers and networks based on neutral atoms as the qubits.

Thu 8/23/07, 3:30PM in Room 184 print event
C. Caves
Quantum-Limited Measurements: One Physicist\'s Crooked Path From Quantum Optics to Quantum Information
Quantum information science has changed our view of quantum mechanics. Originally viewed as a nag, whose uncertainty principles restrict what we can do, quantum mechanics mechanics is now seen as a liberator, allowing us to do things, such as secure key distribution and efficient computations, that could not be done in the realistic world of classical physics. Yet there is one area, that of quantum limits on high-precision measurements, where the two faces of quantum mechanics remain locked in battle. Using my own career as a convenient backdrop, I will trace the history of quantum-limited measurements, from the use of nonclassical light to improve the phase sensitivity of an interferometer to the modern perspective on how quantum entanglement can be used to improve measurement precision.

Tue 5/22/07, 3:30PM in Room 184 print event
R. Stock
Quantum Computing With Identical Atoms and No Loopholes for Bell
Optical clock-transitions such as the ones in Ytterbium and Strontium atoms are prime candidates for encoding qubits for quantum information processing. In this work, we investigate the challenges involved in using these candidates for quantum information applications. We devise entangling operations for identical atoms trapped in optical tweezers, as well as determine the feasibility of rapid qubit rotation and measurement of qubits encoded in these desirable low-decoherence clock transitions. The rapid control of atomic qubits is crucial for high-speed synchronization of quantum information processors, but is also of interest for tests of Bell inequalities that avoid the detection loophole in spacelike separated entangled qubits.

Thu 5/17/07, 3:30PM in Room 184 print event
Prof. Shohini Ghose
Quantum Chaos With Cold Atoms
Cold cesium atoms interacting with lasers and magnetic fields can be used to perform the first experimental realization of the quantum kicked top. We describe the experimental system nd analyze regular and \'chaotic\' dynamics in a deeply quantum rather than the typical semi-classical regime of previous studies. We show that signatures of chaos can be observed in the entanglement between the atom\'s electron and nuclear spin for experimentally feasible parameters. Entanglement dynamics can be experimentally monitored simply by measuring the expectation values of the total spin via Faraday rotation spectroscopy. Signatures of chaos in the dynamics persist in the presence of decoherence due to photon scattering for times much longer than the photon scattering time. Furthermore, the rate of decoherence itself depends on the chaoticity of the system. Finally, we show that the interesting phenomenon of dynamical tunneling between islands separated by KAM surfaces in the classical phase space should be observable in the experimental system.

Thu 5/3/07, 3:30PM in Room 184 print event
K. Wodkiewicz
Gaussian Wave Packets and Sub-Planck Structures
Sub-Planck structures in phase space - an unexpected sign of quantum interference - play a surprisingly important role in the distinguishability of quantum states. It is shown that such sub-Planck structures can occur in linear superpositions of Gaussian wave packets. Gaussian wave packets and optical pulses can exhibit interference, quantum entanglement and quantum sub-Planck structures in phase space. FROG measurement of light pulses can reveal sub-Fourier structures in time and frequency phase space.

Thu 4/26/07, 3:30PM in Room 184 print event
L. Viola
(Some) Principles and Applications of Quantum Information Control:
Developing theoretical and practical methodologies for accurately controlling quantum dynamics is a challenge of growing significance across contemporary physics, engineering, and quantum information science. In spite of intensive effort, a host of challenges remain in devising and validating approaches for control design and analysis able to address \"complex\" dynamical regimes as typically encountered in realistic scenarios. In this talk, I will focus on stabilization problems for both quantum dynamics and quantum states in two distinct complementary settings: open-loop dynamical decoupling methods for non-Markovian quantum evolutions, and continuous-time output-feedback for Markovian master equations. In particular, I will survey recent results on high-level deterministic and randomized dynamical decoupling schemes [1] and their application to electron spin coherence control in semiconductor quantum dots [2], as well as outline Markovian feedback-control strategies for quantum pure state stabilization and noiseless subspace synthesis [3]. References: [1] L.F. Santos & L. Viola, \"Enhanced convergence and robust performance of randomized DD,\" PRL 97, 150501 (2006). [2] W. Zhang, V.V. Dobrovitski, L.F. Santos, L. Viola, B.N. Harmon, \"Dynamical control of electron spin coherence in a quantum dot: A theoretical study,\" PRB-RC (in press, 2007). [3] F. Ticozzi & L. Viola, \"Quantum Markovian dynamics: Invariant subsystems, attractors, and control,\" IEEE Trans. Aut. Control (submitted, 2007).

Thu 4/12/07, 3:30PM in Room 184 print event
Canceled
B. DeMarco
Simulating quantum magnetism (and other Hubbard physics) using optical
Physical simulation as a means for resolving outstanding quantum many-body problems was first proposed by Feynmann in 1981. Since then, physicists have dreamed of using physical quantum simulation as a quantitative tool. Ultra-cold atoms trapped in an optical lattice are now emerging as an ideal tool for quantum simulation of a wide range of many-body quantum models, including the Hubbard model and quantum magnetism. I will review the developing field of quantum simulation using ultra-cold atoms and highlight our progress on simulating quantum magnetism, transport in the Bose-Hubbard model, and the disordered Bose-Hubbard model.

Thu 4/5/07, 3:30PM in Room 184 print event
J. Ye
The Art of Light-based Precision Measurement
Improvements in spectroscopic resolution have been the driving force behind many scientific and technological breakthroughs over the past century, including the invention of the laser and the realization of ultracold atoms. Creating and preserving optical phase coherence are one of the two major ingredients (the other being the control of matter) for this scientific adventure. Lasers with state-of-the-art frequency control can now maintain phase coherence over one second, that is, 1015 optical waves can pass by without losing track of a particular cycle. Translating into distance, such a coherent light wave can traverse the circumference of the Earth 10 times and still interfere with the original light. The recent development of optical frequency combs has allowed this unprecedented optical phase coherence to be established across the entire visible and infrared parts of the electromagnetic spectrum, leading to direct visualization and measurement of light ripples. A new generation of atomic clocks using light has been developed, along with new forms of optical interferometry, with an anticipated measurement precision reaching 1 part in 1018. These developments will have impact to a wide range of scientific problems such as the possible time-variation of fundamental constants and gravitational wave detection, as well as to a variety of technological applications including global position systems and deep space navigation.

Thu 3/29/07, 3:30PM in Room 184 print event

Quantum Walks on Graphs
Quantum mechanical systems can encode and process information in a fundamentally non-classical way. An information-based view of quantum mechanics provides tools both for understanding quantum phenomena, and for exploiting these phenomena to perform information processing tasks. In this talk, I will describe one such tool, the concept of quantum walk. A quantum walk is a quantum mechanical generalization of a random walk. Just as a random walk can be viewed as the diffusion of a classical particle, so a quantum walk can be viewed as the Schrodinger evolution of a quantum particle. The dynamics of random and quantum walks can be very different, since quantum walks exhibit interference, whereas random walks do not. I will illustrate this difference by describing three quantum algorithms based on quantum walks. The first algorithm solves a problem exponentially faster than is possible with any classical process, establishing a dramatic difference not just between random and quantum walk, but between classical and quantum computation. The second algorithm uses a quantum phase transition of a particle on a lattice to quickly find a marked location. And the final algorithm uses a quantum walk on a tree to give a nearly-optimal solution to the problem of evaluating Boolean formulas.

Thu 3/22/07, 3:30PM in Room 184 print event
P. Kwiat
Advanced Quantum Communication Protocols
The usefulness of quantum entanglement for various tasks in secure and efficient communication is now well established. We have been investigating how the usual benefits can be further extended, by incorporating other elements of modern physics (e.g., relativity) and using more complex quantum states (e.g., \"hyperentanglement\"). In the former case, one can double or triple the efficiency of traditional quantum key distribution protocols; in the latter case one can achieve complete Bell-state analysis, and increase the capacity of superdense coding to 2.8 bits/photon. I will discuss our experimental progress in these areas, with an eye toward future developments.

