This series consists of weekly discussion sessions on foundations of quantum Theory and quantum information theory. The sessions start with an informal exposition of an interesting topic, research result or important question in the field. Everyone is strongly encouraged to participate with questions and comments. http://pirsa.org/podcast/S006 Science 2009 http://blogs.law.harvard.edu/tech/rss en-ca Thu, 08 Jan 2009 10:50:52 -0500 sbradwell@perimeterinstitute.ca Thu, 08 Jan 2009 10:50:52 -0500 G 180 sbradwell@perimeterinstitute.ca Steve Bradwell's - Podcast Generator Ernesto Galvao http://streamer.perimeterinstitute.ca/mp3/289fcbe3-3f30-4900-84ff-26120585f6e1.mp3 Science http://streamer.perimeterinstitute.ca/mp3/289fcbe3-3f30-4900-84ff-26120585f6e1.mp3 Wed, 06 Oct 2004 16:00:00 -0400 Robert Spekkens http://streamer.perimeterinstitute.ca/mp3/df9cb554-ff43-4819-a290-707adb0bce00.mp3 Science http://streamer.perimeterinstitute.ca/mp3/df9cb554-ff43-4819-a290-707adb0bce00.mp3 Wed, 27 Oct 2004 16:00:00 -0400 Matthew Leifer http://streamer.perimeterinstitute.ca/mp3/802fca00-a56e-4fdd-ad96-3c96b1c2ae2f.mp3 Science http://streamer.perimeterinstitute.ca/mp3/802fca00-a56e-4fdd-ad96-3c96b1c2ae2f.mp3 Wed, 10 Nov 2004 16:00:00 -0500 Ivette Fuentes-Schuller http://streamer.perimeterinstitute.ca/mp3/31b81ba7-0891-47a0-bd89-5229ae81d5ce.mp3 Science http://streamer.perimeterinstitute.ca/mp3/31b81ba7-0891-47a0-bd89-5229ae81d5ce.mp3 Wed, 17 Nov 2004 15:55:00 -0500 Steven Weinberg http://streamer.perimeterinstitute.ca/mp3/e4051e48-91d4-4231-a34a-a3a4abe9793f.mp3 Science http://streamer.perimeterinstitute.ca/mp3/e4051e48-91d4-4231-a34a-a3a4abe9793f.mp3 Wed, 24 Nov 2004 15:50:00 -0500 Enrique Rico-Ortega http://streamer.perimeterinstitute.ca/mp3/037495b2-8662-4d9e-beb1-3e22ef720abb.mp3 Science http://streamer.perimeterinstitute.ca/mp3/037495b2-8662-4d9e-beb1-3e22ef720abb.mp3 Wed, 19 Jan 2005 16:00:00 -0500 Guife Vidal http://streamer.perimeterinstitute.ca/mp3/0e9cb190-15a7-430d-a4ab-bcfd60a15fe8.mp3 Science http://streamer.perimeterinstitute.ca/mp3/0e9cb190-15a7-430d-a4ab-bcfd60a15fe8.mp3 Wed, 02 Feb 2005 15:50:00 -0500 Robert Alicki http://streamer.perimeterinstitute.ca/mp3/cbef072b-a431-4668-8f06-9935da17d322.mp3 Science http://streamer.perimeterinstitute.ca/mp3/cbef072b-a431-4668-8f06-9935da17d322.mp3 Wed, 23 Feb 2005 16:00:00 -0500 Anthony Aguirre http://streamer.perimeterinstitute.ca/mp3/f50ac504-12c9-41b4-9612-33193464ed63.mp3 Science http://streamer.perimeterinstitute.ca/mp3/f50ac504-12c9-41b4-9612-33193464ed63.mp3 Mon, 28 Feb 2005 11:00:00 -0500 Understanding how galaxies form is a major current goal in physical cosmology: although a basic picture is well-accepted, there are outstanding mysteries to be solved. First, what is the origin of the heavy elements seen outside of galaxies? Given that these elements are created only inside galaxies, there must be a process whereby galaxies can expel gas rather than accrete it. Second, galaxy properties are somewhat different from theory predicts, yet extremely regular -- to the extent that it has been seriously argued that modified gravity, rather than dark matter, explains them. I will discuss these mysteries and the possibility that the same culprit -- galactic winds -- may play a key role in solving both. Anthony Aguirre http://streamer.perimeterinstitute.ca/mp3/e66a4e1f-289b-49d0-ad1e-038922bfcfdb.mp3 Science http://streamer.perimeterinstitute.ca/mp3/e66a4e1f-289b-49d0-ad1e-038922bfcfdb.mp3 Tue, 01 Mar 2005 16:00:00 -0500 J.C Boileau http://streamer.perimeterinstitute.ca/mp3/6e0e8e7e-61aa-4e6f-a909-c561eafde13b.mp3 Science http://streamer.perimeterinstitute.ca/mp3/6e0e8e7e-61aa-4e6f-a909-c561eafde13b.mp3 Wed, 09 Mar 2005 16:05:00 -0500 One of the central critical results in the theory of fault-tolerant quantum computation is that arbitrarily long reliable computation is possible provided the error rate per gate and per time step is below some threshold value. This was proved by a number of groups, but the detailed published proofs are complex and furthermore only hold for concatenation of quantum error-correcting codes able to correct 2 errors per block, while typically the best estimates of the threshold value are based on the 7-qubit code, which only corrects 1 error per block. I will describe recent work by Panos Aliferis, John Preskill, and myself which substantially simplifies existing proofs and applies as well to the concatenated 7-qubit code. The new proof also provides a nice framework in which to attempt to prove relatively high values of the threshold, which so far have only emerged as estimates from simulations Daniel Gottesman http://streamer.perimeterinstitute.ca/mp3/38786db3-eb76-4999-b2c8-873b584ba4f1.mp3 Science http://streamer.perimeterinstitute.ca/mp3/38786db3-eb76-4999-b2c8-873b584ba4f1.mp3 Wed, 23 Mar 2005 16:00:00 -0500 Kaustubh Agashe http://streamer.perimeterinstitute.ca/mp3/ccdc10c4-97da-4bc4-8b5a-36d83c8365b6.mp3 Science http://streamer.perimeterinstitute.ca/mp3/ccdc10c4-97da-4bc4-8b5a-36d83c8365b6.mp3 Thu, 24 Mar 2005 16:00:00 -0500 Valerio Scarani http://streamer.perimeterinstitute.ca/mp3/7c94e7d8-6f20-42a3-a923-dfa1bcbf37ce.mp3 Science http://streamer.perimeterinstitute.ca/mp3/7c94e7d8-6f20-42a3-a923-dfa1bcbf37ce.mp3 Wed, 06 Apr 2005 16:00:00 -0400 Seth Lloyd http://streamer.perimeterinstitute.ca/mp3/7a92bc77-44a5-4651-ab50-a63d68c4ad7d.mp3 Science http://streamer.perimeterinstitute.ca/mp3/7a92bc77-44a5-4651-ab50-a63d68c4ad7d.mp3 Tue, 12 Apr 2005 16:00:00 -0400 Fabio Benatti http://streamer.perimeterinstitute.ca/mp3/5450b3fd-5de7-4f61-8487-a680cd1e283d.mp3 Science http://streamer.perimeterinstitute.ca/mp3/5450b3fd-5de7-4f61-8487-a680cd1e283d.mp3 Wed, 20 Apr 2005 16:00:00 -0400 In this talk I'll discuss recent joint work with Raymond Laflamme, David Poulin and Maia Lesosky in which a unified approach to quantum error correction is presented, called "operator quantum error correction". This scheme relies on a generalized notion of noiseless subsystems and includes the known techniques for the error correction of quantum operations --i.e., the standard model, the method of decoherence-free subspaces, and the noiseless subsystem method--as special cases. Correctable codes in this approach take the form of operator algebras and operator semigroups. The condition from the standard model is shown to be necessary for all of the known methods of error correction, and we'll see this as part of a discussion on conditions that characterize correctability for the general case. David Kribs http://streamer.perimeterinstitute.ca/mp3/05296fd7-d64a-4756-87a8-470296144848.mp3 Science http://streamer.perimeterinstitute.ca/mp3/05296fd7-d64a-4756-87a8-470296144848.mp3 Tue, 24 May 2005 04:00:00 -0400 Rainer Verch http://streamer.perimeterinstitute.ca/mp3/92257ecc-d69e-46dd-8842-ceddbe609ac1.mp3 Science http://streamer.perimeterinstitute.ca/mp3/92257ecc-d69e-46dd-8842-ceddbe609ac1.mp3 Wed, 01 Jun 2005 16:00:00 -0400 David Poulin http://streamer.perimeterinstitute.ca/mp3/317f4964-c7a0-4e39-8b13-2a6c252ceb05.mp3 Science http://streamer.perimeterinstitute.ca/mp3/317f4964-c7a0-4e39-8b13-2a6c252ceb05.mp3 Wed, 08 Jun 2005 04:00:00 -0400 Daniel Lidar http://streamer.perimeterinstitute.ca/mp3/da6880df-b844-47d7-a339-c2d96423583c.mp3 Science http://streamer.perimeterinstitute.ca/mp3/da6880df-b844-47d7-a339-c2d96423583c.mp3 Wed, 15 Jun 2005 04:00:00 -0400 Simon Saunders http://streamer.perimeterinstitute.ca/mp3/cef238a6-742c-4775-8ce6-fca219827b95.mp3 Science http://streamer.perimeterinstitute.ca/mp3/cef238a6-742c-4775-8ce6-fca219827b95.mp3 Wed, 03 Aug 2005 16:00:00 -0400 David Deutsch http://streamer.perimeterinstitute.ca/mp3/d9e577f5-031a-41af-8544-ea1cc4209077.mp3 Science http://streamer.perimeterinstitute.ca/mp3/d9e577f5-031a-41af-8544-ea1cc4209077.mp3 Wed, 24 Aug 2005 16:00:00 -0400 Choi http://streamer.perimeterinstitute.ca/mp3/5acdde0a-a814-47f0-9bbb-2ffbfac9b394.mp3 Science http://streamer.perimeterinstitute.ca/mp3/5acdde0a-a814-47f0-9bbb-2ffbfac9b394.mp3 Wed, 21 Sep 2005 16:00:00 -0400 Hans Westman,Ward Struyve http://streamer.perimeterinstitute.ca/mp3/04b09045-5276-42ad-be44-65d8dbe2166b.mp3 Science http://streamer.perimeterinstitute.ca/mp3/04b09045-5276-42ad-be44-65d8dbe2166b.mp3 Wed, 28 Sep 2005 16:00:00 -0400 Garnet Ord http://streamer.perimeterinstitute.ca/mp3/01a99ea2-1d23-4eb9-b7f0-b5dc3ae3cd13.mp3 Science http://streamer.perimeterinstitute.ca/mp3/01a99ea2-1d23-4eb9-b7f0-b5dc3ae3cd13.mp3 Wed, 05 Oct 2005 15:30:00 -0400 I shall discuss entanglement - assisted invariance (symmetry exhibited by correlated quantum states) and describe how it can be used to understand the nature of ignorance, and, hence, the origin of probabilities in quantum physics. WHZ, Phys. Rev. Lett. 90, 120404 (2003); Rev. Mod. Phys. 75, 715 (2003); Phys. Rev. 71, 052105 (2005) (quant-ph/0405161). Wojciech Zurek http://streamer.perimeterinstitute.ca/mp3/36145b3f-0869-40ce-ab23-2ec32642f038.mp3 Science http://streamer.perimeterinstitute.ca/mp3/36145b3f-0869-40ce-ab23-2ec32642f038.mp3 Wed, 09 Nov 2005 16:00:23 -0500 Optical experiments led the way to quantum information with striking examples of Bell's inequality tests and entangled state synthesis. Early demonstrations of quantum communication proved that optics are important for quantum communication and more recent ideas about linear optic quantum computing raised hopes that this would also be true for computing. I will give an overview of the various elements that are required for optical QIP and the state-of-the-art characteristics. I will then specialize on sources of single photons and entangled photon pairs and show how they need to be adapted to the task at hand. Gregor Weihs http://streamer.perimeterinstitute.ca/mp3/3840fc6e-fc02-4be2-b464-4c422c90c865.mp3 Science http://streamer.perimeterinstitute.ca/mp3/3840fc6e-fc02-4be2-b464-4c422c90c865.mp3 Wed, 23 Nov 2005 16:00:54 -0500 Bohr’s Principle of Complementarity of wave and particle aspects of quantum systems has been a cornerstone of quantum mechanics since its inception. Einstein, Schrödinger and deBroglie vehemently disagreed with Bohr for decades, but were unable to point out the error in Bohr’s arguments. I will report three recent experiments in which Complementarity fails, and argue that the results call for an upgrade of the Quantum Measurement theory. Finally, I will introduce the novel concept of Contextual Null Measurement (CNM) and discuss some of its surprising applications. Web-page: users.rowan.edu/~afshar/ Preprint (published in Proc. SPIE 5866, 229-244, 2005): http://www.irims.org/quant-ph/030503/ Shahriar S. Afshar http://streamer.perimeterinstitute.ca/mp3/bcac7f63-03c4-4029-892a-97510d984699.mp3 Science http://streamer.perimeterinstitute.ca/mp3/bcac7f63-03c4-4029-892a-97510d984699.mp3 Wed, 30 Nov 2005 16:00:15 -0500 will discuss how to realize, by means of non-abelian quantum holonomies, a set of universal quantum gates acting on decoherence-free subspaces and subsystems. In this manner the quantum coherence stabilization virtues of decoherence-free subspaces and the fault-tolerance of all-geometric holonomic control are brought together. Paolo Zanardi http://streamer.perimeterinstitute.ca/mp3/1171ed57-abcb-4b31-b872-00c8b30af7b7.mp3 Science