[quantum-info] Notices

[William Matthews]

**** This Week:

** Talks

Monday Feb. 14, 12:30 pm, Room 2009 (RAC1) IQC
Oleg Gittsovich (IQC)
"Squashing model and applications to quantum key distribution protocols"
We present a framework for describing linear optical measurement devices. This leads to the definition of the squashing map. We demonstrate that this general approach gives strong statements for several measurement devices commonly used in optical implementations of quantum communication protocols.

Wednesday Feb. 16, 2:00 pm, Theatre PI
Zhengcheng Gu (KITP)
"Tensor-net states: a new perspective on many-body quantum systems"
Traditional condensed matter physics is based on two theories: symmetry breaking theory for phases and phase transitions, and Fermi liquid theory for metals. Mean-field theory is a powerful method to describe symmetry breaking phases and phase transitions by assuming the ground state wavefunctions for many-body systems can be approximately described by direct product states. The Fermi liquid theory is another powerful method to study electron systems by assuming that the ground state wavefunctions for the electrons can be approximately described by Slater determinants. From the encoding point of view, both methods only use a polynomial amount of information to approximately encode many-body ground state wavefunctions which contain an exponentially large amount  of information. Moreover, another nice property of both approaches is that all the physical quantities (energy, correlation functions, etc.) can be efficiently calculated (polynomially hard). In this talk, I'll introduce a new class of states: (Grassmann-number) tensor-net states. These states only need polynomial amount of information to approximately encode many-body ground states. Many classes of states, such as Slater determinant states, projective states, string-net states and their generalizations, etc., are subclasses of (Grassmann-number) tensor-net states. However, calculating the physical quantities for these state can be exponentially hard in general. To solve this difficulty, we develop the Tensor-Entanglement Renormalization Group (TERG) method to efficiently calculate the physical quantities. We demonstrate our algorithm by studying several interesting boson/fermion models, including t-J model on a honeycomb lattice.
http://pirsa.org/11020083/

Wednesday Feb. 16, 4:00 pm, Room 405 PI
Sam Ocko (MIT) 
"Quantum codes give counterexamples to the unique pre-image conjecture of the N-representability problem"
It is well known that the ground state energy of many-particle Hamiltonians involving only 2- body interactions can be obtained using constrained optimizations over density matrices which arise from reducing an N-particle state. While determining which 2-particle density matrices are 'N-representable' is a computationally hard problem, all known extreme N-representable 2-particle reduced density matrices arise from a unique N-particle pre-image, satisfying a conjecture established in 1972. We present explicit counterexamples to this conjecture through giving Hamiltonians with 2-body interactions which have degenerate ground states that cannot be distinguished by any 2-body operator. We relate the existence of such counterexamples to quantum error correction codes and topologically ordered spin systems.
http://pirsa.org/11020102/


** Visitors

Sam Ocko from MIT, hosted by Roger Colbeck
February 15th 2011 through February 18th 2011

Lidia del Rio from ETH Zurich, hosted by Roger Colbeck
at PI until February 19th 2011

Zhengcheng Gu from KITP, hosted by Robert Myers
at PI, February 15th 2011 through February 20th 2011

Dr. Jianming Cai from Institut für Theoretische Physik
at IQC until Feb. 27, 2011
hosted by Marco Piani

**** Upcoming (next month, not necessarily complete):

** Talks

Monday March 7, 11:00am, PI Room 405
James Hartle
"Quantum Mechanics with Extended Probabilities"

Tuesday March 8, 4:00pm PI Room 301
Markus Mueller (PI)
"From operational axioms to quantum theory - and beyond?"

Monday March 14, 4:00 pm, PI Room 405 (* the date has changed since the last mailing)
Sergio Boixo (Harvard University)
"TBA"
http://pirsa.org/11030085/

Wednesday March 16, 4:00 pm, PI Room 405,
Pasquale Sodano (PI)
"Entanglement Routers Using Macroscopic Singlets"

** Visitors

Dr. Eric Chitambar, University of Toronto, hosted by Debbie Leung
at IQC, Feb. 28, 2011 through Feb. 28, 2011

Jay Olson from University of Queensland, hosted by Nicolas Menicucci
at PI, February 20th 2011 through March 6th 2011 - Quantum Information

Sergio Boixo from Harvard University, hosted by John Berlinsky
at PI, March 12th 2011 through March 18th 2011 - Quantum Information, Other