June 19, 2018

Toronto Quantum Information Seminars QUINF 2008-09
held at the Fields Institute

The Toronto Quantum Information Seminar - QUINF - is held roughly every two weeks to discuss ongoing work and ideas about quantum computation, cryptography, teleportation, et cetera. We hope to bring together interested parties from a variety of different backgrounds, including math, computer science, physics, chemistry, and engineering, to share ideas as well as open questions.

Talks are held Fridays at 11 am unless otherwise indicated

Upcoming talks

Fri., April 3, 2009
11:10 a.m
Stewart Library

Eric Zhu, Dept. of Electrical and Computer Engineering, University of Toronto
From Type II Upconversion to SPDC: A Path to Broadband Polarization Entanglement in Poled Fibers We report type II sum-frequency and second-harmonic generation in a 24-cm-long periodically-poled silica fiber. Quasi-phase matching is achieved for orthogonally-polarized signal and idler over 1520-1575 nm, demonstrating the path to in-fiber broadband polarization-entangled photon pair generation.

Fri., Mar 20, 2009
11:10 a.m
Stewart Library
Cecilia Lopez, Massachusetts Institute of Technology
Scalable error characterization in quantum information processing
We will introduce the problem of scalable error characterization, and the use of the fidelity decay as a tool to characterize these errors efficiently. We will concentrate then on our scheme to analyze spatial correlations (also termed range or locality, and relevant to evaluate fault-tolerance) and its experimental implementation in liquid state NMR QIP (quant-ph/0805.4825). We will discuss the practical aspects regarding scalability, accuracy and implementation errors.
Fri., Nov. 28, 2008
11:10 a.m
Stewart Library
Payam Abolghasem, Dept. of Electrical and Computer Engineering, University of Toronto
A novel platform for nonlinear and quantum optics in monolithic semiconductor structures
An overview of recent success in phase matching of second order nonlinear processes using Bragg reflection waveguides (BRWs) in compound semiconductors will be reported. The technique utilizes dispersion properties of BRWs to achieve exact phase matching between the interacting frequencies. Characterization of the fabricated devices for doubling mid-infrared light in pulse and continues wave regime will be presented. Potential applications of the method in monolithically integrated optical parametric oscillators and integrated sources of photon-pairs will be highlighted.
Fri., Nov. 14, 2008
11:10 a.m
LM158, Lash Miller Chemical Labs, 80 St. George Street, Toronto
Robert J. Silbey, Massachusetts Institute of Technology
Coherence and Decoherence in the Excited States of Light Harvesting Complexes
Fri., Oct. 31, 2008
11:10 a.m
Stewart Library
Alán Aspuru-Guzik, Dept of Chemistry and Chemical Biology, Harvard University
The Role of Quantum Coherence and the Environment in Chromophoric Energy Transport

Recently, direct evidence of long-lived coherence has been experimentally demonstrated for the dynamics of the Fenna-Matthews-Olson (FMO) protein complex at 77K [Engel et al., Nature 446, 782 (2007)]. It was suggested that quantum coherence was important for exploring many relaxation pathways simultaneously. I will talk about our recent work in developing methods for exploring that question and analyzing the different contributions of the different processes to the efficiency of energy transfer in the complex. We generalized the concept of continuous-time quantum walks to a Liouville space formalism. This helped us analyze these contributions and report that at room temperature, this complex has contribution of coherent dynamics of about 10%. Relaxation processes are responsible for 80% of the efficiency. The quantum transport efficiency can actually be enhanced by the dynamical interplay of the system Hamiltonian with the pure dephasing dynamics induced by a fluctuating environment. This occurs in an intermediate regime between fully coherent hopping and highly incoherent transport. I will finalize with a short discussion of this environment-assisted quantum transport regime.

Fri., Oct. 29, 2008
11:10 a.m
Stewart Library

David Cory, Massachusetts Institute of Technology
Error Finding and Control in Quantum Information Processing

uantum information processing aims to overcome the classical limitations on computation, communication and metrology by harnessing the complexity and apparent nonlocality of quantum dynamics. It is predicated upon the ability to precisely and efficiently control and observe the dynamics of multi-particle quantum systems. The holy grail of the field is a quantum computer, that is a quantum information processor that can be scaled up to an essentially arbitrarily large system. Today we have small quantum processors that serve as test-beds for coherent control. I will discuss how these test-beds can help develop robust quantum computation and how we can test many of the requirements of fault tolerant quantum computation.