http://streamer.perimeterinstitute.ca/mp3/1171ed57-abcb-4b31-b872-00c8b30af7b7.mp3 Tue, 06 Dec 2005 16:00:31 -0500 A swashbuckling tale of greed, deception, and quantum data hiding on the high seas. When we hide or encrypt information, it's probably because that information is valuable. I present a novel approach to quantum data hiding based this assumption. An entangled treasure map marks the spot where a hoard of doubloons is buried, but the sailors sharing this map want all the treasure for themselves! How should they study their map using LOCC? This simple scenario yields a surprisingly rich and counterintuitive game theoretic structure. A maximally entangled map performs no better than a separable one, leaving the treasure completely exposed. But non-maximally entangled maps can hide the information almost perfectly. Warning: contains pirates. Jonathan Walgate http://streamer.perimeterinstitute.ca/mp3/3948eddb-368c-499c-9d6e-a69ff0449135.mp3 Science http://streamer.perimeterinstitute.ca/mp3/3948eddb-368c-499c-9d6e-a69ff0449135.mp3 Wed, 14 Dec 2005 16:00:00 -0500 We consider the problem of bounded-error quantum state identification: given one of two known states, what is the optimal probability with which we can identify the given state, subject to our guess being correct with high probability (but we are permitted to output "don't know" instead of a guess). We prove a direct product theorem for this problem. Our proof is based on semidefinite programming duality and the technique may be of wider interest. Using this result, we present two new exponential separations in the simultaneous message passing model of communication complexity. Both are shown in the strongest possible sense: -- we describe a relation that can be computed with O(log n) classical bits of communication in the presence of shared randomness, but needs n^(1/3) communication if the parties don't share randomness, even if communication is quantum; -- we describe a relation that can be computed with O(log n) classical bits of communication in the presence of shared entanglement, but needs (almost) n^(1/3) communication if the parties share randomness but no entanglement, even if communication is quantum. Dmitry Gavinsky http://streamer.perimeterinstitute.ca/mp3/54109b95-b815-4fab-af00-0b72d3ed6f97.mp3 Science http://streamer.perimeterinstitute.ca/mp3/54109b95-b815-4fab-af00-0b72d3ed6f97.mp3 Wed, 11 Jan 2006 16:00:27 -0500 The information spectrum approach gives general formulae for optimal rates of codes in many areas of information theory. In this talk I shall relate the information spectrum approach to Shannon information theory and explore its relationship to ``entropic'' properties including subadditivity, chain rules, Araki-Lieb inequlities, and monotonicity. Garry Bowen http://streamer.perimeterinstitute.ca/mp3/334ddef4-4a0f-4134-8903-f035cea629cc.mp3 Science http://streamer.perimeterinstitute.ca/mp3/334ddef4-4a0f-4134-8903-f035cea629cc.mp3 Wed, 25 Jan 2006 16:00:02 -0500 Asymptotic statements like the almost-equi-partition law, the theorm of Shannon Mc -Millan-Breiman, the theorem of Sanov have all natural quantum analogs. They all talk about the thermodynamik limit of quantum spin systems. I will try to summarize these results and sketch the main ideas of proof. Ruedi Seiler http://streamer.perimeterinstitute.ca/mp3/896c6283-d6c3-43f4-9a38-a34eec09d582.mp3 Science http://streamer.perimeterinstitute.ca/mp3/896c6283-d6c3-43f4-9a38-a34eec09d582.mp3 Wed, 08 Feb 2006 16:00:55 -0500 Quantum information methods have been recently used for studying the properties of ground state entanglement in several many body and field theory systems. We will discuss a thought experiment wherein entanglement can be extracted from the vacuum of a relativistic field theory into a pair of arbitrarily spatially separated atoms. In order to simulate the detection process, we will consider the ground state of a linear chain of cooled trapped ions, and discuss a scheme for detecting the entanglement between the ion's motional degrees of freedom. Alex Retzker http://streamer.perimeterinstitute.ca/mp3/ed2014c0-4420-40b6-94df-144c70d364b2.mp3 Science http://streamer.perimeterinstitute.ca/mp3/ed2014c0-4420-40b6-94df-144c70d364b2.mp3 Wed, 01 Mar 2006 16:00:18 -0500 The amount of nonlocality in the GHZ state can be quantified by determining how much classical communication is required to bring a local-hidden-variable model into agreement with the predictions of quantum mechanics. It turns out that one bit suffices, and, of course, nothing less will do. I will discuss generalizations of this result to graph states and its relation to the stabilizer formalism. Carlton Caves http://streamer.perimeterinstitute.ca/mp3/0322e707-f907-4368-bba3-5d8cc8a43468.mp3 Science http://streamer.perimeterinstitute.ca/mp3/0322e707-f907-4368-bba3-5d8cc8a43468.mp3 Wed, 08 Mar 2006 16:00:08 -0500 Scott Aaronson http://streamer.perimeterinstitute.ca/mp3/845a4b81-cee8-42f7-8481-a4de2f5b60a6.mp3 Science http://streamer.perimeterinstitute.ca/mp3/845a4b81-cee8-42f7-8481-a4de2f5b60a6.mp3 Wed, 15 Mar 2006 16:00:11 -0500 I will discuss a toy theory that reproduces a wide variety of qualitative features of quantum theory for degrees of freedom that are continuous. The ontology of the theory is that of classical particle mechanics, but it is assumed that there is a constraint on the amount of knowledge that an observer may have about the motional state of any collection of particles -- Liouville mechanics with an epistemic restriction. The formalism of the theory is determined by examining the consequences of this "classical uncertainty principle" on state preparations, measurements, and dynamics. The result is a theory of hidden variables, although it is not a hidden variable model of quantum theory because it is both local and noncontextual. Despite admitting a simple classical interpretation, the theory also exhibits the operational features of Bohr's notion of complementarity. In fact, it includes all of the features of quantum mechanics to which Bohr appeals in his response to EPR. This theory demonstrates, therefore, that Bohr's arguments fail as a defense of the completeness of quantum mechanics. Joint work with Stephen Bartlett and Terry Rudolph Robert Spekkens http://streamer.perimeterinstitute.ca/mp3/f6caaf77-3232-4a05-98a2-d1059255c9f7.mp3 Science http://streamer.perimeterinstitute.ca/mp3/f6caaf77-3232-4a05-98a2-d1059255c9f7.mp3 Wed, 22 Mar 2006 16:00:12 -0500 Alexei Grinbaum http://streamer.perimeterinstitute.ca/mp3/b0957630-07e4-45e2-8756-4cd3751c8173.mp3 Science http://streamer.perimeterinstitute.ca/mp3/b0957630-07e4-45e2-8756-4cd3751c8173.mp3 Wed, 29 Mar 2006 16:00:59 -0500 I will investigate the creation and detection of multipartite entangled states in systems of ultracold neutral atoms trapped in an optical lattice. These setups are scalable, highly versatile and controllable at the quantum level. Thus they provide an ideal test bed for studying the properties of multipartite entangled states. I will first present methods exploiting incoherent dynamics for initializing an atomic quantum register. The immersion of an optical lattice in a Bose-Einstein condensate leads to spontaneous emission of phonons. This process can be used for irreversibly loading and cooling atoms within the lowest Bloch band of the lattice. I will describe loading and cooling schemes based on this mechanism and compare them to conventional loading schemes. I will then show how coherent dynamics in a very strongly interacting 1D optical lattice setup can be used for the efficient generation of arbitrary graph states in the atomic quantum register. This system can be mapped onto an XY spin chain which itself is equivalent to a system of non-interacting fermions. By exploiting the anticommutation relations between these fictitious fermions I will discuss how any graph state can be realized in an efficient and robust way. In the final part of my talk I will present a practical method for detecting and characterizing multipartite entangled states in atomic quantum registers. This scheme is based on measuring violations of entropic inequalities using simple quantum networks involving only two copies of the quantum state under consideration. I will investigate the performance of this method under realistic conditions taking into account the most common sources of experimental errors. Dieter Jaksch http://streamer.perimeterinstitute.ca/mp3/7f68d869-a25c-481b-a877-39efff9d8f25.mp3 Science http://streamer.perimeterinstitute.ca/mp3/7f68d869-a25c-481b-a877-39efff9d8f25.mp3 Wed, 05 Apr 2006 16:00:23 -0400 Hamiltonian oracles are the continuum limit of the standard unitary quantum oracles. In addition to being a potentially useful tool in the study of standard oracles, Hamiltonian oracles naturally introduce the concept of fractional queries and are amenable to study using techniques of differential equations and geometry. As an example of these ideas we shall examine the Hamiltonian oracle corresponding to the problem of oracle interrogation. This talk is intended for all those who wish to apply their knowledge of differential geometry without the risk of creating an event horizon. Carlos Mochon http://streamer.perimeterinstitute.ca/mp3/14089026-3c4b-46b0-8941-a9c5c83e9866.mp3 Science http://streamer.perimeterinstitute.ca/mp3/14089026-3c4b-46b0-8941-a9c5c83e9866.mp3 Wed, 19 Apr 2006 16:00:31 -0400 While modern theories lavishly invoke several spatial dimensions within models that seek to unify relativity theory and quantum mechanics, none seems to consider the possibility that a yet-unfamiliar aspect of time may do the work. I introduce the notion of Becoming and then consider its consequences for physical theory. Becoming portrays a possible aspect of time that is "curled" very much like the extra spatial dimensions in superstring theories. Within the resulting picture of spacetime, some fundamental aspects of quantum mechanics, special and general relativity, thermodynamics and modern cosmology fit in very naturally. The proposed model is not yet a scientific theory as it still lacks a rigorous formalism and experimental predictions, yet it points out an entire family of possible theories that merit serious consideration. Avshalom Elitzur http://streamer.perimeterinstitute.ca/mp3/0ecdfe98-46d9-466c-973d-1fbec948ec3c.mp3 Science http://streamer.perimeterinstitute.ca/mp3/0ecdfe98-46d9-466c-973d-1fbec948ec3c.mp3 Fri, 21 Apr 2006 10:00:13 -0400 Entanglement entropy is currently of interest in several areas in physics, such as condensed matter, field theory, and quantum information. One of the most interesting properties of the entanglement entropy is its scaling behavior, especially close to phase transitions. It was believed that for dimensions higher than 1 the entropy scales like surface area of the subsystem. We will describe a recent result for free fermions at zero temperature, where the entropy in fact scales faster. The latter problem will be related to a mathematical conjecture due to H. Widom (1982). This is a joint work with I. Klich. Dimitri Gioev http://streamer.perimeterinstitute.ca/mp3/d2e3ac31-d94e-466b-976b-b08ddffa5e3e.mp3 Science http://streamer.perimeterinstitute.ca/mp3/d2e3ac31-d94e-466b-976b-b08ddffa5e3e.mp3 