Thu 3/1/07, 3:30PM in Room 184 print event

Quantum Control and Measurement of Large Atomic Spins
A host of techniques for manipulating and observing ultracold atoms make these an attractive platform for testing new ideas in quantum control and measurement. I will discuss recent experiments in which we use AC Stark shifts and magnetic fields to drive non-trivial quantum dynamics of the spin-angular momentum associated with an atomic hyperfine ground state. The nonlinear spin Hamiltonian is sufficiently general to achieve universal quantum control over the 2F+1 dimensional state space, and allows us to both generate any desired spin quantum state and to perform a full quantum state reconstruction of the result. This in turn allows us to implement and track complex quantum dynamics by taking \"snapshots\" of the evolving quantum state. As an example we observe the gradual process of spin squeezing on the Bloch sphere and emergence of a highly non-classical \"Schrödinger kitten\" state. We have also implemented and verified \"optimal control\" to generate a broad variety of spin states, as well as an adiabatic scheme to generate a class of spin-squeezed states that are optimal for precision magnetometry. Broader applications range beyond quantum metrology to quantum information processing and laboratory studies of quantum chaos.

Thu 2/1/07, 3:30PM in Room 184 print event
T. Brun
Quantum Walks on Symmetric Graphs
A quantum walk is a unitary evolution on a Hilbert space corresponding to locations at the vertices of a graph; the particle can only move from one vertex to another if they are connected by an edge. Quantum walks have attracted a great deal of attention recently as a new paradigm for quantum algorithms. We look at the idea of hitting time for a quantum walk--the average time for the walk to reach a particular final vertex--and show that quantum interference can produce both dramatic speed-ups (exponentially shorter hitting times) and slow-downs (infinite hitting times). This is most strongly affected by the symmetry of the underlying graph. For a large class of graphs, we find a sufficient condition for hitting times to be infinite, based on the representations of the automorphism group of the graph; we also show that for some initial conditions, the quantum walk effectively exists on a much smaller quotient graph, which can lead to speed-ups. These are illustrated with examples of simple graphs.

Thu 1/25/07, 3:30PM in Room 184 print event
Patrick Hayden
Un-Distributing Quantum Information
Thermodynamics places surprisingly few fundamental constraints on information processing. In fact, most people would argue that it imposes only one, known as Landauer\'s Principle: a process erasing one bit of information must release an amount kT ln 2 of heat. It is this simple observation that finally led to the exorcism of Maxwell\'s Demon from statistical mechanics, more than a century after he first appeared. Ignoring the lesson implicit in this early advance, however, quantum information theorists have been surprisingly slow to embrace erasure as a fundamental primitive. Over the past year, however, it has become clear that a detailed understanding of how difficult it is to erase correlations leads to a nearly complete synthesis and simplification of the known results of asymptotic quantum information theory. As it turns out, surprisingly many all the tasks of interest, from distilling high-quality entanglement to sending quantum data through a noisy medium to many receivers, can be understood as variants of erasure. I\'ll sketch the main ideas behind these discoveries and end with some speculations on what lessons the new picture might have for understanding information loss in real physical systems.

Thu 1/18/07, 3:30PM in Room 184 print event
D. Gottesman
1-dimensional spin chains are hard
Kitaev showed that the 5-local Hamiltonian problem, which asks for the ground state energy of a Hamiltonian interacting up to 5 qubits in a term, is computationally difficult in precise sense (it is complete for the complexity class QMA, the quantum analogue of NP). A series of later results improved this to show that finding the ground state energy remains QMA-complete even for a Hamiltonian only interacting nearest-neighbor qubits in a 2-dimensional lattice. In this talk, I will sketch a proof that nearest-neighbor interactions in a line are sufficient. The individual particles are not qubits now, but 12-state systems. This result implies that there can exist 1-dimensional systems that cannot relax to their own ground states in a reasonable amount of time.

Thu 0/0/07, 3:30PM in Room 184 print event
Peter D. D. Schwindt
CAS Seminar
Historically, neutral atoms have been the quintessential proving ground for quantum mechanics. However, the intersection between atomic physics and Microsystems has generated a renewed interest in neutral atoms for practical devices such as magnetometers and inertial sensors as well as a medium for quantum information processing. Using the techniques of micromachining, researchers at Sandia and at the National Institute of Standards and Technology have pioneered the drastic miniaturization of vapor cell atomic clocks, magnetometers, and gyroscopes while little has been sacrificed in terms of performance. In particular, I will discuss miniaturized atomic magnetometers. In an effort at Sandia we are miniaturizing a “zero-field” magnetometer while maintaining sub-picotesla sensitivity using a novel tuned cavity approach. In another effort, we are developing improved “atom chips” to cool and trap ultra-cold neutral atoms within tens of microns the chip’s surface. In addition, high-finesse, open-access optical cavities will be integrated onto the atom chip to allow strong atom-photon coupling. Such devices are essential for quantum computers and networks based on neutral atoms as the qubits.

Thu 12/7/06, 3:30PM in Room 184 print event
B. Lev
OH Hyperfine Ground State: From Precision Measurement to Molecular Qubits
We perform precision microwave spectroscopy---aided by Stark deceleration---to reveal the low magnetic field behavior of OH in its ro-vibronic ground state, identifying two field-insensitive hyperfine transitions suitable as qubits and determining a differential Lande g-factor of 1.267(5)X10^-3 between opposite parity components of the Lambda-doublet. The data are successfully modeled with an effective hyperfine Zeeman Hamiltonian, which we use to make a tenfold improvement of the magnetically sensitive, astrophysically important satellite-line frequencies, yielding 1720529887(10) Hz and $1612230825(15) Hz.

Thu 11/30/06, 3:30PM in Room 184 print event
K. Svore
Noise Threshold for a Fault-Tolerant Two-Dimensional Lattice Architecture
In this talk, I will present recent word on a fault-tolerant two-dimensional lattice architecture. We consider a model of quantum computation in which the set of operations is limited to nearest-neighbor interactions on a 2D lattice. We model movement of qubits with noisy SWAP operations. For this architecture we design a fault-tolerant coding scheme using the concatenated [[7,1,3]] Steane code. Our scheme is potentially applicable to ion-trap and solid-state quantum technologies. We calculate a lower bound on the noise threshold for our local model using a detailed failure probability analysis. We obtain a threshold of 1.85 x 10^{-5} for the local setting, where memory error rates are one-tenth of the failure rates of gates, measurement, and preparation steps. For the analogous nonlocal setting, we obtain a noise threshold of 3.61 \\times 10^{-5}. Our results thus show that the additional SWAP operations required to move qubits in the local model affect the noise threshold only moderately.

Thu 11/16/06, 3:30PM in Room 184 print event
J. Yard
Redistributing Quantum Information
I will present the following new result in quantum information theory. Consider many instances of a pure state of four quantum systems ABCR, and suppose that for each copy, Alice holds AC and Bob holds B, while R is an inaccessible reference system. We determine the asymptotic communication and entanglement resources which are necessary and sufficient for Alice to transfer ownership of her C systems to Bob, while negligibly disturbing the purity of the global state. The optimal communication cost gives the first operational interpretation of quantum conditional mutual information on an arbitrary state, and the entanglement cost is positive or negative according to whether entanglement is consumed or generated by the protocol. The corresponding protocol - quantum state redistribution - enjoys two satisfying features: time-reversal symmetry and successive refinability. The protocol includes as special cases the so-called fully quantum Slepian-Wolf, fully quantum reverse Shannon, and state merging protocols. I will thus conclude by showing how state redistribution is a universal protocol from which most known Shannon-theoretic protocols can be derived. This is joint work with Igor Devetak (USC).

Thu 11/9/06, 3:30PM in Room 184 print event
H. Mabuchi
Control theoretic tools in quantum optics and quantum information
One of our group\'s long-term goals is to develop new approaches to studying the interaction of quantum fluctuations with nonlinear dynamics. Of course, historically one of the main model settings in which such studies have been conducted is that of cavity QED. In this talk I will present a new approach to deriving stochastic nonlinear equations to describe semi-classical aspects of cavity QED with strong coupling, and discuss the possibility of using direct numerical simulation of the quantum equations of motion to isolate uniquely quantum-mechanical aspects of nonlinear phenomena such as bifurcations. Our new derivation of semi-classical equations (similar to the Maxwell-Bloch Equations) is based on the control-theoretic technique of geometric filter projection, and illustrates an approach that may be useful in other contexts.

