
SCIENTIFIC PROGRAMS AND ACTIVITIES 

April 19, 2018  
Talks are held Fridays at 11 am unless otherwise indicated Upcoming talks

Fri., April 3, 2009 
Eric Zhu, Dept. of Electrical and Computer Engineering,
University of Toronto 
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 faulttolerance) and its experimental implementation in liquid state NMR QIP (quantph/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 midinfrared 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 photonpairs will be highlighted. 
***
PLEASE NOTE NONSTANDARD PLACE *** 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 AspuruGuzik,
Dept of Chemistry and Chemical Biology, Harvard University The Role of Quantum Coherence and the Environment in Chromophoric Energy Transport Recently, direct evidence of longlived coherence has been experimentally demonstrated for the dynamics of the FennaMatthewsOlson (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 continuoustime 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 environmentassisted quantum transport regime. 
Fri.,
Oct. 29, 2008 11:10 a.m Stewart Library 
David Cory, Massachusetts Institute of Technology 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 multiparticle 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 testbeds for coherent control. I will discuss how these testbeds 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 coordinatefree 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 informationoriented or Bayesian approach to quantum foundations: One good coordinate system may (eventually!) be worth more than a hundred blueintheface arguments. This talk will motivate and chronicle the search for one such class of coordinate systems for finite dimensional operator spaces, the socalled 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 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 chargetomass 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 Hertzlike experiment, i.e., a GR transmitterreceiver 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 We show that the coordinatemomentum commutation relations
and the relativistic and nonrelativistic 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 JaynesCummings 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 singlewalker quantum walk. 
Fri,
Aug. 1, 2008 11:10 a.m Stewart Library 
Masato Koashi, Osaka University 
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 maxentropy We show that the conditional minentropy Hmin(AB) 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 Bpart 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 maxentropy Hmax(AB) 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 maxentropies are known to characterize informationprocessing 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. (**PLEASE NOTE NONSTANDARD DATE) 
Mon,
July 28, 2008 4:10 p.m** Stewart Library 
Stephanie Wehner, California Institute of TechnologyCryptography
from noisy quantum storage (**PLEASE NOTE NONSTANDARD DATE) 
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 everdistantly separated systems as the computation size grew. In this talk, I describe how to achieve quantum walk universality using a Hamiltonian that acts on nearestneighbor spins in one dimension. This opens up the possibility of a new kind of "controlfree" 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. (**PLEASE NOTE NONSTANDARD DATE) 
2008 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 oneway and twoway postprocessing. 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 oneway postprocessing the implication of a symmetric extension is immediate: such a state is not useful. By using twoway procedures in the postprocessing we can break symmetric extensions. Still, a given twoway procedure can be tested to see for which states symmetric extension is actually broken. For example the failure of Gottesman and Lo's twoway procedure to distill key when the the QBER is above 20% and 27,6% for the BB84 protocol and 6state protocol, can be explained by a failure to break a symmetric extension. 