Wed, 26 Apr 2006 16:00:14 -0400 Inspired by the notion that the differences between quantum theory and classical physics are best expressed in terms of information theory, Hardy (2001) and Clifton, Bub, and Halvorson (2003) have constructed frameworks general enough to embrace both quantum and classical physics, within which one can invoke principles that distinguish the classical from the quantum. Independently of any view that quantum theory is essentially about quantum information, such frameworks provide a useful tool for exploring the differences between classical and quantum physics, and the relations between the various properties of quantum mechanics that distinguish it from the classical. In particular, we can ask: on which features of quantum physics do our familiar possibility/impossibility theorems depend? It turns out that it is possible to extend the no-cloning theorem and other results, such as the Holevo bound on acquisition of information by a single measurement, beyond the quantum setting. Wayne Myrvold http://streamer.perimeterinstitute.ca/mp3/f73324ab-59d4-4e3f-a7db-edb76bd76830.mp3 Science http://streamer.perimeterinstitute.ca/mp3/f73324ab-59d4-4e3f-a7db-edb76bd76830.mp3 Wed, 03 May 2006 16:00:15 -0400 We will look at the axioms of quantum mechanics as expressed, for example, in the book by M. A. Nielsen and I. L. Chung ("Quantum Computation and Quantum Information"). We then take a critical look at these axioms, raising several questions as we go. In particular, we will look at the possible informational completeness property of the family of operators that we measure. We will propose physical solutions based on the results of quantum mechanics on phase space and the measurement of quantum particles by quantum mechanical means. We illustrate this with both momentum-position measurements and spin measurements. Franklin Schroeck http://streamer.perimeterinstitute.ca/mp3/2316a6d4-1f4e-4305-b2d5-9f15e7fc50bb.mp3 Science http://streamer.perimeterinstitute.ca/mp3/2316a6d4-1f4e-4305-b2d5-9f15e7fc50bb.mp3 Wed, 10 May 2006 16:00:26 -0400 This talk is concerned with the noise-insensitive transmission of quantum information. For this purpose, the sender incorporates redundancy by mapping a given initial quantum state to a messenger state on a larger-dimensional Hilbert space. This encoding scheme allows the receiver to recover part of the initial information if the messenger system is corrupted by interaction with its environment. Our noise model for the transmission leaves a part of the quantum information unchanged, that is, we assume the presence of a noiseless subsystem or of a decoherence-free subspace. We address the case when the noiseless component cannot contain all the quantum information to be transmitted, and investigate how to best spread the information in a quantum state across the noise-susceptible components. (Joint work with David Kribs and Vern Paulsen.) Bernhard Bodmann http://streamer.perimeterinstitute.ca/mp3/5c759361-1801-48bc-8b86-4aa5926c8e36.mp3 Science http://streamer.perimeterinstitute.ca/mp3/5c759361-1801-48bc-8b86-4aa5926c8e36.mp3 Wed, 17 May 2006 16:00:54 -0400 It is somewhat surprising, but problems in quantum computing lead to problems in algebraic graph theory. I will discuss some instances that I am familiar with, and note a commmon thread. Chris Godsil http://streamer.perimeterinstitute.ca/mp3/ba6e586f-8c19-4fa0-b9f2-d14aeab5a46b.mp3 Science http://streamer.perimeterinstitute.ca/mp3/ba6e586f-8c19-4fa0-b9f2-d14aeab5a46b.mp3 Wed, 24 May 2006 16:00:48 -0400 Clifton, Bub, and Halvorson claim to be able to derive quantum mechanics from information-theoretic axioms. However, their derivation relies on the auxiliary assumption that the relevant probabilities for measurement outcomes can be represented by the observables (self-adjoint operators) and states of a C*-algebra. There are legitimate probability theories that are not so representable --- in particular, the nonlocal boxes of Popescu and Rohrlich. We explain the impact of nonlocal boxes on the interpretation of the CBH derivation, and we discuss possible generalizations of the CBH derivation in the framework of these more general probability theories. Hans Halvorson http://streamer.perimeterinstitute.ca/mp3/9da12bba-deab-4728-809b-b4e873541b84.mp3 Science http://streamer.perimeterinstitute.ca/mp3/9da12bba-deab-4728-809b-b4e873541b84.mp3 Mon, 29 May 2006 14:30:55 -0400 Adiabatic Quantum Computation is not only a possibly more robust alternative to standard quantum computation. Since it considers a continuous-time evolution of the system, it also provides a natural bridge towards studying the dynamics of interacting many-particle quantum systems, quantum phase transitions and other issues in fundamental physics. After a brief review of adiabatic quantum computation, I will show our recent results on the dynamics of entanglement and fidelity for the search and Deutsch algorithms including several variations and optimization. I will show how these studies led to suggesting an alternative definition of entanglement and compare the results, and discuss possible implications for considering entanglement a resource. I will conclude with an outlook on further applications and extensions of adiabatic quantum computation. Daria Ahrensmeier http://streamer.perimeterinstitute.ca/mp3/efd1e7d2-6c6e-4622-887b-a8623ed9c092.mp3 Science http://streamer.perimeterinstitute.ca/mp3/efd1e7d2-6c6e-4622-887b-a8623ed9c092.mp3 Wed, 07 Jun 2006 16:00:29 -0400 The modern view of representing a quantum observable as a semispectral measure as opposed to the traditional approach of using only spectral measures has added a great deal to our understanding of the mathematical structures and conceptual foundations of quantum mechanics. The old questions of 1) how to determine a quantum observable from its classical counter-part (if any), 2) how much statistical information is needed to determine an observable, 3) which observables can be measured together, and 4) are there noiseless measurements, all appear in a new perspective, calling for a study of problems such as: 1) how to obtain a semispectral measure by a quantization map, 2) do the moment operators of a semispectral measure determine the operator measure, 3) are coexistent observables jointly measurable, and 4) does minimal variance occur only in the case of a spectral measure? In my talk I will survey some of the recent developments concerning these questions and problems. Pekka Lahti http://streamer.perimeterinstitute.ca/mp3/3287359c-9197-4232-802e-dd2e38ea0463.mp3 Science http://streamer.perimeterinstitute.ca/mp3/3287359c-9197-4232-802e-dd2e38ea0463.mp3 Wed, 14 Jun 2006 16:00:21 -0400 In this talk, I will outline a quantum generalization of causal networks that are used to analyze complex probabilistic inference problems involving large numbers of correlated random variables. I will review the framework of classical causal networks and the graph theoretical constructions that are abstracted from them, including entailed conditional independence, d-separation and Markov equivalence. I will show how to generalize the definition of causal networks to the quantum case, such that the same graph theoretic constructions apply, and give an explicit representation of the states supported on the graph as the Gibbs states of certain classes of Hamiltonians. Matthew Leifer http://streamer.perimeterinstitute.ca/mp3/0506921c-e760-42d2-94e5-343f5a5cf131.mp3 Science http://streamer.perimeterinstitute.ca/mp3/0506921c-e760-42d2-94e5-343f5a5cf131.mp3 Wed, 28 Jun 2006 16:00:18 -0400 In order to predict the future state of a quantum system, we generally do not need to know the past state of the entire universe, but only the state of a finite neighborhood of the system. This locality is best expressed as a restriction on how information "flows" between systems. In this talk I will describe some recent work, inspired by quantum cellular automata, about the information strucutre of local quantum dynamics. Issues to be discussed include the definition of "locality", some characterization theorems, connections between classical and quantum locality for reversible maps, the relation between local and global dynamics, and the dissection of CNOT. Ben Schumacher http://streamer.perimeterinstitute.ca/mp3/27bdb7ed-198d-4a44-b969-fef69a413cfa.mp3 Science http://streamer.perimeterinstitute.ca/mp3/27bdb7ed-198d-4a44-b969-fef69a413cfa.mp3 Wed, 05 Jul 2006 16:00:12 -0400 In this talk I will present several new results from joint work with Dmitry Gavinsky, Oded Regev and Ronald de Wolf, relating to the model of one-way communication and the simultaneous model of communication. I will describe several separations between various resources (entanglement versus event coin, quantum communication versus classical communication), showing in particular that quantum communication cannot simulate a public coin and that entanglement can be much more powerful than a public coin, even if communication is quantum. I will also present a characterization of the quantum fingerprinting technique. Julia Kempe http://streamer.perimeterinstitute.ca/mp3/8a1a4b74-21d5-4106-89e5-0aeed4dc15c8.mp3 Science http://streamer.perimeterinstitute.ca/mp3/8a1a4b74-21d5-4106-89e5-0aeed4dc15c8.mp3 Wed, 30 Aug 2006 16:00:00 -0400 My field is the foundations of quantum mechanics, in particular Bohmian mechanics, a non-relativistic theory that is empirically equivalent to standard quantum mechanics while solving all of its paradoxes in an elegant and simple way, essentially by assuming that particles have trajectories. Bohmian mechanics possesses a straightforward generalization to relativistic space-time, be it flat or curved, if one assumption against the spirit of relativity is granted: the existence of a "time foliation", i.e., a physical object mathematically represented by a slicing of space-time into spacelike 3-surfaces, which evolves according to a Lorentz-invariant law. On the basis of this kind of theory, describing particles in a background 4-geometry, I propose an extension in which the space-time geometry is dynamically generated, as in general relativity. Whether my model is empirically equivalent to any known type of quantum gravity I don't know. In this model, there is a Lorentzian metric on configuration-space-time, evolving according to the higher-dimensional analog of the Einstein field equation. The 4-metric is obtained from the configuration-space-time metric and the actual particle configuration. Thus, this Bohm-like model generates (up to diffeomorphisms) a 4-metric and particle world lines from a given wave function. Roderich Tumulka http://streamer.perimeterinstitute.ca/mp3/61fa8a3e-b966-41fe-addb-e20a00af533a.mp3 Science http://streamer.perimeterinstitute.ca/mp3/61fa8a3e-b966-41fe-addb-e20a00af533a.mp3 Tue, 19 Sep 2006 04:00:00 -0400 Entanglement is one of the most studied features of quantum mechanics and in particular quantum information. Yet its role in quantum information is still not clearly understood. Results such as (R. Josza and N. Linden, Proc. Roy. Soc. Lond. A 459, 2011 (2003)) show that entanglement is necessary, but stabilizer states and the Gottesman-Knill