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2008
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January
29: IQI Seminar
Sergio Boixo, University of New Mexico
3:00 p.m.,
74 Jorgensen
Title: Quantum
simulated annealing
Abstract:
We develop a quantum algorithm
to solve combinatorial optimization problems through quantum simulation of a classical
annealing process. Our algorithm combines techniques from quantum walks, quantum
phase estimation, and quantum Zeno effect. It can be viewed as a quantum analogue
of the discrete-time Markov chain Monte Carlo implementation of classical simulated
annealing. Our implementation requires order of inverse of the square root of
delta operations to find an optimal solution with bounded error probability, where
delta is the minimum spectral gap of the stochastic matrix used in the classical
simulation. The quantum algorithm outperforms the classical one, which requires
order of the inverse of delta operations.
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January
30: Group Meeting
Charles
Bennett, IBM, Watson Research Center
5:30 p.m.,156
Jorgensen
Title: Dissipation-error
tradeoff in tape copying with imperfect discrimination
Abstract:
A simple,
reversible non-proofreading Brownian tape-copying system, patterned on the chemical
reaction catalyzed by RNA polymerase, displays a remarkable range behavior as
the thermodynamic driving force is varied. In some regimes, it harnesses
the tape's entropy increase, in the form of copying errors, to pump the driving
reactions uphill, with the overall dissipation still remaining positive.
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February12:
IQI Seminar
Darrick Chang, Harvard
University
3:00 p.m., 74 Jorgensen
Title: Nanoscale quantum optics: from single-photon devices
to strongly correlated systems
Abstract:
Recently, a number
of novel photonic waveguide technologies have been developed that allow light
to be confined to dimensions near or below the diffraction limit. The tight
confinement can induce strong interactions between single photons and single optical
emitters in the vicinity of the waveguide and large single-photon nonlinearities
without the use of an optical cavity. When combined with quantum optical
techniques for manipulation, these systems potentially enable many applications
such as single-photon generation on demand and single-photon transistors, and
can also give rise to rich many-body phenomena such as crystallization of photons
and strongly correlated photon transport.
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February
19: IQI Seminar
Panos Aliferis,
IBM, Watson Research Center
3:00 p.m., 74 Jorgensen
Title:
How to quantum compute against biased noise
Abstract:
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 designed to work effectively in this setting [joint work with J. Preskill;
arXiv:0710.1301]. Along the way, I will review basic concepts such as recursive
fault-tolerant simulations and level reduction.
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February
26: IQI Seminar
Berge Englert, National
University of Singapore
3:00 p.m., 74 Jorgensen
Title: Complementarity and wave-particle duality
Abstract:
After formulating Bohr's Principle of Complementarity, the
basic principle of quantum kinematics, in technical terms, a few simple examples
will illustrate it in the context of familiar elementary quantum degrees of freedom.
A quantitative statement about Einstein's Wave-Particle Duality, arguably the
most important phenomenological consequence of complementarity, is then first
presented in the historical context of two-path interferometers, for which the
trade-off between path knowledge and interference strength has been studied in
a number of experiments. Recent extensions to multi-path interferometers raise
new questions and open the door to more research in the future.
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March
4: IQI Seminar
Parsa Bonderson,
Microsoft Station Q
3:00 pm, 74 Jorgensenn
Title: Measurement-only topological quantum computation
Abstract:
The topological approach to quantum computing derives intrinsic
fault-tolerance by encoding qubits in the non-local state spaces of non-Abelian
anyons. The original prescription required topological charge measurement for
qubit readout, and used braiding exchanges of anyons to execute computational
gates. We present an anyonic analog of quantum state teleportation, and use it
to show how a series of topological charge measurements may replace the physical
transportation of computational anyons in the implementation of computational
gates.
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March
18: IQI Seminar
William Wootters,
Williams College
3:00 p.m., 74 Jorgensen
Title: The quantum cost of a nonlocal measurement
Abstract:
In recent years much work has been done on quantifying the entanglement of quantum states. But it has also been recognized that a measurement can be nonlocal in a way that is not captured by the entanglement of the states the measurement aims to distinguish. For example, one can find sets of orthogonal quantum states that cannot be distinguished by any combination of local operations and classical communication even though the states themselves are unentangled. This observation suggests the following general question: for any desired measurement on a spatially distributed system, how much quantum communication does the measurement require? We explore this question by considering simple examples of measurements on bipartite systems, and looking for protocols that minimize the cost in quantum communication.
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April 1: IQI Seminar
Jean Christian Boileau, University of Toronto
3:00 p.m., 74 Jorgensen
Title:The physical underpinnings of privacy
Abstract:
One of the outstanding features of quantum mechanics is the existence of multipartite physical states, known as private states, that upon measurement produce a shared random outcome that cannot in any circumstance be correlated to an external system. Any quantum key distribution (QKD) protocol is in fact an non-coherent version of a private state distillation protocol using decoupled bit and phase error correction codes. To establish the security of a QKD protocol, it is sufficient to construct the latter. However, the most general security proofs avoid a direct correspondence with private state distillation protocol.
Inspired by Koashi's complementarity scenario, I'll give an alternative definition of private state in term of an information tradeoff between conjugate bases and then exploit this definition to present a general private state distillation protocol based on CSS codes that achieves the same key rate as recent, more information-theoretic approaches. Additionally, the same method can be used to establish the hashing inequality for entanglement distillation, as well as the direct part of the quantum coding theorem.
I also discuss a generalization of the Maassen-Uffink entropic uncertainty relation, its connection to our new definition of private state and possible applications to security analysis.
If time permits, I will explain how this method circumvent the need of a random permutation for security analysis of a generic QKD protocol.
