[quantum-info] Talks (one tomorrow!) and Visitors

[William Matthews]

If you notice something you feel is missing from this mail,
or have a correction, please inform the mailing list.

------ This week ------

-- Talks --

Monday, Nov 22nd, 12:30pm to 1:30pm, IQC, RAC1 Room 2009,
Robert Raussendorf, University of British Columbia
Title: The 2D AKLT state is universal for measurement-based quantum computation
Abstract: We demonstrate that the two-dimensional AKLT state on a honeycomb lattice is a universal resource for measurement-based quantum computation [1]. Our argument proceeds by reduction of the AKLT state to a 2D cluster state, which is already known to be universal, and consists of two steps. First, we devise a local POVM by which the AKLT state is mapped to a random planar graph state. Second, we show numerically that the connectivity properties of these random graphs are governed by percolation, and that typical graphs are in the connected phase. The corresponding graph states can then be transformed to 2D cluster states by standard techniques.
Joint work with Tzu-Chieh Wei and Ian Affleck. An analogous result has been obtained by A. Miyake in [2].
[1] TC Wei, I. Affleck and R.Raussendorf, arXiv:1009.2840
[2] A. Miyake, arXiv:1009.3491

Tuesday, Nov 23rd, 4:00 pm, PI, Room 301
Paul Skrzypczyk, University of Bristol
Abstract: The second law of thermodynamics tells that physics imposes a fundamental constraint on the efficiency of all thermal machines. Here I will address the question of whether size imposes further constraints upon thermal machines, namely whether there is a minimum size below which no machine can run, and whether when they are small if they can still be efficient? I will present a simple model which shows that there is no size limitation and no limit on the efficiency of thermal machine and that this leads to a unified view of small refrigerators, pumps and engines.

Wednesday, Nov 24th, 2:00 pm, PI, Room 405,
Carl Bender, Washington University
Title: Making sense of non-Hermitian Hamiltonians
Abstract: The average quantum physicist on the street believes that a quantum-mechanical Hamiltonian must be Dirac Hermitian (invariant under combined matrix transposition and complex conjugation) in order to guarantee that the energy eigenvalues are real and that time evolution is unitary. However, the Hamiltonian $H=p^2+ix^3$, which is obviously not Dirac Hermitian, has a real positive discrete spectrum and generates unitary time evolution, and thus it defines a fully consistent and physical quantum theory. Evidently, the axiom of Dirac Hermiticity is too restrictive. While $H=p^2+ix^3$ is not Dirac Hermitian, it is PT symmetric; that is, invariant under combined space reflection P and time reversal T. The quantum mechanics defined by a PT-symmetric Hamiltonian is a complex generalization of ordinary quantum mechanics. When quantum mechanics is extended into the complex domain, new kinds of theories having strange and remarkable properties emerge. Some of these properties have recently been verified in laboratory experiments. If one generalizes classical mechanics into the complex domain, the resulting theories have equally remarkable properties.

Wednesday, Nov 24th, 4:00 pm, PI, Room 405,
Patrick Hayden, McGill University
Title: From low-distortion embeddings to information locking
Abstract: I'll describe a connection between uncertainty relations, information locking and low-distortion embeddings of L2 into L1. Exploiting this connection leads to the first explicit construction of entropic uncertainty relations for a number of measurements that is polylogarithmic in the dimension d while achieving an average measurement entropy of (1-e) log d for arbitrarily small e. From there, it is straightforward to obtain the first strong information locking scheme that is efficiently computable using a quantum computer. This locking scheme can be interpreted as a method for encrypting classical messages using a key of size much smaller than the message length. Other applications include efficient encodings for amortized quantum identification over classical channels and new string commitment protocols.

Friday, Nov 26th, 2:00 pm, PI, Room 405,
Yutaka Shikano, Tokyo Institute of Technology
Title: Counter-factual Processes in Quantum Mechanics
Abstract: The counter-intuitive phenomena in quantum mechanics are often based on the counter-factual (or virtual) processes. The famous example is the Hardy paradox, which has been recently solved in two dependent experiments. Also, the delayed choice experiment and one of quantum descriptions of the closed time like curves can be also examples of the counter-intuitive phenomena. The counter-factual processes can be characterized by the weak value initiated by Yakir Aharonov and his colleagues. In this talk, I will introduce the weak value from the probability theory and the connection to the counter-factual processes in these examples.

-- Visitors --

Patrick Hayden (McGill University) is visiting PI
until December 15th
hosted by Neil Turok

Giacomo Mauro D'Ariano (Universita degli Studi di Pavia) is visiting PI
until December 2nd 
hosted by Giulio Chiribella

Yutaka Shikano (Tokyo Institute of Technology) is visiting PI
until November 27th
hosted by Michele Mosca

Sergey Bravyi (IBM T. J. Watson Research Center) will be visiting PI
November 30th 2010 through December 2nd 2010
hosted by Lucien Hardy

------ Upcoming (next 4 weeks - not exhaustive!) ------

-- Talks --

Wednesday, Dec 1st, 2:00pm, PI, Room 405
Sergey Bravyi
Colloquium (Title TBA)

Monday, Dec 6th, 4:00 pm, PI, Room 301
Oliver Buerschaper
"Classifying Topological Order: Dualities and Hierarchies"

Wednesday, Dec 7th, 2:00pm, PI, Room 405
David Cory
Colloquium (Title TBA)

-- Visitors --

Xie Chen (MIT) will be visiting PI
November 29th 2010 through December 3rd 2010
hosted by Zhengfeng Ji

Oliver Buerschaper (Max-Planck-Institut für Quantenoptik)
December 4th 2010 through December 11th 2010
hosted by Michele Mosca

Lianao Wu (University of the Basque Country)
December 6th 2010 through December 18th 2010 
hosted by Daniel Gottesman