Fri., Oct. 24, 2008
11:10 a.m
Stewart Library
Christopher Fuchs, Perimeter Institute for Theoretical Physics
Charting the Shape of Quantum State Space

Physicists have become accustomed to the idea that a theory's content is always most transparent when written in coordinate-free language. But sometimes the choice of a good coordinate system can be quite useful for settling deep conceptual issues. This is particularly so for an information-oriented or Bayesian approach to quantum foundations: One good coordinate system may (eventually!) be worth more than a hundred blue-in-the-face arguments. This talk will motivate and chronicle the search for one such class of coordinate systems for finite dimensional operator spaces, the so-called Symmetric Informationally Complete measurements. The desired class will take little more than five minutes to define, but the quest to construct these objects will carry us down a 35 year journey, with the most frenzied activity only recently. If time permits, I will turn the tables and discuss how one might hope to get the formal content of quantum mechanics out of the very existence of such a coordinate system.

Fri., Oct. 17, 2008
11:10 a.m
Stewart Library

Raymond Y. Chiao, U.C. Merced
Looking back on the laser of Schawlow and Townes, and looking forwards to the generation of gravitational radiation.

In their historic 1958 paper, Schawlow and Townes proposed the use of stimulated emission for generating macroscopically coherent light. Here it is proposed that the use of charged, macroscopically coherent quantum matter can lead to the efficient generation of gravitational waves by means of transduction from electromagnetic waves.

The interaction of charged, macroscopically coherent quantum systems, such as a pair of charged superconducting spheres, with both electromagnetic (EM) and gravitational (GR) waves, will be considered. When the charge-to-mass ratio of a pair of identical, levitated superconducting spheres is adjusted so as to satisfy the "criticality" condition $Q/M=\sqrt{4{\pi}{\epsilon_{0}}G}$, where $\epsilon_{0}$ is the permittivity of free space, and $G$ is Newton's gravitational constant, the gravitational force of attraction will be balanced against the electrostatic force of repulsion between the two spheres, which are freely floating in space. At criticality, when these two spheres are set in simple harmonic motion relative to each other by, say, a passing GR wave, they will radiate equal amounts of quadrupolar GR and EM radiation. The superconducting spheres possess an energy gap (the BCS gap) separating the ground state from all excited states. At sufficiently low temperatures with respect to the BCS gap, all dissipative degrees of freedom of the spheres will be frozen out by the Boltzmann factor. Then at criticality, there will be an equipartition of both kinds of incident radiation upon scattering. This implies that a Hertz-like experiment, i.e., a GR transmitter-receiver experiment, should be experimentally feasible. I will present theoretical and experimental progress on this problem.

Fri., Sept. 26, 2008
11:10 a.m
Stewart Library

Moshe Shapiro, Weizmann Institute of Science and University of British Columbia
Canonical Invariance as a unifying symmetry of nature: derivation of the coordinate-momentum commutation relation, the time-dependent Schroedinger equation, and Maxwell equations.

We show that the coordinate-momentum commutation relations and the relativistic and non-relativistic quantum dynamical equations can all be derived from the classical principle of Canonical Invariance and the linearity of the correspondence between physical observables and quantum operators. The implications of this derivation to accelerating quantum relativistic systems, the third law of thermodynamics, and what may be viewed as the "beginning of time" are discussed.

Fri., Sept. 12, 2008
11:10 a.m
Stewart Library
Peng Xue, Institute for Quantum Information Science, University of Calgary
Quantum walk on circles in phase space via superconducting circuit QED

We show how a quantum walk in phase space can be implemented via cavity or circuit quantum electrodynamics (CQED) where only the resonator field (i.e. the walker) needs to be driven and measured. The atom or Cooper pair box (i.e. the coin) is controlled indirectly via Jaynes-Cummings coupling. Decoherence can be tuned so that the transition from quantum to classical walk can be controlled, which confirms the quantum nature of the walk. In contrast to previous proposals for CQED realizations, the walker is not confined to one circle in phase space (fixed mean energy) but rather leaps to other circles in phase space. Despite this complication, the quantum enhanced diffusion of walker's phase can be cleanly observed and rigorously explained, thereby enabling the first experimental realization of a single-walker quantum walk.