theorem (for example) imply that it is far from sufficient. I will discuss three aspects of entanglement. First, a quantum circuit with a "vanishingly small" amount of entanglement that admits an apparent exponential speed-up over the classical case. Second, I will discuss techniques for lower-bounding the amount of entanglement in bipartite quantum states. Finally, I will discuss the role of entanglement in quantum metrology. Specifically, I will show that entangling ancillas can make no difference to the accuracy of a quantum parameter estimation, regardless of the nature of the coupling Hamiltonian. I will conclude by discussing strategies for improving the scaling of quantum parameter estimation. Steve Flammia http://streamer.perimeterinstitute.ca/mp3/08a3e08f-4b21-48b9-8908-3ca1472d8bd1.mp3 Science http://streamer.perimeterinstitute.ca/mp3/08a3e08f-4b21-48b9-8908-3ca1472d8bd1.mp3 Thu, 21 Sep 2006 16:00:00 -0400 Quantum mechanics is a non-classical probability calculus -- but hardly the most general one imaginable. In this talk, I'll discuss some familiar non-classical properties of quantum-probabilistic models that turn out to be features of {em all} non-classical models. These include a generic no-cloning theorem obtained in recent work with Howard Barnum, Jon Barrett and Matt Leifer. Alexandre Wilce http://streamer.perimeterinstitute.ca/mp3/5cec474c-9cb3-4fbe-8542-41b0e82b847d.mp3 Science http://streamer.perimeterinstitute.ca/mp3/5cec474c-9cb3-4fbe-8542-41b0e82b847d.mp3 Fri, 22 Sep 2006 15:00:00 -0400 We describe a protocol for distilling maximally entangled bipartite states between random pairs of parties (``random entanglement'') from those sharing a tripartite W state, and show that this may be done at a higher rate than distillation of bipartite entanglement between specified pairs of parties (``specified entanglement''). Specifically, the optimal distillation rate for specified entanglement for the W has been previously shown to be the asymptotic entanglement of assistance of 0.92 EPR pairs per W, while our protocol can distill 1 EPR pair per W between random pairs of parties, which we conjecture to be optimal. We further extend this to a more general class of W-like states and show by increasing the number of parties in the protocol that there exist states with fixed lower-bounded distillable random entanglement for arbitrarily small specified entanglement. [Work done in collaboration with Benjamin Fortescue. Preprint available at http://arxiv.org/abs/quant-ph/0607126 ] Hoi-Kwong Lo http://streamer.perimeterinstitute.ca/mp3/cf538d5c-47d2-4305-b0e4-d8ed51b892ca.mp3 Science http://streamer.perimeterinstitute.ca/mp3/cf538d5c-47d2-4305-b0e4-d8ed51b892ca.mp3 Mon, 25 Sep 2006 16:00:00 -0400 TBA Itamar Pitowsky http://streamer.perimeterinstitute.ca/mp3/022564b9-17b1-439e-b5e9-7b24d2928d59.mp3 Science http://streamer.perimeterinstitute.ca/mp3/022564b9-17b1-439e-b5e9-7b24d2928d59.mp3 Tue, 03 Oct 2006 09:00:00 -0400 TBA Dorit Aharonov http://streamer.perimeterinstitute.ca/mp3/bd491b81-256f-47f0-a7fa-747ac769f597.mp3 Science http://streamer.perimeterinstitute.ca/mp3/bd491b81-256f-47f0-a7fa-747ac769f597.mp3 Wed, 11 Oct 2006 04:00:00 -0400 I will discuss various different ways of quantifying the differences between two quantum observables (POVMs). Each of these approaches gives rise to a notion of approximately measuring one observable by means of measuring some other observable. This will be illustrated in the case of position and momentum by studying the question which POVMs on phase space can reasonably be said to represent a joint approximate determination of these observables. A new, universally valid trade-off relation for the associated inaccuracies will be rigorously formulated. I will sketch the proof which is an adaptation of some interesting techniques and properties of covariant phase space observables used recently by R Werner in a related project. Recommended reading (optional): quant-ph/0405184 (R Werner), quant-ph/0609185 (PB et al), and also for further background information quant-ph/0309091 (M Hall), quant-ph/0310070 (M Ozawa), quant-ph/9803051 (DM Appleby). Paul Busch http://streamer.perimeterinstitute.ca/mp3/0c6669ce-5baa-4331-ac35-8ec603a646c5.mp3 Science http://streamer.perimeterinstitute.ca/mp3/0c6669ce-5baa-4331-ac35-8ec603a646c5.mp3 Wed, 18 Oct 2006 16:00:00 -0400 Complexity class MA is a class of yes/no problems for which the answer `yes' has a short certificate that can be efficiently checked by a classical randomized algorithm. We prove that MA has a natural complete problem: stoquastic k-SAT. This is a quantum-mechanical analogue of the satisfiability problem such that k-bit clauses are replaced by k-qubit projectors with non-negative matrix elements. Complexity class AM is a generalization of MA in which the certificate may include a short conversation between Prover and Verifier. We prove that AM also has a natural complete problem: stoquastic Local Hamiltonian with a quenched disorder. The problem is to evaluate expectation value of the ground state energy of disordered local Hamiltonian with non-positive matrix elements. Sergey Bravyi http://streamer.perimeterinstitute.ca/mp3/dc04b230-ae18-4574-b7fe-70bd7cb6f971.mp3 Science http://streamer.perimeterinstitute.ca/mp3/dc04b230-ae18-4574-b7fe-70bd7cb6f971.mp3 Wed, 08 Nov 2006 16:00:00 -0500 In this talk, I will show how to efficiently generate graph states based on realistic linear optics (with imperfect photon detectors and source), how to do scalable quantum computation with probabilistic atom photon interactions, and how to simulate strongly correlated many-body physics with ultracold atomic gas. Luming Duan http://streamer.perimeterinstitute.ca/mp3/27393a94-b535-417b-9b7d-8a372edfa8a3.mp3 Science http://streamer.perimeterinstitute.ca/mp3/27393a94-b535-417b-9b7d-8a372edfa8a3.mp3 Wed, 15 Nov 2006 16:00:00 -0500 Most modern discussions of Bell's theorem take microscopic causality (the arrow of time) for granted, and raise serious doubts concerning realism and/or relativity. Alternatively, one may allow a weak form of backwards-in-time causation, by considering "causes" to have not only "effects" at later times but also "influences" at earlier times. These "influences" generate the correlations of quantum entanglement, but do not enable information to be transmitted to the past. Can one realize this scenario in a mathematical model? If macroscopic time-asymmetry is introduced by imposing initial conditions, such a model can not be deterministic. Stochastic Quantization (Parisi and Wu,1981) is a non-deterministic approach known to reproduce quantum field theory. Based on this, a search for models displaying quantum nonlocal correlations, while maintaining the principles of realism, relativity and macroscopic causality, is proposed. Nathan Argaman http://streamer.perimeterinstitute.ca/mp3/901ee082-00f1-47bf-8986-c647ab98377e.mp3 Science http://streamer.perimeterinstitute.ca/mp3/901ee082-00f1-47bf-8986-c647ab98377e.mp3 Wed, 22 Nov 2006 04:00:00 -0500 We explore the role of rotational symmetry of quantum key distribution (QKD) protocols in their security. Specifically, in the first part of the talk, we consider a generalized QKD protocol with discrete rotational symmetry. Note that, before our work, each QKD protocol seems to have a different security proof. Given that the techniques of those proofs are similar, it will be interesting to have a unified proof for QKD protocols with symmetry (e.g., the BB84 protocol and the SARG04 protocol). This is exactly what we achieve in our work. We show that rotational symmetry plays an important role in the unified security proof of QKD protocols with symmetry, leading to simple and structural security relations. In the second part, we consider a QKD protocol that does not possess rotational symmetry and analyze its security. Interestingly, even without any rotational symmetry, this protocol can still be proven secure. However, the security relation is not as simple as those in the first part, due to the lack of symmetry. Therefore, although rotational symmetry is not required in a QKD protocol to ensure its security, rotational symmetry does provide significant simplification in the security analysis, leading to simple security relations. Fred Fung http://streamer.perimeterinstitute.ca/mp3/f1110ee2-406a-46af-be6e-cf41746a54e6.mp3 Science http://streamer.perimeterinstitute.ca/mp3/f1110ee2-406a-46af-be6e-cf41746a54e6.mp3 Wed, 29 Nov 2006 16:00:00 -0500 Although entanglement constitutes one of the most remarkable differences between classical and quantum mechanics, and entanglement does have directly observable consequences, entanglement is not a regular observable like momentum or energy. It is rather a non-linear functional of a typically large set of such observables. Therefore, one often needs to perform many different measurements, in order to determine the degree of entanglement of a given quantum state. We show, how the number of measurement can be reduced -- sometimes to a single one -- if collective observables are measured, that respect the natural invariance properties of entangled states. Finally, we discuss the implementation of this approach in a laboratory experiment. Nature 440, 1022 (2006) PRL 79, 050501 (2006) quant-ph/0605250 Florian Mintert http://streamer.perimeterinstitute.ca/mp3/0e201666-c181-4406-a4ba-7693e23fc8ad.mp3 Science http://streamer.perimeterinstitute.ca/mp3/0e201666-c181-4406-a4ba-7693e23fc8ad.mp3 Wed, 06 Dec 2006 16:30:00 -0500 Yi Zhao http://streamer.perimeterinstitute.ca/mp3/d35f05d8-aace-4836-a7b6-5cf2b43ccd0c.mp3 Science http://streamer.perimeterinstitute.ca/mp3/d35f05d8-aace-4836-a7b6-5cf2b43ccd0c.mp3 Wed, 13 Dec 2006 16:00:00 -0500 Kolmogorov complexity is a measure of the information contained in a binary string. We investigate the notion of quantum Kolmogorov complexity, a measure of the information required to describe a quantum state. We show that for any definition of quantum Kolmogorov complexity measuring the number of classical bits required to describe a pure quantum state, there exists a pure n-qubit state which requires exponentially many bits of description. This is shown by relating the classical communication complexity to the quantum Kolmogorov complexity. Furthermore we give some examples of how quantum Kolmogorov complexity can be applied to prove results in different fields, such as quantum computation and communication. Caterina-Eloisa Mora http://streamer.perimeterinstitute.ca/mp3/b4fa9815-d76c-4281-b617-f0c43853bd61.mp3 Science http://streamer.perimeterinstitute.ca/mp3/b4fa9815-d76c-4281-b617-f0c43853bd61.mp3 Wed, 10 Jan 2007 16:00:00 -0500 A multi-partite entanglement measure is constructed via the distance or angle of the pure state to its nearest unentangled state. The extention to mixed states is made via the convex-hull construction, as is done in the case of entanglement of formation. This geometric measure is shown to be a monotone. It can be calculated for various states, including arbitrary two-qubit states, generalized Werner and isotropic states in bi-partite systems. It is also calculated for various multi-partite pure and mixed states, including ground states of some physical models and