Thu 11/2/06, 3:30PM in Room 184 print event
L. Hollberg
Cold Atoms plus Stable Lasers for Precision Measurements
A growing synergism between precision laser spectroscopy, laser-cooling of atoms, and ultra-fast science is enabling scientific searches in unexplored regions, as well as technological advances of practical importance. Reasonably well developed tools now include: frequency-stabilized cw lasers with spectral (energy) resolving power > 1 part in 1015, cold atoms with absolute motional temperatures < 1 microkelvin, and the revolutionary “optical frequency combs” based on mode-locked lasers. Some of the promising applications are optical atomic clocks with unprecedented performance, searches for time-variation of fundamental “constants”, rapid and precise spectroscopy of atoms and simple molecules over broad spectral regions, pulses with ultra-low timing jitter (< 1fs) and generation of microwave signals with extremely low phase-noise. This field is growing and advancing extremely rapidly because of the new technologies, but also because of new understandings and perspectives. Optics and atomic physics bring several advantages and unique capabilities (very low loss, high-Q’s, quantum information storage for long times), and some fundamental limitations. Reproducible laboratory experiments can be performed to test fundamental theories and postulates, and in some cases can set more stringent bounds on validity.

Thu 10/26/06, 3:30PM in Room 184 print event
J. Renes
Quantum key distribution for the lazy and careless: Noisy preprocessing and twisted states
Quantum key expansion allows two separated parties to accomplish what seems logically impossible: unconditionally-secure expansion of a shared secret key using an untrusted means of quantum communication along with public classical communication. On the quantum side, the parties need only prepare, transmit, and measure individual quantum states. All subsequent \"key distillation\" operations are then performed on the raw classical output data. The security of the procedure can be traced back to entanglement by describing the entire procedure in quantum-mechanical terms. Key distillation becomes entanglement distillation, though the entanglement is virtual and not physical. Since maximal entanglement cannot be shared by more than two parties, keys generated from it are unknown to any eavesdropper. However, maximally entangled states are not the only states which produce private keys; these are generally termed \"twisted states\" or private states. In this talk I will show how to modify the usual picture of key expansion as entanglement distillation to include twisted states. Fortunately, the usual methods of entanglement distillation suffice for this purpose, with the added benefit that some portion of the phase errors need not be corrected at all. This modification also nicely resolves a recently-discovered paradoxical effect that making errors while distilling the key can actually improve the protocol\'s overall resistance to eavesdropping.

Thu 10/19/06, 3:30PM in Room 184 print event
D. Poulin
Iterative Decoding of Quantum Codes
The optimal decoding of an error correction code is an NP-complete problem, and easy instances of this problem typically result in poor codes. Iterative decoding is a technique used to achieve sub-optimal but yet \"good enough\" decoding, and is at the heart of most modern (nearly capacity-achieving) coding techniques. In this talk I will present some recent developments of iterative decoding of quantum error correction codes. Ollivier and Tillich first used this technique for the decoding of quantum LDPC codes. Using the language of operator quantum error correction, I will discuss the underlying principle call the \"conditional renormalization\" of a quantum channel. I will illustrate its use with sever examples: concatenated quantum block codes, quantum convolutional codes, and quantum turbo codes. The results obtained from concatenation show a significant improvement over the blockwise decoding technique typically used in this context, and also indicate that the 5-qubit code\'s threshold is the hashing bound. Results obtained from quantum turbo codes show a behavior quite similar to their classical cousins.

Thu 10/12/06, 3:30PM in Room 184 print event
Jack Burns Center for Astrophysics & Space Astronomy University of Colorado
Simulating the Universe: Giant Light Cones and Cool Cores
Advanced numerical simulations with gas physics are an essential complementary tool with new observations to enable precision cosmology. Using the adaptive mesh refinement hydro/N-body simulation tool Enzo, we are constructing very large synthetic light cones to study upcoming sky surveys from X-ray, Sunyaev-Zeldovich Effect (SZE), and weak lensing observations of galaxy clusters. I will describe some of our initial results of these light cone calculations that mimic SZE surveys with Planck and the South Pole Telescope. In addition, I will discuss simulations that illustrate a new idea for the formation and evolution of rich galaxy clusters that contain cool cores and non-cool cores. We propose a model such that non-cool core clusters (e.g., Coma) have undergone major mergers at an early epoch (z>0.5) which have destroyed the embryonic cool cores. In contrast, we believe that cool core clusters have grown through continuous accretion of smaller mass halos that have not disrupted the central cool cores. I will describe the consequences of this model for using cool core cluster mass estimates as precision cosmology tools.

Fri 10/6/06, 8:30AM in Embassy Suites Hotel , Albuquerque, NM print event

2006 LCS Workshop
5th Annual Workshop on Laser Cooling of Solids and 2nd Annual Review on MURI Project Consortium for Laser Cooling in Solids (CLCS)

Thu 9/28/06, 3:30PM in Room 184 print event
B. Reichardt
To Be Announced
To Be Announced

Thu 9/28/06, 3:30PM in Room 184 print event
B. Reichardt University of California
A probabilistic mixing lemma and quantum fault tolerance
The fragile nature of quantum superpositions makes it particularly important to design robust schemes for fault-tolerant quantum computation. Over the last couple of years, new schemes for achieving fault tolerance based on error detection, rather than error correction, appear to tolerate as much as 3-6% noise per gate -- an order of magnitude better than previous schemes, although with higher overhead. But proof techniques could not show that these promising fault-tolerance schemes tolerated any noise at all. With an analysis based on decomposing probability distributions, we prove the existence of constant tolerable noise rates (\"noise thresholds\") for error-detection-based schemes. The talk will survey these recent developments and present the probabilistic mixing technique. (The talk will not assume any background in fault-tolerant quantum computation.)

Thu 9/21/06, 3:30PM in Room 184 print event
M. Raymer
Entanglement of optical wave-packet modes and collective atomic-ensembles by stimulated Raman scattering
Efficient entanglement generation, distribution and storage are the basic requirements for development of successful quantum information technologies, including proposals for realizing quantum information networks. They include entanglement generation by Raman scattering, storage in collective polarization of atomic ensembles and release by anti-Stokes Raman scattering. Entanglement of light and atomic electronic polarization excited during single-pass stimulated Raman scattering is decomposable into multiple bosonic mode pairs, each pair undergoing independent evolution into a two-mode squeezed state. Experiments are underway. Verification of entanglement is difficult at the macro-level.

Thu 9/7/06, 3:30PM in Room 184 print event
D. Kotchetkov
The end-cap muon spectrometer for the ATLAS detector
The ATLAS detector at the Large Hadron Collider is designed to study proton-proton collisions at the energy of 14 TeV. Measurements of muons are crucial to satisfy the reach of new physics at the energy frontier. The ATLAS muon system consists of the barrel and the end-cap spectrometers optimized for good hermeticity and momentum resolution. The end-cap spectrometer is built in four wheels, Inner, Extra, Middle, and Outer, to cover the pseudorapidity range 1 < |h| < 2.7. The wheels are located at 7, 10, 14, and 21-23 m from the interaction point at both sides of ATLAS. Every wheel is constructed from sector-frames, each carrying one or several chambers. Monitored Drift Tube (MDT) chambers will be installed on all four wheels with 16-fold azimuthal symmetry to perform precision momentum measurements of the muon tracks with pseudorapidities 1 < |h| < 2. The expected resolution of MDT chambers is 10% for transverse momenta pT = 1 TeV. The MDT chambers are assembled from layers of 30 mm aluminum-made drift tubes filled with Ar-CO2 mixture with 2x104 gas gain. Multiwire Cathode Strip Chambers (CSCs) in the Inner station-wheel, with 2.54 mm wire spacing and 4x104 gas gain, are used to cover pseudorapidities 2 < |h| < 2.7 and withstand demanding rates and background conditions. An optical alignment system has been designed to meet requirements of the mechanical accuracy and survey of the momenta measuring chambers. The trigger system covers the pseudorapidity range 1 < |h| < 2.4 and is provided by multiwire Thin Gap Chambers (TGCs), which are installed in the Inner and the Middle wheels and have a thin gas gap of 2.8 mm. Assembly and installation of the Middle wheels are underway at European Organization for Nuclear Research (CERN).