Fri, Aug. 1, 2008
11:10 a.m
Stewart Library

Masato Koashi, Osaka University
Security of quantum key distribution using an entangled pair of pulses

While quantum key distribution (QKD) is possible without producing entangled states, there is an advantage in actually preparing an entangled pair of pulses and distributing them to two receivers. The advantage is the fact that we can totally forget about the credibility of the light source: any fault in or any malicious tampering with the
light source will be automatically caught as an increase of observed error rates. This is certainly true if we use ideal photon detectors, and a nontrivial question is whether we can still enjoy this auto-detecting capability with practical detectors, which cannot
distinguish photon numbers or selecting out a single optical mode. In this talk, I will discuss two independent approaches: (1) Using a security proof based on complementarity, it can be shown that if one of the receivers uses an ideal detector, the other one can use almost any kind of detectors. (2) By analyzing properties of polarizations of photons, we can show that auto-detecting capability is available when
the receivers use practical detectors modeled by a threshold detector.

Mon, July 28, 2008
2:10 p.m**
Stewart Library
Christian Schaffner, CWI (Centre for Mathematics and Computer Science), The Netherlands
The operational meaning of min- and max-entropy

We show that the conditional min-entropy Hmin(A|B) of a bipartite state rho_AB is directly related to the maximum achievable overlap with a maximally entangled state if only local actions on the B-part of rho_AB are allowed. In the special case where A is classical, this overlap corresponds to the probability of guessing A given B. In a similar vein, we connect the conditional max-entropy Hmax(A|B) to the maximum fidelity of rho_AB with a product state that is completely mixed on A. In the case where A is classical, this corresponds to the security of A when used as a secret key in the presence of an adversary holding B. Because min- and max-entropies are known to characterize information-processing tasks such as randomness extraction and state merging, our results establish a direct connection between these tasks and basic operational problems. For example, they imply that the (logarithm of the) probability of guessing A given B is a lower bound on the number of uniform secret bits that can be extracted from A relative to an adversary holding B.


Mon, July 28, 2008
4:10 p.m**
Stewart Library

Stephanie Wehner, California Institute of TechnologyCryptography from noisy quantum storage

We show how to implement cryptographic primitives based on the realistic
assumption that quantum storage of qubits is noisy. To illustrate the
power of this new model, we show that a protocol for oblivious transfer
(OT) is secure for any amount of quantum-storage noise, as long as
honest players can perform perfect quantum operations. We then address
a more realistic setting, where the honest players also experience
noise. We show trade-offs between the amount of storage noise, the
amount of noise in the operations performed by the honest players and
the security of oblivious transfer with individual-storage attacks. As
an example, we show that for the case of depolarizing noise in storage
we can obtain secure oblivious transfer as long as the quantum bit-error
rate of the channel does not exceed 11% and the noise on the channel is
strictly less than the quantum storage noise. Finally, we show that our
analysis easily carries over to quantum protocols for secure identification.


Thurs, July 24, 2008
11:10 a.m**
Stewart Library
Andrew Landahl, University of New Mexico
Universal quantum walks driven by local Hamiltonians

That quantum walks can be made universal for quantum computation has been known for over twenty years. Previous constructions required Hamiltonians to act on ever-distantly separated systems as the computation size grew. In this talk, I describe how to achieve quantum walk universality using a Hamiltonian that acts on nearest-neighbor spins in one dimension. This opens up the possibility of a new kind of "control-free" quantum computing architecture. To run an algorithm in this architecture, one prepares the initial state of the spin system so that it describes the program of interest and the data on which it is to act, waits for the quantum walk to apply the program to the data, and then measures the data to get an answer. A corollary of this quantum walk construction is that there is no efficient classical algorithm for simulating generic spin chain dynamics in one dimension if the state space of each spin is eight or greater.

Based on joint work with Brad Chase in arXiv:0802.1207.


Fri Jul 11

11:10 a.m
Stewart Library
Geir Ove Myhr, Institute for Quantum Computing, University of Waterloo
Symmetric extension and quantum key distribution

I will talk about how to characterize states with a symmetric extension and why symmetric extensions are relevant to quantum key distribution, both with one-way and two-way post-processing.

A bipartite state shared between Alice and Bob is said to have a symmetric extension if it can be extended to a triparite state, such that the third party has a part of the state equivalent to Bob's. For the case when Alice and Bob each holds a qubit, the characterization simplifies tremendously, and I present a conjectured simple formula which we have proven in special cases. In higher dimension the characterization is necessarily more complicated, but we can still get necessary conditions.

For quantum key distribution with one-way postprocessing the implication of a symmetric extension is immediate: such a state is not useful. By using two-way procedures in the postprocessing we can break symmetric extensions. Still, a given two-way procedure can be tested to see for which states symmetric extension is actually broken. For example the failure of Gottesman and Lo's two-way procedure to distill key when the the QBER is above 20% and 27,6% for the BB84 protocol and 6-state protocol, can be explained by a failure to break a symmetric extension.