states generated from quantum alogrithms, such as Grover's. A specific application to a spin model with quantum phase transistions will be presented in detail.The connection of the geometric measure to other entanglement properties will also be discussed. Tzu-Chieh Wei http://streamer.perimeterinstitute.ca/mp3/4cf69fd0-51da-4cae-a14e-c263a2a574cf.mp3 Science http://streamer.perimeterinstitute.ca/mp3/4cf69fd0-51da-4cae-a14e-c263a2a574cf.mp3 Wed, 17 Jan 2007 16:00:00 -0500 If a large quantum computer (QC) existed today, what type of physical problems could we efficiently simulate on it that we could not simulate on a conventional computer? In this talk, I argue that a QC could solve some relevant physical "questions" more efficiently. First, I will focus on the quantum simulation of quantum systems satisfying different particle statistics (e.g., anyons), using a QC made of two-level physical systems or qubits. The existence of one-to-one mappings between different algebras of observables or between different Hilbert spaces allow us to represent and imitate any physical system by any other one (e.g., a bosonic system by a spin-1/2 system). We explain how these mappings can be performed showing quantum networks useful for the efficient evaluation of some physical properties, such as correlation functions and energy spectra. Second, I will focus on the quantum simulation of classical systems. Interestingly, the thermodynamic properties of any d-dimensional classical system can be obtained by studying the zero-temperature properties of an associated d-dimensional quantum system. This classical-quantum correspondence allows us to understand classical annealing procedures as slow (adiabatic) evolutions of the lowest-energy state of the corresponding quantum system. Since many of these problems are NP-hard and therefore difficult to solve, is worth investigating if a QC would be a better device to find the corresponding solutions. Rolando Somma http://streamer.perimeterinstitute.ca/mp3/1d92605f-eb9b-43c3-a253-42c8bb9c0f42.mp3 Science http://streamer.perimeterinstitute.ca/mp3/1d92605f-eb9b-43c3-a253-42c8bb9c0f42.mp3 Mon, 22 Jan 2007 16:00:00 -0500 We propose an extended quantum theory, in which the number of degrees of freedom K behaves as FOURTH power the number N of distinguishable states. As the simplex of classical N--point probability distributions can be embedded inside a higher dimensional convex body of mixed quantum states, one can further increase the dimensionality constructing the set of extended quantum states. The embedding proposed corresponds to an assumption that the physical system described in N dimensional Hilbert space is coupled with an auxiliary subsystem of the same dimensionality. The extended theory is shown to be a non-trivial generalisation of the standard quantum theory for which K=N^2. Imposing certain restrictions on initial conditions and dynamics allowed in the quartic theory one obtains quadratic theory as a special case. We discuss the question, how the theory of information processing looks like in the framework of the generalised quantum theory. In particular we propose a scheme of extended dense coding, in which one transmits two qubits by sending one extended bit, provided it was initially entangled with the extended bit of the receiver. Karol Zyczkowski http://streamer.perimeterinstitute.ca/mp3/cee1c425-8428-4512-a62e-481f930377b7.mp3 Science http://streamer.perimeterinstitute.ca/mp3/cee1c425-8428-4512-a62e-481f930377b7.mp3 Wed, 21 Feb 2007 16:00:00 -0500 We show that singlets composed of multiple multi-level quantum systems can naturally arise as the ground state of a physically-motivated Hamiltonian. The Hamiltonian needs to be one which simply exchanges the states of nearest neighbours in any graph of interacting d-level quantum systems (qudits) as long as the graph also has d sites. We point out that local measurements on some of these qudits, with the freedom of choosing a distinct measurement basis at each qudit randomly from an infinite set of bases, project the remainder onto a singlet state. One implication of this is that the entanglement in these states is very robust (persistent), while an application is in establishing an arbitrary amount of entanglement between well-separated parties (for subsequent use as a communication resource) by local measurements on an appropriate graph. Based on quant-ph/0602139. Christopher Hadley http://streamer.perimeterinstitute.ca/mp3/85da255f-581a-4165-8301-6846d7bf9c6a.mp3 Science http://streamer.perimeterinstitute.ca/mp3/85da255f-581a-4165-8301-6846d7bf9c6a.mp3 Wed, 07 Mar 2007 16:00:00 -0500 We give a communication problem between two players, Alice and Bob, that can be solved by Alice sending a quantum message to Bob, for which any classical interactive protocol requires exponentially more communication. Dmitry Gavinsky http://streamer.perimeterinstitute.ca/mp3/593ef9f3-f2d8-466f-9ef6-79e01ca98a45.mp3 Science http://streamer.perimeterinstitute.ca/mp3/593ef9f3-f2d8-466f-9ef6-79e01ca98a45.mp3 Wed, 21 Mar 2007 16:00:00 -0400 Consider a discrete quantum system with a d-dimensional state space. For certain values of d, there is an elegant information-theoretic uncertainty principle expressing the limitation on one's ability to simultaneously predict the outcome of each of d+1 mutually unbiased--or mutually conjugate--orthogonal measurements. (The allowed values of d include all powers of primes, and at present it is not known whether any value of d is excluded.) In this talk I show how states that minimize uncertainty in this sense can be generated via a discrete phase space based on finite fields. I also discuss some numerically observed features of these minimum-uncertainty states as the dimension d gets very large. William Wootters http://streamer.perimeterinstitute.ca/mp3/b5a7ccf0-c2f7-4b10-981e-3e9694b077e2.mp3 Science http://streamer.perimeterinstitute.ca/mp3/b5a7ccf0-c2f7-4b10-981e-3e9694b077e2.mp3 Wed, 28 Mar 2007 16:00:00 -0400 After a brief overview of the three broad classes of superconducting quantum bits (qubits)--flux, charge and phase--I describe experiments on single and coupled flux qubits. The quantum state of a flux qubit is measured with a Superconducting QUantum Interference Device (SQUID). Single flux qubits exhibit the properties of a spin-1/2 system, including superposition of quantum states, Rabi oscillations and spin echoes. Two qubits, coupled by their mutual inductance and by screening currents in the readout SQUID, produce a ground state |0> and three excited states |1>, |2> and |3>. Microwave spectra reveal an anticrossing between the |1>and |2> energy levels. The level repulsion can be reduced to zero by means of a current pulse in the SQUID that changes its dynamic inductance and hence the coupling between the qubits. The results are in good agreement with predictions. The ability to switch the coupling between qubits on and off permits efficient realization of universal quantum logic. This work was in collaboration with T. Hime, B.L.T. Plourde, P.A. Reichardt, T.L. Robertson, A. Ustinov, K.B. Whaley, F.K. Wilhelm and C.-E. Wu, and supported by AFOSR, ARO and NSF. John Clarke http://streamer.perimeterinstitute.ca/mp3/35e95108-06f9-46af-9bfc-c71163234ade.mp3 Science http://streamer.perimeterinstitute.ca/mp3/35e95108-06f9-46af-9bfc-c71163234ade.mp3 Wed, 04 Apr 2007 16:00:00 -0400 Inelastic collisions occur in Bose-Einstein condensates, in some cases, producing particle loss in the system. Nevertheless, these processes have not been studied in the case when particles do not escape the trap. We show that such inelastic processes are relevant in quantum properties of the system such as the evolution of the relative population and entanglement. Moreover, including inelastic terms in the models of multimode condensates allows for an exact analytical solution.  Ivette Fuentes-Schuller http://streamer.perimeterinstitute.ca/mp3/b37fac60-affd-4696-b0bd-0ed24a006e24.mp3 Science http://streamer.perimeterinstitute.ca/mp3/b37fac60-affd-4696-b0bd-0ed24a006e24.mp3 Wed, 11 Apr 2007 16:00:00 -0400 I will survey recent feasibility results on building multi-party cryptographic protocols which manipulate quantum data or are secure against quantum adversaries. The focus will be protocols for secure evaluation of quantum circuits. Along the way, I'll discuss how quantum machines can (and can't) prove knowledge of a secret to a distrustful partner. The talk is based on recent unpublished results, as well as older joint work with subsets of Michael Ben-Or, Claude Crepeau, Daniel Gottesman, and Avinatan Hasidim (STOC '02, FOCS '02, Eurocrypt '05, FOCS '06). Adam Brown http://streamer.perimeterinstitute.ca/mp3/c1fe643c-2582-4e6a-9af9-b689bbf8318f.mp3 Science http://streamer.perimeterinstitute.ca/mp3/c1fe643c-2582-4e6a-9af9-b689bbf8318f.mp3 Wed, 25 Apr 2007 16:00:00 -0400 In nearly every quantum algorithm which exponentially outperforms the best classical algorithm the quantum Fourier transform plays a central role. Recently, however, cracks in the quantum Fourier transform paradigm have begun to emerge. In this talk I will discuss one such development which arises in a new efficient quantum algorithm for the Heisenberg hidden subgroup problem. In particular I will show how considerations of symmetry for this hidden subgroup problem lead naturally to a different transform than the quantum Fourier transform, the Clebsch-Gordan transform over the Heisenberg group. Clebsch-Gordan transforms over finite groups thus appear to be an important new tool for those attempting to find new quantum algorithms. [Part of this work was done in collaboration with Andrew Childs (Caltech) and Wim van Dam (UCSB)] Dave Bacon http://streamer.perimeterinstitute.ca/mp3/d063dc80-9290-44be-8f8e-902fc95c8d61.mp3 Science http://streamer.perimeterinstitute.ca/mp3/d063dc80-9290-44be-8f8e-902fc95c8d61.mp3 Wed, 16 May 2007 16:00:00 -0400 In this talk we discuss how large classes of classical spin models, such as the Ising and Potts models on arbitrary lattices, can be mapped to the graph state formalism. In particular, we show how the partition function of a spin model can be written as the overlap between a graph state and a complete product state. Here the graph state encodes the interaction pattern of the spin model---i.e., the lattice on which the model is defined---whereas the product state depends only on the couplings of the model, i.e., the interaction strengths. As main examples, we find that the 1D Ising model corresponds to the 1D cluster state, the 2D Ising model without external field is mapped to Kitaev's toric code state, and the 2D Ising model with external field corresponds to the 2D cluster state---but the mappings are completely general in that arbitrary graphs, and also q-state models can be treated. These mappings allow one to make connections between concepts in (classical) statistical mechanics and quantum information theory and to obtain a cross-fertilization between both fields. As a main application, we will prove that the classical Ising model on a 2D square lattice (with external field) is a "complete model", in the sense that the partition function of any other spin model---i.e., for q-state spins on arbitrary