Thu 8/31/06, 3:30PM in Room 184 print event
C. Moore
A Tale of Two Cultures: Phase Transitions in Physics and Computer Science
Certain problems in computer science, such as Satisfiability and Graph Coloring, are classified as NP-complete. These are the hardest search problems, in the sense that if we can find an efficient algorithm to solve them, we could solve thousands of other hard problems as well (and the nature of mathematical truth would be completely different from what we now believe). In the early 1990s, workers in the field of Artificial Intelligence noticed that if we create random instances of these problems, they make a sharp transition, akin to a phase transition in physics such as the freezing of water, in which they abruptly become unsolvable. Over the past 15 years, a marvelous collaboration between mathematicians, computer scientists, and statistical physicists has sprung up to understand these transitions, using a mixture of numerical experiment, physical intuition, and rigorous proof. I will describe some recent results in the area, and discuss how ideas from each field have gained respect and acceptance in the other.

Thu 8/24/06, 3:30PM in Room 184 print event
Krzysztof Wódkiewicz
Directed Spontaneous Emission from an Extended Ensemble of N Atoms
Directed spontaneous emission from entangled Dicke states is investigated. It is shown that a single photon absorbed by the N atoms will be followed by spontaneous emission in the same direction. Furthermore, phase matched emission is found when one photon is absorbed by N atoms followed by two-photon down-conversion.

Thu 6/22/06, 3:30PM in Room 184 print event
Dr. Hong Tang
Nanoelectromechanical System – Mechanical Devices to Study Nanoscale Physics, Weigh Molecules, and Sniff Nerve Gases
The emerging nanoelectromechanical system provides great advantages for both fundamental studies and technological applications. I will first focus on GaAs based nanomechanical devices in the study of confined quantum system, and the mechanical measurement of thin film Spintronics materials. I will then move beyond vacuum and describe the operation of silicon based NEMS in gaseous and fluidic environments. At ambient pressure, in contract to common belief that nanomechanical devices lose their mechanical quality factors, instead, our nanomechanical resonators have demonstrated sensitive mass detection at the level of individual molecules, with mass sensitivity reaching sub-attogram (10-19g). As a specific application, these nanomechanical sensors have shown promise in detecting chemical warfare agents at part per trillion level (ppt). In the fluidic environment of the nano-bio interface, nanomechanical devices face close-to-zero Reynolds number fluidic damping. I will present our approaches to overcome this problem and demonstrate mechanical detection of individual pathogens. Finally, I will discuss some exciting prospects of extending NEMS to cross-disciplinary areas of nanoscale biochemistry, microfluidics and nanophotonics.

Fri 5/26/06, 3:30PM in Room 184 print event
S. Bravyi
Computational Complexity of Spin Hamiltonian Problems
Evaluation of the smallest eigenvalue of a quantum spin Hamiltonian with interactions involving only a few spins is known as the "Local Hamiltonian Problem" (LHP). We study computational complexity of LHP in a special case: all off-diagonal matrix elements of a Hamiltonian in the standard basis must be non-positive real numbers. Equivalently, a Hamiltonian must have non-negative Gibbs matrix (NGM) for any temperature. We prove that LHP with NGM belongs to the complexity class AM --- probabilistic version of NP with two rounds of communication between the prover and the verifier. Also we show that NGM LHP is hard for the class MA. With the additional promise of Hamiltonian having a polynomial spectral gap, we show that NGM LHP belongs to the class POSTBPP --- a generalization of BPP in which a postselective readout is allowed. This is a joint work with David DiVincenzo and Barbara Terhal.

Thu 5/18/06, 3:30PM in Room 184 print event
V. Gupta
Towards a Theory of Coupled Processes on Ordered Channel Networks
Many physical variables in river basins, for example, channel network geometry (basin areas, channel lengths), hydraulic-geometry (channel widths, depths, velocities, slopes), and peak flows for individual rainfall-runoff events, show presence of power laws with respect to drainage areas, and statistical fluctuations around them. On-going work by Bruce Milne suggests that this observation extends to patches of riparian vegetation along a river network. Power laws arise as a solution of a functional equation that results from an invariance property under a change of scale. In the present context, power laws represent mean behavior of physical and ecological variables with respect to drainage areas. Mean statistics of drainage areas and stream lengths obey Horton relationship on ordered networks that has been widely known for over fifty years. Recent work by us has generalized the Horton relationship from means to full probability distributions of rescaled drainage areas and stream lengths. It represents statistical self-similarity in these variables on ordered networks. Power laws in physical variables can be combined with generalized Horton relationship for drainage areas to infer statistical self-similarity in probability distributions of rescaled physical variables on ordered networks. Existing data sets are inadequate to directly test statistical self-similarity in peak flows and hydraulic-geometry on ordered networks. Can the generalized Horton relationship in physical and ecological variables be understood from physical and biological processes? This is a big question, and progress is slowly being made towards answering it. Specifically, a mathematical theory of coupled processes on ordered channel networks (COPON) is being developed to predict statistical self-similarity in probability distributions from physical and biological processes. Statistical self-similarity is an asymptotic property of a system, which is not built into the physical equations. I will illustrate how statistical self-similarity in peak flow statistics on ordered networks arises from a coupled system of conservation equations for a large number of hillslope-links into which a basin is partitioned. I will briefly describe a field program in the 1100 km2 Whitewater basin, Kansas, which is developing a unique database to test COPON theory. This theory will enable us to predict basin-wide alterations of atmospheric, hydrologic, landscape and ecological responses to natural hydro-climate fluctuations and changes resulting from human practices.

Wed 5/17/06, 4:00PM in Room 184 print event
Professor Robert G. Clark
SINGLE ATOM NANOELECTRONICS AND SPINTRONICS FOR SILICON-BASED QUBITS
The demanding challenge of demonstrating silicon-based quantum computing at the few qubit level, in which the qubits are comprised of engineered phosphorus atoms embedded in a silicon host with quantum information encoded onto the electron charge or spin state of these atoms, has required the development of single-atom-based fabrication strategies combined with aligned control gates and sensitive readout devices (electrometry). Fully configured qubit devices have been constructed in a top-down fabrication strategy, in which the phosphorus atoms are implanted individually at low energy into the high purity silicon substrate using nanoscale apertures and on-chip verification (atom counting), with qubit readout provided by radiofrequency single electron transistors. A high precision bottom up approach, involving STM lithography and silicon MBE has been pursued in parallel. Following a systematic development program from atom clusters down to single atoms, in which amongst other things we have demonstrated Si:P as a new (buried) double quantum dot system and quantum cellular automata, we have recently measured several 2-P-atom devices and a 4-P-atom device. RF-SET measurements of the 2-P-atom devices demonstrate an ability to controllably transfer a single electron between the two atoms in a P-P+ charge qubit configuration, and in pulsed gate voltage measurements of a device with exactly two P atoms prepared using apertures spaced by 50nm, Fourier-transformed oscillatory structure in charge relaxation data between qubit levels is quantitatively consistent with (acoustic) phonon-assisted tunneling with wavelength matched to a 52nm donor separation. Following the calibration of 0-40 GHz microwave spectroscopy on superconducting Cooper pair box charge qubit devices and single Cooper pair transistors (fabricated in-house) by observation of sideband structure (driven Rabi oscillation), we have carried out microwave spectroscopy on the 4-P-atom device. The frequency, power and magnetic field dependance of the data is striking, and is entirely consistent with a particular level structure of this engineered multiple atom device. Spectroscopy measurements in progress on a 2-P-atom device with a close atom spacing are anticipated to provide charge qubit demonstration. Finally, in the move from charge to spin qubits we describe the development of nano-Schottky technology combined with single atom engineering for single spin readout and control, and a P-atom-rail \'bus\' architecture (2D) for coherent spin transport, namely Coherent Transport Adiabatic Passage (CTAP). A key target (6-P-atom) device (2xCTAP3) will be described for the demonstration of the essential elements of a scaleable architecture for spin qubits in silicon. Here the qubit devices are combined with on-chip control electronics at low temperature, for which we have designed, fabricated (via Peregrine Semiconductors) and measured a low power-dissipation rf-CMOS pulsed voltage source for mK operation.

