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The schedule
can be found here.
Complete List of attendees: (titles and abstracts follow below, titles link to talk slides)
Designing Spin Lattice Hamiltonians with Polar Molecules in Optical Lattices by Gavin Brennen (with A.M. Micheli, P. Zoller), Universität Innsbruck Motivated by theoretical discoveries of emergent exotic phases from quasi-local spin interactions we show how to simulate a broad class of lattice spin models in one or more dimensions using trapped polar molecules in an optical lattice. The spin corresponds to electronic degrees of freedom of a molecule in its ground rotational state. Spin dependent couplings between molecules are obtained using a microwave field to mix ground states with dipole-dipole coupled rotationally excited states. The interaction strengths are fast relative to decoherence times and any permutation symmetric two spin coupling is designable using several microwave fields. We illustrate how to build two models: one on a 2D square lattice with an energy gap providing for protected quantum memory, and another on stacked triangular lattices leading to topological quantum computing.
Classical Approaches to Quantum Decoherence by Paul Brumer, University of Toronto We demonstrate that the "classical Wigner approach", wherein an initial state is described quantum mechanically, but where the subsequent mechanics is computed classically, provides a useful tool for calculating rates of decoherence. The basis of this approach, as well as applications to several systems, will be described. A comparison of computed rates with simplified Master equation models demonstrates the failure of these models and the need for realistic computations.
Tackling the challenges of quantum computing with trapped Cd+ ions by Louis Deslaurier, University of Michigan Trapped atomic ions have a number of desirable features that make them well suited for quantum information applications. Pairs of hyperfine ground states can behave as an ideal quantum bit, and entanglement between multiple ions can proceed through Coulomb interaction mediated by appropriate laser fields. Many of the critical components of a trapped ion quantum computer have now been demonstrated here at Michigan as well as by a few other groups around the world. I will review the Michigan effort which consists of three main thrusts: Ion-ion entanglement, ion-photon entanglement, and scalable trap development.
Our calculations are
based on
the Time-Convolutionless Projector (TCL) technique, as well as more
standard
Born-Markov methods. For quantum-dot charge qubits we will argue that
recent
experiments [1-3] are close to the intrinsic material limits, and show how
these limits scale with the size of the system down to the dimensions of
atomic
defects. For spin qubits, we show how coupling them through
localised
excitations with well-defined level structures [4] can provide
particularly
effective ways of minimising decoherence while producing entangled spin
states.
Decoherence and Nonadiabatic Rate Processes by Raymond Kapral, University of Toronto The computation of the rates of quantum mechanical reactions in condensed phase environments entails sampling from suitable initial states and quantum evolution of operators that characterize metastable sets. The talk will focus on the simulation of such rates using quantum-classical Liouville dynamics that describes the evolution of a quantum subsystem coupled to a classical bath. Simulations of nonadiabatic quantum-classical dynamics can be carried out in terms of ensembles of surface-hopping trajectories that involve evolution on single adiabatic surfaces, interspersed with quantum transitions to coherently coupled adiabatic surfaces where the trajectory contribution carries a phase, followed by transitions to single adiabatic surfaces that destroy the coherence. The presence of these coherent segments and the destruction of the coherence by the environment play an important role in determining the rates and mechanisms of quantum reactions in the condensed phase. The role of decoherence on quantum rate processes will be illustrated by computations on proton transfer reactions in solution and on model two-level systems.
High fidelity Josephson phase qubits . winning the war on decoherence by Nadav Katz, Physics Department and California NanoSystems Institute, UCSB Superconducting qubits are considered one of the leading technologies for scalable quantum computation. The Josephson phase qubit can be thought of as a .tunable atom. where the energy levels used as the qubit are tuned via external controls. Decoherence due to coupling to various material defects has been identified as a major source of decoherence. Recently we have dramatically improved the coherence of our qubits by quantifying various decoherence pathways, and removing them via optimized fabrication. I will present some recent results, including quantum state tomography of the qubit state, analysis of qubit evolution due to partial measurement, and the generation of various Bell states in coupled qubits.