lattices---can be obtained as a special instance of the (q=2) 2D Ising partition function with suitably tuned (complex) couplings. This result is obtained by invoking the above mappings from spin models to graph states, and the property that the 2D cluster states are universal resource states for one-way quantum computation. Joint work with Wolfgang Duer and Hans Briegel, see PRL/ 98 117207 (2007)/ and quant-ph/0708.2275. For related work, see also S. Bravyi and R. Raussendorf, quant-ph/0610162. Maarten Van den Nest http://streamer.perimeterinstitute.ca/mp3/e4dc9f56-22a9-4542-af57-8bc6fb2e2026.mp3 Science http://streamer.perimeterinstitute.ca/mp3/e4dc9f56-22a9-4542-af57-8bc6fb2e2026.mp3 Wed, 12 Sep 2007 16:00:00 -0400 It is known that finite fields with d elements exist only when d is a prime or a prime power. When the dimension d of a finite dimensional Hilbert space is a prime power, we can associate to each basis state of the Hilbert space an element of a finite or Galois field, and construct a finite group of unitary transformations, the generalised Pauli group or discrete Heisenberg-Weyl group. Its elements can be expressed, in terms of the elements of a Galois field. This group presents numerous applications in Quantum Information Science e.g. tomography, dense coding, teleportation, error correction and so on. The aim of our talk is to give a general survey of these properties and to present recently obtained results in connection with three problems: -the so-called ''Mean King's problem'' in prime power dimension, -discrete Wigner distributions, -and quantum tomography . Finally we shall discuss a limitation of the possible dimensions in which the so-called epistemic interpretation can be consistently formulated, in relation with the existence of finite affine planes, Euler's conjecture and the 36 officers problem. Thomas Durt http://streamer.perimeterinstitute.ca/mp3/ab38d6dd-dca1-4062-ab9a-c1c0994a3a53.mp3 Science http://streamer.perimeterinstitute.ca/mp3/ab38d6dd-dca1-4062-ab9a-c1c0994a3a53.mp3 Wed, 10 Oct 2007 16:00:00 -0400 Entanglement plays a fundamental role in quantum information processing and is regarded as a valuable, fungible resource, The practical ability to transform (or manipulate) entanglement from one form to another is useful for many applications. Usually one considers entanglement manipulation of states which are multiple copies of a given bipartite entangled state and requires that the fidelity of the transformation to (or from) multiple copies of a maximally entangled state approaches unity asymptotically in the number of copies of the original state. The optimal rates of these protocols yield two asymptotic measures of entanglement, namely, entanglement cost and distillable entanglement. It is not always justified, however, to assume that the entanglement resource available, consists of states which are multiple copies, i.e.,tensor products, of a given entangled state. More generally, an entanglement resource is characterized by an arbitrary sequence of bipartite states which are not necessarily of the tensor product form. In this seminar, we address the issue of entanglement manipulation for such general resources and obtain expressions for the entanglement cost and distillable entanglement. Nilanjana Datta http://streamer.perimeterinstitute.ca/mp3/19cb4af1-3bb3-45f1-8c71-f531b43ea690.mp3 Science http://streamer.perimeterinstitute.ca/mp3/19cb4af1-3bb3-45f1-8c71-f531b43ea690.mp3 Wed, 17 Oct 2007 16:00:00 -0400 We give a convenient representation for any map which is covariant with respect to an irreducible representation of SU(2), and use this representation to analyze the evolution of a quantum directional reference frame when it is exploited as a resource for performing quantum operations. We introduce the moments of a quantum reference frame, which serve as a complete description of its properties as a frame, and investigate how many times a quantum directional reference frame represented by a spin-j system can be used to perform a certain quantum operation with a given probability of success. We provide a considerable generalization of previous results on degradation of reference frame, from which follows a classification of the dynamics of spin-j system under the repeated action of any covariant map with respect to SU(2). Joint work with Lana Sheridan, Martin Laforest and Stephen Bartlett Jean Christian Boileau http://streamer.perimeterinstitute.ca/mp3/ec8f60e3-d42d-41a6-80ec-d39eb9caab3f.mp3 Science http://streamer.perimeterinstitute.ca/mp3/ec8f60e3-d42d-41a6-80ec-d39eb9caab3f.mp3 Wed, 24 Oct 2007 16:00:00 -0400 The one clean qubit model is a model of quantum computation in which all but one qubit starts in the maximally mixed state. One clean qubit computers are believed to be strictly weaker than standard quantum computers, but still capable of solving some classically intractable problems. I'll discuss my recent work in collaboration with Peter Shor which shows that evaluating a certain approximation to the Jones polynomial at a fifth root of unity for the trace closure of a braid is a complete problem for the one clean qubit complexity class. Stephen Jordan http://streamer.perimeterinstitute.ca/mp3/26e116cc-d08f-412e-b5d3-6341b3a17f58.mp3 Science http://streamer.perimeterinstitute.ca/mp3/26e116cc-d08f-412e-b5d3-6341b3a17f58.mp3 Wed, 31 Oct 2007 16:00:00 -0400 Renner's global quantum de Finetti theorem establishes that if the state of a quantum system is invariant under permutations of its systems, then almost all of its subsystems are almost in the same state and independent of each other. Motivated by this result, we show that the most straightforward classical analogue of Renner's theorem is false. Joint work with Matthias Christandl (Cambridge). Ben Toner http://streamer.perimeterinstitute.ca/mp3/8174731b-58de-4bf9-bd33-45eda37b5ccf.mp3 Science http://streamer.perimeterinstitute.ca/mp3/8174731b-58de-4bf9-bd33-45eda37b5ccf.mp3 Wed, 07 Nov 2007 16:00:00 -0500 We will compare quantum phase estimation from the point of view of quantum computation and quantum metrology. In the simplest cases, the former can be simplified to a sequential (unentangled) protocol, while the latter is parallel (entangled). We show that both protocols can be formally related with circuit identities and that they respond in exactly the same way to decoherence. We present sequential protocols for optimal estimation and frame synchronization in DQC1. Finally, we introduce new estimation protocols based on nonlinear Hamiltonians. We show that both optimal input states and product states with separable measurements improve the scaling of linear Hamiltonians. We will comment on the effect of decoherence in nonlinear protocols, and the role of entanglement in nonlinear protocols with product states. Sergio Boixo http://streamer.perimeterinstitute.ca/mp3/0e6192a6-3c83-47fc-90b3-1be1de473862.mp3 Science http://streamer.perimeterinstitute.ca/mp3/0e6192a6-3c83-47fc-90b3-1be1de473862.mp3 Wed, 14 Nov 2007 16:00:00 -0500 A class of operations distinct to entangled states shared between more than two parties is their conversion to entangled states shared between fewer parties. The extent to which these can be achieved in the regime of local operations and classical communication provides an operational characterisation of multiparty states, for example in the "entanglement of assistance" and related quantities. I will give a brief overview of this topic and discuss our results showing qualitatively different behaviour when the parties receiving the final state are not chosen beforehand. Benjamin Fortescue http://streamer.perimeterinstitute.ca/mp3/d8dc2652-5866-4e51-8ec1-5bed4dc10759.mp3 Science http://streamer.perimeterinstitute.ca/mp3/d8dc2652-5866-4e51-8ec1-5bed4dc10759.mp3 Wed, 21 Nov 2007 16:00:00 -0500 We attempt at characterizing the correlations present in the quantum computational model DQC1, introduced by Knill and Laflamme [Phys. Rev. Lett. 81, 5672 (1998)]. The model involves a collection of qubits in the completely mixed state coupled to a single control qubit that has nonzero purity. Although there is little or no entanglement between two parts of this system, it provides an exponential speedup in certain problems. On the contrary, we find that the quantum discord across the most natural split is nonzero for typical instances of the DQC1 ciruit. Nonzero values of discord indicate the presence of nonclassical correlations. We propose quantum discord as figure of merit for characterizing the resources present in this computational model. This might be a complementary measure for counting resources in quantum information science. Animesh Datta http://streamer.perimeterinstitute.ca/mp3/f513ec06-d10f-4b27-96f5-8146316d8a74.mp3 Science http://streamer.perimeterinstitute.ca/mp3/f513ec06-d10f-4b27-96f5-8146316d8a74.mp3 Wed, 28 Nov 2007 16:00:00 -0500 Roughly speaking, the more Alice is entangled with Bob, the harder it is for her to send her state to Charlie. In particular, it will be shown that the squashed entanglement, a well known entanglement measure, gives the fastest rate at which a quantum state can be sent between two parties who share arbitrary side information. Likewise, the entanglement of formation and entanglement cost is shown to be the fastest rate at which a quantum state can be sent when the parties have access to side-information which is maximally correlated. A further restriction on the type of side-information implies that the rate of state transmission is given by the quantum mutual information. This suggests a new paradigm for understanding entanglement and other correlations in terms of quantum Shannon theroy. Different types of side-information correspond to different types of correlations with the squashed entanglement and the mutual information being two extremes. Furthermore, there is a dual paradigm: if one distributes the side-information as aliciously as possible so as to make the sending of the state as difficult as possible, one finds maximum rates which give interpretations to known quantities as well as new ones. Jonathan Oppenheim http://streamer.perimeterinstitute.ca/mp3/29ad893a-c147-4077-9fc8-b8f405186ab9.mp3 Science http://streamer.perimeterinstitute.ca/mp3/29ad893a-c147-4077-9fc8-b8f405186ab9.mp3 Wed, 12 Dec 2007 16:00:00 -0500 We use a Bayesian approach to optimally solve problems in noisy binary search. We deal with two variants: 1. Each comparison can be erroneous with some probability 1 - p. 2. At each stage k comparisons can be performed in parallel and a noisy answer is returned. We present a (classic) algorithm which optimally solves both variants together, up to an additive term of O(log log (n)), and prove matching information theoretic lower bounds. We use the algorithm with the results of Farhi et al. (FGGS99)presenting a quantum search algorithm in an ordered list of expected complexity less than log(n)/3, and some improved quantum lower bounds on noisy search, and search with an error probability. Joint work with Michael Ben-Or. Avinatan Hassidim http://streamer.perimeterinstitute.ca/mp3/4c0f36c7-d558-462c-812e-8b4dd7d64231.mp3 Science http://streamer.perimeterinstitute.ca/mp3/4c0f36c7-d558-462c-812e-8b4dd7d64231.mp3 