Fri 5/12/06, 1:00PM in Room 184 print event
M. Leifer
Quantum Causal Networks
In this talk, I will outline quantum generalizations of causal networks that are used in the classical case to analyze complex decision scenarios involving large numbers of correlated random variables. Firstly, I will motivate the work by outlining potential applications, including the simulation of quantum systems and the equivalence and verification of quantum protocols. I will then review the framework of classical causal networks, including entailed conditional independence, d-separation and Markov equivalence. I will show how to generalize the definition of causal networks to the quantum case in two distinct ways, depending on whether the networks represent states or entire processes. In the former, most of the results from the classical case have straightforward generalizations. For the latter, I introduce a modified variant of the usual isomorphism between quantum states and completely positive maps, which allows them to be partially classified along the lines of the classical results.

Thu 5/11/06, 3:30PM in Room 184 print event
S. Kokkelmans
Fermionic Superfluidity with Positive Scattering Length
Superfluidity in an ultracold Fermi gas is usually associated with either a negative scattering length, or the presence of a two-body bound state. We show that none of these ingredients is necessary to achieve superfluidity. Using a narrow Feshbach resonance with strong repulsive background interactions, the effective interactions can be repulsive for small energies and attractive for energies around the Fermi energy, similar to the effective interactions between electrons in a metallic superconductor. This can result in BCS-type superfluidity while the scattering length is positive.

Thu 4/27/06, 3:30PM in Room 184 print event
Deborah Evans
Exactly Solvable Models in Condensed-Phase Dynamics
The calculation of the dynamics of a small quantum system coupled to condensed phase bath is extremely important in chemical physics. We will show how to compute the reduced system density matrix exactly for a large class of "system-bath" Hamiltonians, namely those for which the system Hamiltonian and the system factor in the system-bath coupling term commute. For this class of problems, the Markovian limit of the equations of motion form a positive semigroup and for bilinear coupling to a harmonic bath, local second order perturbation theory is exact, even for strong system-bath coupling. An analytically solvable model of a multi-level condensed phase quantum system relevant to vibrational relaxation and electron transfer is presented. Exact solutions are derived for the reduced system density matrix dynamics of a degenerate N-level quantum system coupled to a dissipative harmonic oscillator bath. We demonstrate that for N>2 the long-time steady state system site occupation probabilities are not the same for all sites, i.e., they are distributed in a non-Boltzmann manner which depends on the initial conditions and the number of levels in the system. These ideas are then applied to an analysis of electron transfer in non-rigid molecular systems where motion of the molecular "bridge" is strongly coupled to the electron transport.

Thu 4/20/06, 3:30PM in Room 184 print event
L. Faoro
Controlling Decoherence in Superconducting Qubits
Despite remarkable experimental breakthroughs of recent years, the realization of quantum computer using small superconducting circuits containing Josephson junctions remains far away. Neither of these circuits in fact satisfies the stringent requirements on the amount of allowed decoherence imposed by the fault-tolerant quantum computation. Thus, it is crucial to identify (and possibly eliminate) the microscopic origin of the noise in these devices. It is commonly believed that the most important source of noise for superconducting qubits is the motion of charges of unknown origin in the substrate and the insulating barrier that is responsible for both the charge noise and critical current fluctuations. Recent experiments performed on SET and superconducting qubits provide an almost complete characterization of the charge and critical current power spectrum. The talk consists of two parts. In the first one, I will briefly overview the different superconducting qubits designs and illustrate the experimental status of the art. In the second part, I will discuss two microscopic mechanisms that might be responsible for the charge noise. Specifically, first we consider a model of dephasing due to the environmentof weakly interacting quantum TLSs and we show that such environment might provide a significant source of dephasing but the detailed characteristics of the noise power spectrum are in a qualitative and quantitative disagreement with the experimental data. Then we propose a novel microscopic mechanism of decoherence based on Kondo-like impurities and argue that within this model we can explain most features observed in the experiments.

Thu 4/13/06, 3:30PM in Room 184 print event
Aephraim Steinberg
Manipulating and Measuring Quantum Information: Some Experiments
Throughout the 20th century, the question of quantum measurement has confused and intrigued physicists. At the dawn of the 21st, these issues have taken on new practical importance due to the birth of the interdisciplinary science of \"quantum information.\" The realization that quantum mechanics allows communications more secure than one could ever have classically, and computation exponentially more efficient than any known classical algorithms, has incited a huge amount of research into this new area, which has in turn provided an exciting new perspective on quantum mechanics. Motivated in part by these considerations, my lab has been carrying out a variety of experiments on controlling simple quantum systems and comparing different techniques for \"measuring\" their wave functions, density matrices, or phase-space distributions. I will describe some of the current issues in measurement and characterisation of quantum systems, and show the results of some of our recent experiments, including the generation of multi-photon entangled states for \"super-resolution\" and the optimisation of pulse sequences for control of coherence in optical lattices. In addition, if I speak quickly enough, I may discuss how many seeming paradoxes in quantum mechanics appear to be resolved if one considers the outcomes of so-called \"weak measurements\"... but only at the expense of accepting some truly surreal results.

Thu 4/6/06, 3:30PM in Dane Smith, Room 100 print event
William Phillips
A Bose Condensate in an Optical Lattice: Cold Atomic Gases Meet Solid State Physics
An atomic-gas Bose-Einstein Condensate, placed in the periodic light-shift potential of an optical standing wave, exhibits many features that are similar to the familiar problem of electrons moving in the periodic potential of a solid-state crystal lattice. Differences include the distance scale of the lattice (100s of nanometers compared to a few Ångstroms) and the fact that the BEC represents a wavefunction whose coherence extends over the entire lattice, with what is essentially a single quasi momentum. Recent experiments at NIST-Gaithersburg explore the behavior of a BEC in an optical lattice and interpret the sometimes surprising results using traditional band theory.

Tue 3/28/06, 3:30PM in Regener, Room 103 print event
Roy Glauber
A Hundred Years of Light Quanta

Thu 3/9/06, 3:30PM in Room 184 print event
Paul Julienne
Resonance Control of Collisions in Atomic Gases
Magnetically or optically tunable scattering resonance states can be used to tune the collision properties of atoms in ultracold atomic gases. Such tuning has been used for precise control of the scattering length as well as the production of ultracold molecules for both bosonic or fermionic atomic species. For example, extremely large molecules of the Li-6 dimer with mean interatomic separation larger than 100 nm can be made, Bose condensed, and quantitatively probed by radiofrequency photodissociation. Recent experimental and theoretical work shows that collisions of Group II elements like Sr should be subject to optical Feshbach control by using very narrow molecular photoassociation resonances near the atomic intercombination line.

Thu 3/2/06, 3:30PM in Room 184 print event
Andrew Childs
Quantum Algorithms by Optimal State Estimation
One of the major challenges facing quantum computation is to better understand what problems can be solved faster by quantum computers than by classical ones. In particular, we would like to develop new algorithmic tools for obtaining quantum speedup. In this talk, I will present an approach to quantum algorithms based on optimal measurements for distinguishing quantum states. This approach has led to new quantum algorithms with exponential speedup for certain instances of the hidden subgroup problem and other related problems. These algorithms implement joint measurements on several copies of the states to be distinguished, which is significant since independent measurements are provably insufficient in some cases. I will present such an algorithm in detail for the hidden subgroup problem over the Heisenberg group, and I will survey other recent developments. This talk is based on joint work with Dave Bacon (University of Washington) and Wim van Dam (UC Santa Barbara).