I will also present how a shield forged from error correction, can be used to improve the key rate of a generic QKD protocol. The shield is actually a system that does not contribute the key, but that is not under the eavesdropper's control. The latter argument is a generalization, from a different perspective, of an observation from Kraus, Branciard and Renner to improve the secret key generation rates of SARG04 by considering a different symmetrization.
This is joint work with Joseph M. Renes.
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April 8: IQI Seminar
Ulrich Schollwock, Technical
University of Aachen
3:00 p.m., 74 Jorgensen
Title: Out-of-equilibrium ultracold atoms in optical lattices
Abstract:
Ultracold atoms in optical lattices provide a unique playground for the study of coherent quantum dynamics in essentially closed, strongly interacting quantum systems. On the one hand, old questions of quantum many-body physics can finally be accessed experimentally in a controlled fashion; on the other hand, completely new ways of manipulation have arisen. From a theoretical perspective, time- dependent density-matrix renormalization-group methods provide a powerful tool to simulate such experiments. After a brief introduction into the experimental and theoretical background, I want to discuss old many-body phenomena such as spin-charge separation, discuss new ways of cooling ultracold atoms down to the currently inaccessible low temperatures needed for many strong correlation phenomena by adiabatic state transformations, and - to explore the opposite limit - discuss relaxation in closed quantum systems after a sudden quench.
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April 15: IQI Seminar
Wim van Dam, UCSB
3:00 p.m., 74 Jorgensen
Title: Algebraic quantum circuit
Abstract:
A model of algebraic quantum circuit computation is introduced where the wires carry superpositions over the elements of GF(q), and its gates are the Fourier transform over GF(q) and a finite set of controlled phase rotations. Taking our cue from the theory of classical algebraic circuits, we want to develop methods of analyzing such quantum circuits in a way that ignores as much as possible the specifics of the field GF(q) and instead focusses on its algebraic properties. In this talk I will show how the amplitudes of an algebraic quantum circuit C can be expressed as an exponential sum for a multivariate polynomial f_C with integer coefficients and how this polynomial can be used for further analysis. Specifically, using earlier work on exponential sums I show that the acceptance probabilities of a circuit C converge to either 0 or to 1 in the limit of large q.
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April 29: IQI Seminar
Pawel Wocjan, University of Central Florida
3:00 p.m., 74 Jorgensen
Title: Speed-up via quantum sampling
Abstract:
The Markov Chain Monte Carlo method is at the heart of many fully-polynomial randomized approximation schemes for #P-complete problems such as estimating the permanent or the volume of a polytope.
It is therefore very natural and important to determine whether quantum computers can speed-up classical mixing processes based on Markov chains. To this end, we present a new quantum algorithm, making it possible to prepare a quantum sample, i.e., a coherent version of the stationary distribution of a reversible Markov chain. We show that our methods provide a speed-up over a recently proposed method for preparing quantum samples of Boltzmann-Gibbs distributions of (classical) Hamiltonians. We also show that they yield a speed-up of a classical algorithm for approximately counting the number of perfect matchings in dense bipartite graphs.
This is work in progress with Anura Abeyesinghe.
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May 6: IQI Seminar
Avi Zadok, Caltech
3:00 p.m., 74 Jorgensen
Title: Secure optical key generation using an ultra-long fiber laser
Abstract:
Most cryptographic systems are based on coding of the confidential information with a secret key, shared only by legitimate users. The secure generation and distribution of this secret key are the weakest points of the shared-key encryption protocols. Physical layer encryption schemes, most notably quantum cryptography schemes, are promising solutions to the key distribution problem. Quantum key distribution (QKD), based on the quantum mechanical properties of single photons, could theoretically provide unconditional security. However, the practical implementation of quantum cryptography systems remains technologically challenging. This talk will present an experimental proof of concept of a classical key distribution system, based on establishing laser oscillation between two parties and realized using standard fibre-optic components. In our Ultra-long Fibre Laser (UFL) system, each of the two end users places a randomly chosen, spectrally selective mirror at his/her end of a fibre laser, with the choice of mirrors representing a single key bit [1]. We demonstrate the ability of each user to extract the mirror choice of the other using a simple analysis of the lasing signal established in the UFL. An adversary tapping the link could not reconstruct the transmitted key, using time or frequency domain analysis. The simplicity and the enhanced performance of this system render it a promising alternative for secure and practical key distribution in the optical domain.
[1] J. Scheuer and A. Yariv, Phys. Rev. Lett. 97, 140502, 2006
Biography: Avi Zadok received his B.Sc. in Physics and Mathematics at the Hebrew University, Jerusalem in 1994, and the M.Sc. and Ph.D. degrees in Electrical Engineering at Tel-Aviv University in 1999 and 2007. His Ph.D. research areas included dynamic optical filters, statistical optics, optical communications and slow light. He is presently a post-doctoral scholar with the group of prof. Amnon Yariv at Caltech, where his work concentrates on active Silicon photonics and secure optical communications.
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May 13: IQI Seminar
Liang Jiang, Harvard University
3:00 p.m., 74 Jorgensen
Title: Anyonic interferometry and protected memories in atomic spin
Abstract:
Systems with topological order can exhibit remarkable phenomena such as quasi-particles with anyonic statistics and might be used for naturally error-free quantum computation. Here we describe how to unambiguously detect and characterize such states in recently proposed spin lattice realizations using ultra-cold atoms or molecules trapped in an optical lattice. We propose an experimentally feasible technique to access non-local degrees of freedom by performing global operations on trapped spins mediated by an optical cavity mode. We show how to reliably read and write topologically protected quantum memory using an atomic or photonic qubit. Furthermore, our technique can be used to probe statistics and dynamics of anyonic excitations.
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