Wed, 09 Jan 2008 16:00:00 -0500 In this talk, we will investigate the distinguishability of quantum operations from both discrete and continuous point of view. In the discrete case, the main topic is how we can identify quantum measurement apparatuses by considering the patterns of measurement outcomes. In the continuous case, we will focus on the efficiency of parameter estimation of quantum operations. We will discuss several methods that can achieve Heisenberg Limit and prove in some other cases the impossibility of breaking the Standard Quantum Limit. The general treatment of estimation of quantum operations also allows an investigation of the effect of noise on estimation efficiency. Zhengfeng Ji http://streamer.perimeterinstitute.ca/mp3/42548b49-d417-4731-957c-1b3bb731e1fd.mp3 Science http://streamer.perimeterinstitute.ca/mp3/42548b49-d417-4731-957c-1b3bb731e1fd.mp3 Wed, 16 Jan 2008 16:00:00 -0500 The renormalization group (RG) is one of the conceptual pillars of statistical mechanics and quantum field theory, and a key theoretical element in the modern formulation of critical phenomena and phase transitions. RG transformations are also the basis of numerical approaches to the study of low energy properties and emergent phenomena in quantum many-body systems. In this colloquium I will introduce the notion of \"entanglement renormalization\" and use it to define a coarse-graining transformation for quantum systems on a lattice [G.Vidal, Phys. Rev. Lett. 99, 220405 (2007)]. The resulting real-space RG approach is able to numerically address 1D and 2D lattice systems with thousands of quantum spins using only very modest computational resources. From the theoretical point of view, entanglement renormalization sheds new light into the structure of correlations in the ground state of extended quantum systems. I will discuss how it leads to a novel, efficient representation for the ground state of a system at a quantum critical point or with topological order. Guifre Vidal http://streamer.perimeterinstitute.ca/mp3/cb38a649-2c00-4d5d-9b5f-deb5cbd48cc2.mp3 Science http://streamer.perimeterinstitute.ca/mp3/cb38a649-2c00-4d5d-9b5f-deb5cbd48cc2.mp3 Wed, 23 Jan 2008 16:00:00 -0500 We construct a simple translationally invariant, nearest-neighbor Hamiltonian on a chain of 10-dimensional qudits that makes it possible to realize universal quantum computing without any external control during the computational process, requiring only initial product state preparation. Both the quantum circuit and its input are encoded in an initial canonical basis state of the qudit chain. The computational process is then carried out by the autonomous Hamiltonian time evolution. After a time greater than a polynomial in the size of the quantum circuit has passed, the result of the computation can be obtained with high probability by measuring a few qudits in the computational basis. This result also implies that there cannot exist efficient classical simulation methods for generic translationally invariant nearest-neighbor Hamiltonians on qudit chains, unless quantum computers can be efficiently simulated by classical computers (or, put in complexity theoretic terms, unless BPP=BQP). This is joint work with Daniel Nagaj. Pawel Wocjan http://streamer.perimeterinstitute.ca/mp3/7917822b-b46e-4101-9b2a-cd9baf6635b4.mp3 Science http://streamer.perimeterinstitute.ca/mp3/7917822b-b46e-4101-9b2a-cd9baf6635b4.mp3 Wed, 30 Jan 2008 16:00:00 -0500 The \"frequency comb\" defined by the eigenmodes of an optical resonator is a naturally large set of exquisitely well defined quantum systems, such as in the broadband mode-locked lasers which have redefined time/frequency metrology and ultra precise measurements in recent years. High coherence can therefore be expected in the quantum version of the frequency comb, in which nonlinear interactions couple different cavity modes, as can be modeled by different forms of graph states. We show that is possible to thereby generate states of interest to quantum metrology and computing, such as multipartite entangled cluster and Greenberger-Horne-Zeilinger states. Olivier Pfister http://streamer.perimeterinstitute.ca/mp3/ea50dd02-ec63-4d10-9811-b961055d0b73.mp3 Science http://streamer.perimeterinstitute.ca/mp3/ea50dd02-ec63-4d10-9811-b961055d0b73.mp3 Wed, 06 Feb 2008 16:00:00 -0500 I should like to show how particular mathematical properties can limit our metaphysical choices, by discussing old and new theorems within the statistical-model framework of Mielnik, Foulis & Randall, and Holevo, and what these theorems have to say about possible metaphysical models of quantum mechanics. Time permitting, I should also like to show how metaphysical assumptions lead to particular mathematical choices, by discussing how the assumption of space as a relational concept leads to a not widely known frame-invariant formulation of classical point-particle mechanics by Föppl and Zanstra, and related research topics in continuum mechanics and general relativity. Luca (Piero G) Mana http://streamer.perimeterinstitute.ca/mp3/4964230a-592e-4d62-83df-493707da4cfc.mp3 Science http://streamer.perimeterinstitute.ca/mp3/4964230a-592e-4d62-83df-493707da4cfc.mp3 Wed, 13 Feb 2008 16:00:00 -0500 Decoherence attempts to explain the emergent classical behaviour of a quantum system interacting with its quantum environment. In order to formalize this mechanism we introduce the idea that the information preserved in an open quantum evolution (or channel) can be characterized in terms of observables of the initial system. We use this approach to show that information which is broadcast into many parts of the environment can be encoded in a single observable. This supports a model of decoherence where the pointer observable can be an arbitrary positive operator-valued measure (POVM). This generalization makes it possible to characterize the emergence of a realistic classical phase-space. In addition, this model clarifies the relations among the information preserved in the system, the information flowing from the system to the environment (measurement), and the establishment of correlations between the system and the environment. Cedric Beny http://streamer.perimeterinstitute.ca/mp3/ae72b710-7a2d-4d55-956b-25a1195e8352.mp3 Science http://streamer.perimeterinstitute.ca/mp3/ae72b710-7a2d-4d55-956b-25a1195e8352.mp3 Wed, 27 Feb 2008 16:00:00 -0500 Coin flipping by telephone (Blum '81) is one of the most basic cryptographic tasks of two-party secure computation. In a quantum setting, it is possible to realize (weak) coin flipping with information theoretic security. Quantum coin flipping has been a longstanding open problem, and its solution uses an innovative formalism developed by Alexei Kitaev for mapping quantum games into convex optimization problems. The optimizations are carried out over duals to the cone of operator monotone functions, though the mapped problem can also be described in a very simple language that involves moving points in the plane. Time permitting, I will discuss both Kitaev's formalism, and the solution that leads to quantum weak coin flipping with arbitrarily small bias. Carlos Mochon http://streamer.perimeterinstitute.ca/mp3/d05aa992-c46d-49a6-a164-5e694a3b9dde.mp3 Science http://streamer.perimeterinstitute.ca/mp3/d05aa992-c46d-49a6-a164-5e694a3b9dde.mp3 Wed, 05 Mar 2008 16:00:00 -0500 The talk concerns a generalization of the concept of a minimum uncertainty state to the finite dimensional case. Instead of considering the product of the variances of two complementary observables we consider an uncertainty relation involving the quadratic Renyi entropies summed over a full set of mutually unbiased bases (MUBs). States which achieve the lower bound set by this inequality were introduced by Wootters and Sussman, who proved existence for every prime power dimension, and by Appleby, Dang and Fuchs who showed that in prime dimension the fiducial vector for a for a symmetric informationally complete positive operator valued measure (SIC-POVM) covariant under the Weyl-Heisenberg group is a state of this kind. Subsequently Sussman proved existence for a class of odd prime power dimensions. The purpose of this talk is to complete the existence proof by showing that minimum uncertainty states exist in every prime power dimension, without exception. Along the way we establish a number of properties of the Clifford group, and Galois extension fields, which might be of some independent interest. Marcus Appleby http://streamer.perimeterinstitute.ca/mp3/ac201f51-7225-4e9d-9d0f-27730061b92a.mp3 Science http://streamer.perimeterinstitute.ca/mp3/ac201f51-7225-4e9d-9d0f-27730061b92a.mp3 Wed, 26 Mar 2008 16:00:00 -0400 The Hamiltonian of traditionally adopted detector models features out of diagonal elements between the vacuum and the one particle states of the field to be detected. We argue that reasonably good detectors, when written in terms of fundamental fields, have a more trivial response on the vacuum. In particular, the model configuration ``detector in its ground state + vacuum of the field' generally corresponds to a stable bound state of the underlying theory (e.g. the hydrogen atom in a suitable QED with electrons and protons) and therefore should be also an eigenstate of the model Hamiltonian. As a concrete example, we study a consistent ``fundamental' toy field theory where a stable particle can capture a light quantum and form a quasi-stable state. To such stable particle correspond eigenstates of the full theory, as is shown explicitly by using a dressed particle formalism at first order in perturbation theory. We then write the corresponding Hamiltonian for a model detector (at rest) where the stable particle and the quasi-stable configurations correspond to the two internal levels, ``ground' and ``excited', of the detector. The accelerated version of this Hamiltonian is inevitably model dependent emph{i.e.