Thu 2/23/06, 3:30PM in Room 184 print event
Shohini Ghose
Quantum Dynamics of Chaotic Systems
The study of quantum systems with a chaotic classical limit is of importance both for understanding the correspondence between the quantum and classical world as well as in the context of controlling complex quantum systems for information processing applications. I will describe our studies of dynamical entanglement in quantum chaotic systems. We analyze a multiqubit system that collectively behaves as a quantum kicked top, and identify signatures of classical chaos in the bipartite and pairwise entanglement dynamics. Furthermore, we define a global entangling power that can be related to the classical Lyapunov exponent. We show that generic features of unitarily evolving quantum chaotic systems can be explained by examining the spectral properties of the evolution operator. A physical system of trapped atoms in a magneto-optical lattice is discussed, in which quantum chaotic dynamics can be studied in a controlled manner. Our analysis of the classical limit of this system shows that large entanglement can persist in the classical regime.

Wed 2/22/06, 3:30PM in Room 184 print event
Shohini Ghose
Quantum dynamics of chaotic systems
The study of quantum systems with a chaotic classical limit is of importance both for understanding the correspondence between the quantum and classical world as well as in the context of controlling complex quantum systems for information processing applications. I will describe our studies of dynamical entanglement in quantum chaotic systems. We analyze a multiqubit system that collectively behaves as a quantum kicked top, and identify signatures of classical chaos in the bipartite and pairwise entanglement dynamics. Furthermore, we define a global entangling power that can be related to the classical Lyapunov exponent. We show that generic features of unitarily evolving quantum chaotic systems can be explained by examining the spectral properties of the evolution operator. A physical system of trapped atoms in a magneto-optical lattice is discussed, in which quantum chaotic dynamics can be studied in a controlled manner. Our analysis of the classical limit of this system shows that large entanglement can persist in the classical regime.

Thu 2/16/06, 3:30PM in Room 184 print event
Dietrich Leibfried
Prospects For a Large Scale Quantum Information Processor Based On Trapped Ions
Atomic ions confined in an array of interconnected traps represent a potentially scalable approach to quantum information processing. All basic requirements have been experimentally demonstrated in one and two qubit experiments. The remaining task is to scale the system to hundreds and later thousands of qubits and minimize errors in the system. While this requires extremely challenging technological improvements, no fundamental roadblocks are currently foreseen. The talk will give a survey of recent progress in implementing simple quantum algorithms with up to 6 qubits in ion trap arrays. The prospects and challenges of scaling this particular approach towards a large scale computing device will also be summarized. * Work supported by DTO and NIST

Thu 2/9/06, 3:30PM in Room 184 print event
JM Geremia
TBA
TBA

Thu 2/9/06, 3:30PM in Room 184 print event
JM Geremia
Deterministic Fock State Generation via Quantum Feedback Control
It is possible to generate desired number states of an optical cavity mode on demand by embedding a continuous, backaction-evading photon number measurement within a real-time quantum feedback control loop. As the continuous measurement gradually reduces the cavity mode to a Fock state, feedback drives the cavity amplitude quadrature to stabilize the photon number to the desired Fock state with high certainty, even in the presence of cavity decay. I will present a theoretical analysis of the stability of the proposed feedback control procedure and describe an experiment, about to begin at the University of New Mexico, to implement this feedback-mediated source of deterministic photon number states.

Thu 2/2/06, 3:30PM in Room 184 print event
Jim Harrington
A Fault-tolerant One-way Quantum Computer
Fault tolerance encompasses the study of robustly carrying out tasks (such as a complex computation) on a device made up of faulty parts. In this work (found at quant-ph/0510135) we improve the value of the accuracy threshold for fault-tolerant universal quantum computation on a particular architecture. Specifically, we consider a three-dimensional cluster state, which falls under the class of one-way quantum computers introduced by Raussendorf and Briegel. Following the connection between toric codes and cluster states established in [Phys. Rev. A 71, 062313 (2005)], we determine a threshold for both a simple depolarizing error model (1.4%) and a more realistic error model containing preparation, gate, storage, and measurement errors (0.11%).

Mon 12/12/05, 4:00PM in Room 190 print event
Alan Aspuru-Guzik
Quantum Computation for Quantum Chemistry
The calculation time for the energy of atoms and molecules scales exponentially with system size on a classical computer, but polynomially using quantum algorithms. We demonstrate that such algorithms can be applied to problems of chemical interest using modest numbers of quantum bits. Calculations of the H2O and LiH molecular ground-state energies have been carried out on a quantum computer simulator using a recursive phase estimation algorithm. The recursive algorithm reduces the number of quantum bits required for the read-out register from approximately twenty to four. Mappings of the molecular wave function to the quantum bits are described. An adiabatic method for the preparation of a good approximate ground-state wave function is described and demonstrated for stretched H2. The number of quantum bits required scales linearly with the number of basis functions used and the number of gates required grows polynomially with the number of quantum bits.

Thu 12/8/05, 3:30PM in Room 184 print event
P. Nussenzveig
Bright Entangled Optical Beams of Different Frequencies
In this talk, I will describe the ongoing experimental effort in Sao Paulo to generate and characterize bright entangled optical beams for applications to quantum information. Our experiments are carried out with a nondegenerate optical parametric oscillator (OPO), operating above threshold. In 1988, it was predicted that this system would produce entangled light beams, but this hadn\\\'t been measured to date, owing to the difficulty in measuring phase noise. Typically, the twin beams have different frequencies, which are very difficult to control. Hence, one does not have local oscillators for homodyne measurements. We overcome this difficulty by a self-homodyne technique and were able to obtain data which meet the two most commonly used criteria for continuous-variable entanglement.

Thu 12/1/05, 3:30PM in Room 184 print event
L. Zane
The Paradox of the Amazing Shrinking Space Ship
As a space ship in Earth’s reference frame speeds up, its Earth-frame length contracts. Consequently Earth-frame observers see the passenger at the rear of the space ship travel further than the captain piloting the ship from the front. This apparently allows the average speed of the passenger to be greater than the speed of light in Earth’s frame. In this presentation, the motion of the Amazing Shrinking Space Ship will be carefully analyzed and the paradox resolved.

Thu 11/17/05, 3:30PM in Room 184 print event
D. Berkeland
Quantum Simulations in Ion Traps
A quantum simulator is necessary to solve many-body quantum problems that would be intractable using a classical computer, even with advanced numerical techniques. Quantum simulators can solve only a limited set of problems, but building one would represent an important milestone in the road to universal quantum computation. We are using an array of strontium ions confined in a linear rf trap to build a multi-body quantum simulator. In our experiment, each ion simulates a single spin system, while Coulomb and optical forces simulate spin-spin interactions and magnetic fields. This system can simulate the most basic models of condensed matter physics: the Ising model and the Heisenberg XY model. In collaboration with Sandia National Laboratories, we are building more complex ion traps that will let us work with tens of ions, and with two-dimensional arrays of ions. Ultimately, these systems will show us how to perform complex simulations in two dimensions, investigate new ordered states of matter, and further develop technology for a universal quantum computer.

Thu 11/10/05, 3:30PM in Room 184 print event
D. Blume
Coupled-Channel Pseudo-Potential Description of Feshbach Resonance in Two Dimensions
The success in creating Bose-Einstein condensates and Fermi degenerate gases has resulted in a renewed interest in the scattering between two atoms. At very low temperatures the de Broglie wavelengths of the atoms are large compared to the typical van der Waals length of atom-atom potentials. As a result, atoms at ultracold temperatures do not probe the detailed structure of the atom-atom interaction potential. Consequently, the shape-dependent atom-atom interaction potential can be replaced by an appropriate zero-range pseudo-potential, which is characterized by a single parameter, the generalized scattering length. The foremost advantage of using zero-range pseudo-potentials is that a number of two-body and some many-body problems become analytically solvable, thus highlighting the physical meaning of a few key parameters. This talk derives pseudo-potentials that describe the scattering between two atoms in two spatial dimensions for any partial wave m, whose scattering strength is parameterized in terms of the m-dependent phase shift. Using our proposed pseudo-potential for the lowest partial wave, i.e., for m=0, we develop a coupled channel model with 2D zero-range interactions, which describes the two-body physics across a 2D Feshbach resonance. Implications for on-going experiments will be discussed.

