March 2011 - ITP-quantumseminare

Special Lecture for PhD and Master Students
by Ilona Blatter 17 Mar '11

17 Mar '11

Dear all The NCCR QSIT organizes periodically a 'Special Lecture for Master and PhD Students'. This semester the lecture will be given by Carlo Beenakker, who is recognized to give outstanding and comprehensive talks. Date: Tuesday March 22, 2011 Place: ETH Science City, HPV G 5 Time: 15:45 - 17:30 Coffee and tea at 15:15 What is special about graphene Carlo Beenakker, Leiden University, The Netherlands. Abstract: see flyer. With best regards, Ilona Blatter --------------------------------------------------- ETH Zurich Dr. Ilona Blatter QSIT Coordinator Laboratory for Solid State Physics HPF E 17 Schafmattstrasse 16 8093 Zurich ilona.blatter(a)phys.ethz.ch www.qsit.ethz.ch www.nccr-qsit.ethz.ch Phone: +41 44 633 36 06 Home: +41 44 844 42 50

2 1

Seminar talks by Fernando Brandao and Niek Bouman
by Stefan Wolf 14 Mar '11

14 Mar '11

Hello, Tomorrow Tuesday, we will have two seminar talks, by Fernando Brandao and by Niek Bouman. We therefore start already at 16:30. Both talks will be on Hoenggerberg. Best regards, Stefan ________________________________________________________________________________________ Tuesday, March 15, 16:30 - 18:00, HIT J 51. Fernando Brandao: "The quantum one-time pad and superactivation" A classical one-time pad allows two parties to send private messages over a public classical channel -- an eavesdropper who intercepts the communication learns nothing about the message. In this talk I will discuss a quantum analogue of this task: two parties initially share a quantum state and use an insecure quantum channel to communicate privately. I will show that the optimal rate is given by an additive and single-letter expression, a direct quantum analogue of the celebrated classical formula. The rate turns out to be equal to the quantum capacity assisted by symmetric side-channels, a quantity which had a key role in showing superactivation of the quantum capacity, but which lacked a clear operational meaning. In this way, the symmetric side-channel receives an interpretation as a quantum public channel. I will also discuss how taking this relation further gives new insight into superactivation of the channel capacity by showing the equivalence of (noisy) quantum privacy and distillable entanglement under public quantum communication. This is based on joint work with Jonathan Oppenheim (arXiv:1004.3328, arXiv:1005.1975) ________________________________________________________________________________________ Niek Bouman: "Sampling in a Quantum Population" Random sampling can be used to estimate a property from a population by merely looking at some randomly selected elements of that population. Classically, this is very well understood and useful tools (such as exponential-tail bounds on the accuracy of the estimate) are available. For example, the relative Hamming weight of a bit string (the fraction of non-zero positions in the string) can be estimated by determining the relative Hamming weight of a random subset of the bits. Suppose that we apply random sampling to a multi-qubit quantum system: we select some qubit positions at random and measure these qubits in some reference basis. Now, the main question is: What can we conclude, given this measurement outcome, about the remaining part of the quantum state? In this talk, we present our solution to this question, and formalize the intuition that the random sample should contain some information about the remaining qubits. As a corollary to our results, we get a lower-bound on the min-entropy (conditioned on an auxiliary quantum system that could be entangled to the qubits) of the outcomes obtained when measuring the remaining qubits in some specific basis. As part of our "framework", we define a "quantum error probability", which is, informally speaking, the probability that our conclusions about the remaining qubits, drawn based on the sample, are incorrect. We furthermore show that an upper bound on this quantum error probability can be easily computed from the error probability of the corresponding classical sampling procedure (by which we mean the same sampling distribution and method to compute the estimate, but then applied to a classical population). As a consequence of this, it suffices to analyze the classical sampling procedure, which is much easier. We also demonstrate that our techniques are useful as a proof technique for BB84 quantum key distribution and quantum oblivious transfer. Both protocols contain a "checking phase" where a random subset of the qubits is selected and measured. Then, in both cases, the min-entropy bound on future measurement outcomes that follows from these sample measurements almost directly leads to a security proof. See also http://arxiv.org/abs/0907.4246 ________________________________________________________________________________________

1 0

Fwd: [QMtl] Fwd: Summer School on Quantum Information
by Frédéric Dupuis 04 Mar '11