Measuring and
Reducing the
1/f Noise in Josephson Junctions for Potential use as qubits by Jan Kycia,
Department of Physics and The Institute for Quantum Computing, University
of Waterloo
Stretching the electron as far as it will go by Gordon Semenoff, UBC Vancouver It is argued that zero modes of majorana fermions can mediate a teleportation-like process with actual transfer of electronic material between well separated points. The process is illustrated using a quasi-realistic, exactly solvable model of a quantum wire embedded in a p-wave superconductor.
Restoring Coherence Lost to a Mesoscopic Bath by LuJ. Sham (with Wang Yao and Ren-Bao Liu), UCSD We present a quantum solution to the electron spin decoherence without stochastic assumptions. The many-body problem of a mesoscopic system of N interacting nuclear spins in the presence of a single electron spin is solved by a nuclear pair-correlation approximation, shown to be valid when N is bounded. Computation results on free-induction decay of the single electron spin coherence and the restoration of coherence will be presented. The question of the applicability of our theory to a wider class of physical systems will be presented for discussion.
Many-body EPR breakup and fragments' decoherence by Moshe Shapiro, UBC Vancouver We explore EPR correlations in the breakup of polyatomic molecules into two mutually entangled fragments. We discuss how this entanglement is related to "coherent control" and give a derivation based on the properties of the dissociated wave function that no information is transferred, not even at a speed smaller than the speed of light, from one entangled partner to the other concerning its measurement or lack thereof. We also show how one can attain a variable degree of entanglement, using which, we explain some experimental results as due to the gradual switching off of entanglement.
Quantum superconductor-metal transition be Boris Spivak, University of Washington I will discuss a theory of quantum superconductor-metal transition. To do so I will consider a system of superconducting grains embedded in a normal metal. At zero temperature this system exhibits a quantum superconductor-normal metal phase transition as a function of grain size and grain concentration. In the framework of this model the transition can take place at arbitrarily large conductance of the normal metal. As a result, it is possible to construct an asymptotic theory of the phenomenon. I will concentrate on properties of the exotic metallic phase and will discuss a relation of the theory to the theory of 2D localization.
Measuring and manipulating coherence in photonic and atomic systems by Aephraim Steinberg, University of Toronto The manipulation and "complete" characterisation of quantum states have become major topics in experimental and theoretical physics, with crucial roles to play in quantum information & control. I will discuss experimental progress in these areas drawing from two of our experiments, one on the production of highly entangled n-photon states (where n>2), and the other on the manipulation of the quantum vibrational states of atoms trapped in an optical lattice. In the first, we have realized that in typical single-mode systems (for instance, polarisation-entangled photons in optical fibre), one must be careful to differentiate between loss of coherence (between different quantum states) and loss of indistinguishability (of "different" photons). The standard tomographic approach breaks down when photons become partially distinguishable due to degrees of freedom not directly probed experimentally. We extend the ideas of tomography to develop a theory of quasi-complete state characterisation for such systems, and demonstrate it on a two-photon system. In the second, we have developed techniques for state control & characterisation for an ensemble of atoms trapped in an optical lattice, and measure the decoherence, due to a combination of inhomogeneities and inter-well tunneling. We present progress on "pulse echo" or "dynamical decoupling" techniques for restoring coherence, and on using superoperator extraction to optimize such pulse sequences.
Role of Conduction Electrons in the Decoherence of Impurity-Bound Electrons in a Semiconductor by Kuljit S. Virk (with J.E. Sipe), University of Toronto We study the dynamics of impurity bound electrons (qubits) interacting with a bath of conduction band electrons in a semiconductor. Only the exchange interaction is considered. This is applicable to the specific system of a silicone lattice at nonzero temperatures doped with a density nD of phosphorus atoms. Each P atom donates an electron, which either becomes a conduction electron or is captured by another ionized P atom forming an .atom. with hydrogen-like properties. The captured electrons are usually in s-states with a .Bohr radius. of about 25 Å and a binding energy of about 44 meV. The conduction electrons form a gas of approximately free particles, the density of which builds up (from zero at T = 0) as temperature rises and more donors are ionized. This gas forms a gas with which the qubits interact by scattering conduction electrons, and consequently undergo decoherence. We derive master equations for the density matrices of single and two-qubit systems under the usual Born and Markov approximations. The bath mediated RKKY interaction in the two-qubit case arises naturally. It leads to an energy shift significant only when the ratio (RT) of the inter-qubit distance to the thermal deBroglie wavelength of the bath electrons is small. This bath mediated interaction is shown to be an important factor in determining decoherence times; the effect decreases monotonically with RT. We also discuss the effect of this on the purity and fidelity of the Bell states in the case of two-qubit systems.