Thu 11/3/05, 3:30PM in Room 184 print event
Professor S. Forrest
Computation in the Wild
\"Our software infrastructure confronts a situation increasingly similar to the challenges faced by living organisms in a biological ecosystem. Highly dynamic, complex, and hostile environments are placing new demands on our computational infrastructure. Biological design principles can potentially change the way we engineer, maintain, and evolve large dynamic software infrastructures. Examples of such principles include: adaptability, homeostasis, redundancy, and diversity. The talk will illustrate how biological design principles are providing new insights and approaches in the field of computer security. The talk will describe recent progress on several related projects that are investigating the problem of automated response. \"

Thu 10/27/05, 3:30PM in Room 184 print event
T. Porto
Controlled Collisions in Dynamic Optical Lattices.
I will describe the experimental realization of two optical lattices that allow for dynamic control of the lattice topology and spacings. The first, a 2D double well lattice, is able to isolate neighboring pairs of atoms within a lattice. This lattice allows for a variety of experiments involving controlled two-atom collisions, and in particular provides a test bed for two qubit gates in neutral atoms. It also could be useful in realizing interesting extended Bose-Hubbard models and is well suited for realizing quantum cellular automata. The second, a stretchable \"accordion\" lattice, allows for dynamic control of the lattice spacing, and can be used to study commensurate/incomensurate states in two period lattices or to isolate 2D planes of atoms.

Thu 10/20/05, 3:30PM in Room 184 print event
M. Lilly
Experiments to Explore the Role of Interactions and Coherent Transport in One-Dimensional Systems
The observation of quantized conductance steps in ballistic semiconductor quantum wires is an early example of the now very broad field of nanoelectronics. Although the physics of plateaus at quantized values of G0 = 2e2/h is easily understood using non-interacting quantum mechanics, Coulomb interactions are expected to play a key role in 1D systems. In this talk, transport experiments on single and double quantum wires will be presented. In the first part, the interplay between disorder and interactions in long single wires and wires with a variable density will be tested with a variety of conductance measurements. The second part of the talk focuses on tunneling in a system of vertically coupled quantum wires. These nanostructures are fabricated from bilayer electron samples with electron beam lithography on both top and bottom defining the double wire. Parallel conductance as a function of split gate voltages provide a map of the 1D subband occupations; tunneling measurements can be made with any combination of subbands occupied in each wire. The full tunneling spectroscopy is measured using both a voltage between the wires and a parallel magnetic field to learn about both the energy and momentum dependence of tunneling events. We compare the data to a non-interacting model of tunneling. Deviations from the simple picture may require analysis of the 1D systems as Luttinger liquids. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000.

Thu 9/15/05, 3:30PM in Room 184 print event
S Habib
Quantum Active Cooling
Cooling dynamical systems to very temperatures can be achieved by a variety of methods, both passive and active. In this talk I will discuss two more or less realistic recent proposals for dynamical cooling of single quantum degrees of freedom -- (i) the center of mass motion of a trapped atom in a high-finesse cavity and (ii) a high-Q nanoscillator -- via active feedback techniques. The essential quantum nature of the cooling process both in terms of the underlying dynamics and measurement backaction will be emphasized.

Thu 9/8/05, 3:00PM in Room 184 print event
Mr. H. Haselgrove
Noise thresholds for optical quantum computers
I shall report on efforts to calculate the noise threshold for optical quantum computing. We are considering the recent proposal for combining aspects of the original KLM scheme for optical quantum computation, with the so-called cluster state model of computation. I will describe how one may go about designing an error-correction protocol in this scheme, for resistance to a range of noise types. Numerical threshold results will be given showing the amount of noise permissible for reliable optical quantum computing to occur.

Thu 9/1/05, 3:30PM in Room 184 print event
Robert Nelson
Nuclear “Bunker Busters,” “Mini-nukes” and the future of the U.S. Nuclear Stockpile
Senior members of the Bush Administration and some members of America’s nuclear weapons laboratories have advocated that the United States develop a new generation of nuclear weapons: nuclear “bunker busters” for destroying deeply buried targets, high-precision “mini-nukes” for reduced collateral damage, and agent defeat weapons for destroying underground stockpiles of chemical and biological (CB) agents. Because they are intended to detonate below ground and have lower yields than would be required for a surface detonation, nuclear advocates sometimes describe these weapons as having “minimal collateral damage,” and could be used near densely populated areas. However, simple physical arguments show that no earth penetrating weapon can penetrate deeply enough to contain a nuclear explosion and will necessarily produce especially intense and deadly radioactive fallout. Furthermore, even an earth-burrowing nuclear warhead is unlikely to sterilize buried chemical or biological agents. The explosion may simply disperse active CB agents into the environment. A more sensible strategy would be to use conventional means to seal all entrances and exits to the facility, and keep them sealed until the territory can be captured and the agents carefully neutralized.

Wed 8/31/05, 10:00AM in Room 1131, Physics & Astronomy print event
Professor Robert G. Clark
Progress in the Demonstration of Single Atom Qubits in Silicon

Thu 8/25/05, 3:30PM in Room 184 print event
Assistant Professor A. Landahl
Designing quantum computers with static spin-spin interactions
Quantum spins are natural candidates for storing quantum bits. To process the spins, current spin-based quantum computers modulate spin-spin interactions dynamically. In this talk, I explain how spin-based quantum computers can be designed so that they only use static spin-spin interactions. By converting the dynamical problem to a static one, quantum circuit errors can be corrected in the fabrication process rather than in the computing process. The analysis uses some recent theoretical developments in quantum information science such as quantum walks. It also relies on some classic quantum mechanics gems by Wigner and Feynman

Wed 8/24/05, 10:00AM in Room 184 print event
Robert Nelson is a Senior Scientist at the Union of Concerned Scientists and a Visiting Research Scientist at the Princeton University Program on Science and Global Security. He is a theoretical astrophysicist by training
Nuclear “Bunker Bunkers,” “Mini-nukes” and the future of the U.S. Nuclear Stockpile
Senior members of the Bush Administration and some members of America’s nuclear weapons laboratories have advocated that the United States develop a new generation of nuclear weapons: nuclear “bunker busters” for destroying deeply buried targets, high-precision “mini-nukes” for reduced collateral damage, and agent defeat weapons for destroying underground stockpiles of chemical and biological (CB) agents. Because they are intended to detonate below ground and have lower yields than would be required for a surface detonation, nuclear advocates sometimes describe these weapons as having “minimal collateral damage,” and could be used near densely populated areas. However, simple physical arguments show that no earth penetrating weapon can penetrate deeply enough to contain a nuclear explosion and will necessarily produce especially intense and deadly radioactive fallout. Furthermore, even an earth-burrowing nuclear warhead is unlikely to sterilize buried chemical or biological agents. The explosion may simply disperse active CB agents into the environment. A more sensible strategy would be to use conventional means to seal all entrances and exits to the facility, and keep them sealed until the territory can be captured and the agents carefully neutralized.

Thu 5/5/05, 3:30PM in Room 184 print event
E.B. Feldman
From Celestial Mechanics to Quantum Computing through Spin Dynamics
Nuclear spin systems have proven to be the most useful models for the investigation of fundamental problems in statistical mechanics. Dynamics in the presence of rapidly oscillating forces has attracted much interest for the last 300 years. We discuss the history of the problem of averaging rapidly oscillating interactions from the 18th century celestial mechanics till modern spin dynamics. We begin with the peculiar dynamics of Kapitza's pendulum which is the simplest system subjected to rapidly oscillating forces. Then we consider time reversal and spin diffusion reversal experiments which seemingly contradict the second law of thermodynamics, and discuss Bogolyubov's method of averaging that serves as the theoretical foundation of the field. We also address recent developments based on exactly solvable models of multiple-quantum dynamics. Possibilities of multiple-quantum NMR for extracting physical-chemical information are discussed. A special attention is atracted to the connection of correlations and multiple-quantum coherences, and to NMR applications to quantum information processing.

Thu 4/28/05, 3:30PM in Room 120 print event

Marlan O. Scully, Princeton and Texas A&M
It is generally believed that the old quantum theory, as presented by Niels Bohr in 1913, fails when applied to few-electron systems, such as the H2 molecule. We have found new solutions within the original Bohr theory that describe the potential energy curve for the lowest singlet and triplet states of H2 about as well as the early wave mechanical treatment of Heitler and London. We also develop a new interpolation scheme which substantially improves the agreement with the exact ground-state potential curve of H2 and provides a good description of more complicated molecules. The present approach emulates dimensional scaling methods of quantum chromodynamics.