04 Mar '11

For those of you who plan to go to the summer school in Montreal, the registration deadline is coming up... --Frédéric -------- Original Message -------- Subject: [QMtl] Fwd: Summer School on Quantum Information Date: Thu, 3 Mar 2011 13:21:30 -0500 From: Patrick Hayden <patrick(a)cs.mcgill.ca> To: qmtl(a)cs.mcgill.ca Begin forwarded message: > *From: *David Poulin <david.poulin(a)usherbrooke.ca > <mailto:david.poulin@usherbrooke.ca>> > *Date: *March 3, 2011 12:03:04 PM EST > *To: *undisclosed-recipients:; > *Subject: **Summer School on Quantum Information* > > Dear colleague, > > (with apologies if you receive this message multiple times) > > This is a reminder that the early registration deadline for the 11th > Canadian Summer School on Quantum Information and the 8th Canadian > Student Conference on Quantum Information is in one week, *March 11th. > *Places are limited and based on first come first served. Please pass > on this information to your students and colleagues. Thanks to our > generous sponsors, we will cover 50% of the local expenses (meals and > lodging) of all students, and additional funds are available for > students with greater needs. > > Best wishes, > David > * > * > *Registration now open:* > *11th Canadian Summer School on Quantum Information (June 6th-15th)* > *8th Canadian Student Conference on Quantum Information (June 16th-17th)* > *Jouvence, Québec, Canada* > > The 11th edition of the Canadian Summer School on Quantum Information > and the 8th Canadian Student Conference on Quantum Information will be > held from June 6th to 17th 2011 at the Centre de Villegiature de > Jouvence, located in a Quebec National Park. Once again this year, > many of the world's leading experts on quantum information will > present series of lectures targeted at prospective and current > graduate students, as well as postdocs, in the areas of physics, > mathematics, computer science and engineering. The Student Conference > is a unique occasion for students to present their work and exchange > with their fellow students. More information about these events can be > found at the web page > > http://www.crm.umontreal.ca/QI11/index_e.php > > Registration is now open. Early registration ends March 11th, 2011. > *Places are limited, first come first served. Hurry up!* > Deadline to submit a contribution to the Student Conference: April 20th > > Looking forward to see you in Jouvence. > > Organizers: > David Poulin > Alexandre Blais > Michel Pioro-Ladrière > Bertrand Reulet >

1 0

QSIT Seminar by Peter Domokos: Tuesday March 8, 2011
by Ilona Blatter 04 Mar '11

04 Mar '11

Dear all Please be informed about the following QSIT Seminar. With best regards, Ilona Blatter -------------------------------------------------- Date: Tuesday March 8, 2011 Time: 13:00 Place: HIL E 7 Host: Tilman Esslinger Non-equilibrium quantum phase transition with atoms in optical resonators Peter Domokos Research Institute for Solid State Physics and Optics, Budapest, Hungary Laser-driven ultracold atoms in a Bose-Einstein condensate coupled to the single mode of a high-finesse optical cavity exhibit a phase transition from the homogeneous into a periodically patterned distribution above a critical driving strength. This process of self- organization is closely related to the quantum phase transition of the spin-boson Dicke-model from the normal to the superradiant phase. However, the cavity QED system is intrinsically open and the system is not in equilibrium even at zero temperature. We will discuss the influence of the irreversible photon loss through the cavity mirrors on the main features of the criticality. --------------------------------------------------- ETH Zurich Dr. Ilona Blatter QSIT Coordinator Laboratory for Solid State Physics HPF E 17 Schafmattstrasse 16 8093 Zurich ilona.blatter(a)phys.ethz.ch www.qsit.ethz.ch www.nccr-qsit.ethz.ch Phone: +41 44 633 36 06 Home: +41 44 844 42 50

1 0

Talk by Norbert Schuch
by Stefan Wolf 01 Mar '11

01 Mar '11

Good morning, Next week's seminar talk will be by Norbert Schuch. The seminar will be on March 8 on Hoenggerberg: Tuesday, March 8, 17:00, HIT J 51. Best regards, Stefan _______________________________________________________________ Classifying quantum phases using Matrix Product States and PEPS > > joint work with D. Perez-Garcia and I. Cirac > > We apply the framework of Matrix Product States (MPS) and their parent > Hamiltonians to the classification of gapped quantum phases in > one-dimensional systems and beyond. We discuss both the case of systems > with unique ground states and with degenerate ground states, i.e., > symmetry breaking, both in the absence and in the presence of symmetries. > We find that without symmetries, all systems (up to ground state > degeneracy) are in the same phase. If symmetries are imposed, the > classification is based on the different types of projective > representations of the symmetry group, together with its permutation > action on the symmetry broken ground states. We also discuss how to > extend our results to two dimensional systems using Projected Entangled > Pair States (PEPS), and show that one cannot expect the same type of > symmetry protection as in one-dimension. As a central tool, we introduce > the isometric form of an MPS which gives a rigorous framework for studying > phases using renormalization fixed points, without a need to actually > renormalize the system. _______________________________________________________________

1 0