Quantum computation in the chaotic regime by Joshua Wilke, SFU Even in the absence of external influences the operability of a quantum computer can fail as a result of internal imperfections. We examine the dynamics of a CNOT gate performed on two qubits which interact through residual interactions with the rest of a quantum computer which has one-- and two--body flaws. Contrary to expectation we observe less decoherence in the chaotic regime where two--body flaws are strongest. In addition, we find that poor fidelity is primarily caused by a coherent shift rather than decoherence or dissipation. Our results suggest that greater attention should be given to internal sources of error and associated correction schemes.
Holonomic quantum computation in decoherence-free subspaces by Lian-Ao Wu, University of Toronto We show how to realize, by means of non-abelian quantum holonomies, a set of universal quantum gates acting on decoherence-free subspaces and subsystems. In this manner we bring together the quantum coherence stabilization virtues of decoherence-free subspaces and the fault-tolerance of all-geometric holonomic control. We discuss the implementation of this scheme in the context of quantum information processing using trapped ions and quantum dots.
Entanglement between solid state systems, by Tony Leggett , Univ of Illinois (Urbana-Champaign), and PITP, Vancouver Examples of types of qubit where the electromagnetic-field and solid-state ingredients are both important include flux-mode Josephson systems, quantum-optical cavities (where the metallic walls are essential to the confinement of the modes), and most spectacularly surface plasmons, where the metallic degrees of freedom are essential for the very existence of the mode. It has frequently been argued in the literature that in all these systems the "solid-state" degrees of freedom are strongly entangled. I shall argue that with the parameters typical of existing experiments, this is true only for the flux-more SQUID case; in particular, in existing experiments involving surface plasmons, the entanglement of the solid-state degrees of freedom is extremly small.
Dynamics of the Nuclear Spin bath in molecular magnets: a test for decoherence, by Andrea Morello, UBC, Vancouver Any conceivable application of the quantum properties of large spin systems requires a thorough understanding of their coupling with the environment, in particular the nuclear spin bath. Molecular nanomagnets, forming crystalline arrays of identical and well-characterized high-spin clusters, are benchmark systems to study this problem in detail and make accurate comparisons between experimental results and theoretical predictions. I shall review our recent experiments on the spin dynamics and thermodynamics in a variety of nanomagnets at ultra-low temperatures, where both the nuclear-driven electron spin dynamics and the electron-driven nuclear spin dynamics can be studied, and illustrate how these results challenge our current understanding of decoherence in quantum spins.
Coherence Windows in solid-state qubit systems, by Philip Stamp, UBC and PITP, Vancouver I discuss the competition between spin bath decoherence at low energy scales and oscillator bath-mediated decoherence at higher energy scales, and the .coherence window. that exists at intermediate energy scales. The implications for experiments on magnetic and superconducting qubits are described. Then in the second part of the talk, I discuss new results on 2 problems. The first is decoherence in a large class of lattice models with particles coupled to an Ohmic oscillator bath- these models include the .Schmid. model, and the dissipative Hofstadter model. Second, I discuss new results on decoherence in quantum walks- this is a problem of quantum diffusion on some graph which represents paths in a computational Hilbert space.
Frustration of Decoherence and Entanglement-sharing in the Spin-bath, by Andrew Hines, PITP and PIMS, Vancouver The monogamous nature of entanglement has been illustrated by the derivation of entanglement-sharing inequalities-bounds on the amount of entanglement that can be shared among the various parts of a multipartite system. Motivated by recent studies of decoherence, we demonstrate an interesting manifestation of this phenomena that arises in system-environment models where there exists interactions between the modes or subsystems of the environment. I will discuss this phenomenon in the spin-bath environment, constructing an entanglement-sharing inequality bounding the entanglement between a central spin and the environment in terms of the pairwise entanglement between individual bath spins. The relation of this result to decoherence will be illustrated using simplified system-bath models of decoherence. I will also discuss possible extensions to oscillator-bath models.
Electron spin for quantum computation: what we've learned, and where we go from here, by Joshua Folk, UBC Vancouver No ABSTRACT yet |