Wed 4/27/05, 4:00PM in Room 125 print event
Marlan O. Scully
2005 CAS Distinguished Lecture II: Frozen Light - the Tip of the Iceberg
Recent experiments have slowed light to a few meters per second, stored the quantum state of light and even demonstrated the time reversing of the quantum state of the radiation field. In other words, it has been shown that it is (somewhat ironically) possible to go further toward freezing light by using hot atoms rather than cold. These experiments now pave the way for a wide range of novel applications of ultra-slow light. These include
1. Quantum information and quantum computing devices
2. Ultra-sensitive magnetometers
3. Squeezed states of the radiation field
4. Anomalous Brillouin scattering is possible when the group velocity of light is less than the velocity of sound
5. Dramatic increase of optical nonlinearities associated with coherent media and its application to the generation of ultrashort pulses
6. Phase conjugate nonlinear optics involiving a new kind of ultra-stable oscillator in which the line width is much narrower than could be obtained from a laser
7. Complete quantum teleportation by using the ultra large Kerr nonlinearity associated with ultra slow light.
These are some of the examples of possible applications for this novel effect. It is a fascinating chapter in modern optics.

Tue 4/26/05, 3:30PM in Room 120 print event
Marlan O. Scully
2005 CAS Distinguished Lecture I: Quantum Eraser - Impacting the Future by Erasing the Past
The quantum eraser dramatically underscores the difference between our classical conceptions of time and how quantum processes can unfold in time. Such eyebrow-raising features of time in quantum mechanics have been labeled "the fallacy of delayed choice and quantum eraser" on the one hand and described "as one of the most intriguing effects in quantum mechanics" on the other. In this talk, I will discuss how the availability or erasure of information generated in the past can affect how we interpret data in the present. The quantum eraser concept has been studied and extended in many different experiments and scenarios, for example, the entanglement quantum eraser, the kaon quantum eraser, and the use of quantum eraser entanglement to improve microscopic resolution.

Wed 4/20/05, 4:00PM in NW Wing Conference Room print event
Sonia Daffer
Generating optimal states for a homodyne Bell test
We present a protocol that produces a conditionally prepared state that can be used for a Bell test based on homodyne detection. Based on the results of Munro [PRA 1999], the state is near-optimal for Bell-inequality violations based on quadrature-phase homodyne measurements that use correlated photon-number states. The scheme utilizes the Gaussian entanglement distillation protocol of Eisert et. al. [Annals of Phys. 2004] and uses only beam splitters and photodetection to conditionally prepare a non-Gaussian state from a source of two-mode squeezed states with low squeezing parameter, permitting a loophole-free test of Bell inequalities.

Thu 4/14/05, 3:30PM in Room 184 print event
Krzysztof Wodkiewicz
Cloning of spin coherent states
In this talk I will discuss the optimal cloning of the spin coherent states in Hilbert spaces of different dimensionality. The main resuts that will be presented deal with the optimal cloning transformation for spin coherent states in 3-dimensional space, analytical results for the fidelity of the optimal cloning in d=3 and d=4 as well as numerical results for higher dimensions.

Thu 3/10/05, 3:30PM in Room 184 print event
Ben Schumacher
What is wrong with many worlds?
In many ways, the Everett or "many worlds" interpretation of quantum mechanics is the most ambitious and the most attractive. It aims at providing a complete account of quantum phenomena, including measurement processes and the emergence of classical physics, entirely in terms of the unitary dynamics of a quantum universe. I will argue that this project, so formulated, cannot succeed. I will also discuss a few alternatives, from variants on the "Copenhagen" interpretation to possible new forms of quantum mechanics itself.

Thu 3/3/05, 3:30PM in Room 184 print event
Sergei Maslov
Detecting topological patterns in protein networks
Bio-molecular networks lack the top-down design. Instead, selective forces of biological evolution shape them from raw material provided by random events such as gene duplications and single gene mutations. As a result individual connections in these networks are characterized by a large degree of randomness. One may wonder which connectivity patterns are indeed random, while which arose due to networks's growth, evolution, and/or its fundamental design principles and limitations?

Here we introduce a general method [1,2] allowing one to construct a random version of a given network while preserving the desired set of its low-level topological features, such as, e.g., the number of neighbors of individual nodes, the average level of modularity, numbers of small network motifs, etc. Such a null-model network can then be used to detect and quantify non-random topological patterns. In particular, we measure correlations between numbers of neighbors of interacting nodes in protein binding and regulatory networks in yeast [1]. It was found that in both these networks, links between highly connected proteins are systematically suppressed.

We proceed by presenting a set of empirical findings about how gene duplications shape protein interaction and genetic regulatory networks in several organisms [3]. It is shown that molecular networks in yeast combine the plasticity of regulatory connections with a relative stability of protein functions manifested in the set of their binding partners. We believe this to be a general feature affecting the evolvability of bio-molecular networks.

1. S. Maslov and K. Sneppen, Specificity and Stability in Topology of Protein Networks, Science 296, 910-913, (2002).

2. S. Maslov, K. Sneppen, and A. Zaliznyak, Pattern Detection in Complex Networks: Correlation Profile of the Internet, Preprint at arXiv.org e-Print archive available at http://arxiv.org/abs/cond-mat/0205379, (2002); Physica A 333, 529-540 (2004).

3. S. Maslov, K. Sneppen, and K. Eriksen, Upstream Plasticity and Downstream Robustness in Evolution of Molecular Networks; http://arxiv.org/abs/q-bio.MN/0310028 (2003); BMC Evolutionary Biology, 4:9, pp. 1-12 (2004).

Thu 2/10/05, 3:30PM in Room 184 print event
David Emin
Novel Thermoelectric Materials
The Seebeck and Peltier effects are thermoelectric phenomena upon which many electric-power-generation and electric-refrigeration devices are based. I will begin this talk by describing the underlying physics of the Peltier and Seebeck effects. Then I will indicate how these effects are utilized in thermoelectric devices. The utility of thermoelectric devices can be greatly expanded through the use of materials with unconventional transport. I will illustrate this point by describing the exceptional properties of boron carbides. In particular, I will describe how their distinctive properties can be exploited for surprisingly efficient high-temperature thermoelectric power-generation. Finally, I will describe some materials with unusual low-temperature transport properties. These materials promise to extend the range of thermoelectric coolers to cryogenic temperatures.

Thu 2/10/05, 3:30PM in Room 184 print event
David Emin
Novel Thermoelectric Materials

Thu 1/27/05, 3:30PM in Room 184 print event
Tom Sanford
Progress in z-pinch driven dynamic-hohlraums for high-temperature radiation-flow and ICF experiments at Sandia National Laboratories
Z is the world's most powerful x-ray source. Presently Z is being used to demonstrate the feasibility of a number of ICF concepts. This seminar discusses one such concept, the dynamic hohlraum, which is also used to drive a second concept: the static-wall hohlraum. The dynamic hohlraum has recently demonstrated the production of more than 10**10 D-D thermonuclear neutrons. Included will be a discussion of the power flow through the accelerator to the dynamic-hohlraum load, and scaling of the radiation envirnments produced to those that might be achieved at higher currents.

Thu 1/27/05, 3:30PM in Room 184 print event
Tom Sanford
Progress in z-pinch driven dynamic-hohlraums for high-temperature radiation-flow and ICF experiments at Sandia National Laboratories

Thu 1/20/05, 3:30PM in Room 184 print event
Ken A. Dill
How Proteins Solve Their Folding Problems
Currently, trying to compute the 3-dimensional structure of a protein from its amino-acid sequence is considered a grand challenge. A protein molecule is a polymer with a large number of conformations, so a computer would have to search an astronomical number of them to find the native structure. Yet, some proteins can fold up to their unique native structures in microseconds. Hence, proteins don't appear to regard folding as a "grand challenge." How do they fold so quickly? If we knew the answer, we could design fast computational methods for protein structure prediction. We have explored how proteins fold up, and how to use that information to make faster computer folding algorithms.

Thu 1/20/05, 3:30PM in Room 184 print event
Ken A. Dill
How Proteins Solve Their Folding Problems

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