} it will generally depend on how the stable particle/detector is forced along the accelerated trajectory. However, in its most basic version, the accelerated detector doesn't see Unruh radiation. Federico Piazza http://streamer.perimeterinstitute.ca/mp3/acb0b7c0-890c-4e94-8986-68614a32fcf8.mp3 Science http://streamer.perimeterinstitute.ca/mp3/acb0b7c0-890c-4e94-8986-68614a32fcf8.mp3 Wed, 30 Apr 2008 16:00:00 -0400 Wigner-Dirac relativistic quantum theory is applied to decay laws of an unstable particle in different reference frames. It is shown that decay slows down from the point of view of the moving observer, as expected. However, small deviations from Einstein's time dilation formula are also found. The origin of these deviations is discussed, as well as possibilities for their experimental detection. Eugene Stefanovich http://streamer.perimeterinstitute.ca/mp3/de8fc09c-5412-488a-9aa3-4527aa6718ac.mp3 Science http://streamer.perimeterinstitute.ca/mp3/de8fc09c-5412-488a-9aa3-4527aa6718ac.mp3 Wed, 07 May 2008 11:00:00 -0400 What does a fractional quantum Hall liquid and Kitaev's proposals for topological quantum computation have in common? It turns out that they are physical systems that exhibit degenerate ground states with properties seemingly different than ordinary (Landau-type) vacua, such as the ground states of a Heisenberg magnet. For example, those (topologically quantum ordered)states cannot be characterized by (local) order parameters such as magnetization. How does one characterize this new order? I will present a unifying framework which will allow us to engineer physical systems displaying topological quantum order. What are the physical properties of these new orders? How robust are they to temperature effects? What are they useful for? Topologically quantum ordered states of matter seem to be ideal physical systems to store and manipulate quantum information since they are believed to be robust against decoherence with an environment, and thus appropriate for building a quantum computer and quantum memories. I will discuss the role of temperature in the protection of quantum information. Have we finally found a technological application for quantum Hall liquids? Gerardo Ortiz http://streamer.perimeterinstitute.ca/mp3/cbc8c7a3-a522-438b-b042-1c0dd8c8196d.mp3 Science http://streamer.perimeterinstitute.ca/mp3/cbc8c7a3-a522-438b-b042-1c0dd8c8196d.mp3 Wed, 07 May 2008 16:00:00 -0400 In an effort to better understand the class of operations on a bipartite system which preserve positivity of partial transpose (PPT operations), we have investigated the (non-asymptotic) transformation of pure states to pure states by operations in this class. Under local operations and classical communication (LOCC) Nielsen's majorization criterion provides a necessary and sufficient condition for such a transformation. This can be used to show that under LOCC a phenomenon called catalysis can occur, where an otherwise impossible transformation can be made possible by the provision of an entangled catalyst state, which must be recovered unchanged after the transformation (hence the name). I will present some recent work where we have found a necessary condition for obtaining a given pure state from a maximally entangled state via PPT operations. This condition is conjectured to be sufficient also, and we can prove this for the case where the goal state has Schmidt rank three. We have also shown that catalysis occurs under PPT operations, and have derived a necessary and sufficient condition for PPT pure state transformations where both the initial state and the catalyst are maximally entangled. Will Matthews http://streamer.perimeterinstitute.ca/mp3/5e4f3ce5-f50a-4f49-9970-f37be9625430.mp3 Science http://streamer.perimeterinstitute.ca/mp3/5e4f3ce5-f50a-4f49-9970-f37be9625430.mp3 Wed, 14 May 2008 16:00:00 -0400 We discuss two methods to encode one qubit into six physical qubits. Each of our two examples corrects an arbitrary single-qubit error. Our first example is a degenerate six-qubit quantum error-correcting code. We explicitly provide the stabilizer generators, encoding circuits, codewords, logical Pauli operators, and logical CNOT operator for this code. We also show how to convert this code into a non-trivial subsystem code that saturates the subsystem Singleton bound. We then prove that a six-qubit code without entanglement assistance cannot simultaneously possess a Calderbank-Shor-Steane (CSS) stabilizer and correct an arbitrary single-qubit error. A corollary of this result is that the Steane seven-qubit code is the smallest single-error correcting CSS code. Our second example is the construction of a non-degenerate six-qubit CSS entanglement-assisted code. This code uses one bit of entanglement (an ebit) shared between the sender and the receiver and corrects an arbitrary single-qubit error. The code we obtain is globally equivalent to the Steane seven-qubit code and thus corrects an arbitrary error on the receiver's half of the ebit as well. We prove that this code is the smallest code with a CSS structure that uses only one ebit and corrects an arbitrary single-qubit error on the sender's side. We discuss the advantages and disadvantages for each of the two codes. Bilal Shaw http://streamer.perimeterinstitute.ca/mp3/b495de84-4a66-41d9-867e-b320a9cbecb1.mp3 Science http://streamer.perimeterinstitute.ca/mp3/b495de84-4a66-41d9-867e-b320a9cbecb1.mp3 Wed, 09 Jul 2008 16:00:00 -0400 One of the quintessential features of quantum information is its exclusivity, the inability of strong quantum correlations to be shared by many physical systems. Likewise, complementarity has a similar status in quantum mechanics as the sine qua non of quantum phenomena. We show that this is no coincidence, and that the central role of exclusivity in quantum information theory stems from the phenomenon of complementarity. We adopt an information-theoretic approach to complementarity, which leads to a new and simple definition of private states and new proofs of the achievable asymptotic rates of both secret key and entanglement distillation. From the latter follows a new proof of the direct part of the quantum noisy channel coding theorem. Joe Renes http://streamer.perimeterinstitute.ca/mp3/215528bd-b779-4c87-b4a5-ed76049e7e8c.mp3 Science http://streamer.perimeterinstitute.ca/mp3/215528bd-b779-4c87-b4a5-ed76049e7e8c.mp3 Wed, 13 Aug 2008 16:00:00 -0400 I will report on efforts to implement a new method for simulating concatenated quantum error correction, where many levels of concatenation are simulated together explicitly. That is, the approach involves a Monte Carlo simulation of a noisy circuit involving many thousands of qubits, rather than tens of qubits previously. The new approach allows the threshold and resource usage of concatenated quantum error correction to be determined more accurately than before. Also, the approach makes it possible to better study the effects of circuit optimizations and message-passing algorithms [Poulin, PRA, 2006] on the performance of fault-tolerant concatenated quantum error correction. Such studies are necessary in order make a proper comparison with competing families of error-correction protocols, such as those involving surface codes. In the talk, a range of new numerical results will be presented. Henry Haselgrove http://streamer.perimeterinstitute.ca/mp3/a9f5d6ab-d1f8-4322-a16e-ef16212260ae.mp3 Science http://streamer.perimeterinstitute.ca/mp3/a9f5d6ab-d1f8-4322-a16e-ef16212260ae.mp3 Wed, 17 Sep 2008 16:00:00 -0400 A quantum channel models a physical process in which noise is added to a quantum system via interaction with its environment. Protecting quantum systems from such noise can be viewed as an extension of the classical communication problem introduced by Shannon sixty years ago. A fundamental quantity of interest is the quantum capacity of a given channel, which measures the amount of quantum information which can be protected, in the limit of many transmissions over the channel. In this talk, I will show that certain pairs of channels, each with a capacity of zero, can have a strictly positive capacity when used together, implying that the quantum capacity does not completely characterize a channel's ability to transmit quantum information. As a corollary, I will show that a commonly used lower bound on the quantum capacity - the coherent information, or hashing bound - is an overly pessimistic benchmark against which to measure the performance of quantum error correction because the gap between this bound and the capacity can be arbitrarily large. Jon Yard http://streamer.perimeterinstitute.ca/mp3/f4834b0b-218b-4d27-927e-82a4a87297f9.mp3 Science http://streamer.perimeterinstitute.ca/mp3/f4834b0b-218b-4d27-927e-82a4a87297f9.mp3 Wed, 24 Sep 2008 16:00:00 -0400 In this talk I will give an introduction to the simulation of quantum many-body systems using the so-called tensor networks. After a brief historical review, I will introduce the basics on tensor network representations of quantum states, and will explain some recent developments. In particular, in the last part of my talk I will focus on recent results obtained in the simulation of 2-dimensional quantum lattice systems of infinite size. Roman Orus http://streamer.perimeterinstitute.ca/mp3/fafdfd3b-a523-4e7b-bd53-b10ce39d8b23.mp3 Science http://streamer.perimeterinstitute.ca/mp3/fafdfd3b-a523-4e7b-bd53-b10ce39d8b23.mp3 Wed, 08 Oct 2008 16:00:00 -0400 I would like to provide a short, possibly elementary, introduction to the problem of computing string amplitudes at higher genus for superstrings. Essentially, I will recall which is the mathematical problem in defining the path integral measure (which has a well defined algebraic geometry realization for bosonic strings) and the solution proposed by d~@~YHocker and Phong for the genus 2 case. Their main results are the chiral splitted form of the measure, and its explicit expression in genus two. They proposed the splitting form to work at any genus and assumed some restriction for the explicit form which however did not permitted them to find a solution for genera higher then 2. I will tell something about the technology which permitted us to find explicit solutions for genus 3 and four. Indeed, we showed that the restrictions imposed by d~@~YHocker and Phong have no solution whereas the most general form compatible with modular invariance and clustering provide a unique solution, at least for genus 3 and 4. I will try to be as less technical as possible. Sergio Cacciatori http://streamer.perimeterinstitute.ca/mp3/ad345c70-6617-42ef-9851-fdcc364fa197.mp3 Science http://streamer.perimeterinstitute.ca/mp3/ad345c70-6617-42ef-9851-fdcc364fa197.mp3 Thu, 09 Oct 2008 14:00:00 -0400 The rise of quantum information science has been paralleled by the development of a vigorous research program aimed at obtaining an informational characterization or reconstruction of the quantum formalism, in a broad framework for stochastic theories that encompasses quantum and classical theory, but also a wide variety of other theories that can serve as foils to them. Such a reconstruction, at its most ambitious, is envisioned as playing a role in quantum physics similar to Einstein's reconstruction of the dynamics and kinetics of macroscopic bodies, and later of their gravitational interactions, on the basis of simple principles with clear operational meanings and experimental consequences. But short of such an ambitious goal, it could still lead to a principled understanding of the features of quantum mechanics that account for its greater-than-classical information-processing power, an understanding which could help guide the search for new quantum algorithms and protocols. I will summarize a convex operational framework for possible physical theories, and present results from a project to characterize quantum mechanics in terms of principles tightly linked to the possibility or impossibility of various information processing protocols. Previous results identified properties, like the existence of information-disturbance tradeoffs and restrictions on cloning and broadcasting, common to all nonclassical theories. In this talk I will focus on recent results involving protocols that are less generic. These are: the existence of exponentially secure bit commitment in non-classical theories without entanglement, the consequences for theories of the existence of a conclusive teleportation scheme, and sufficient conditions for the existence of a deterministic teleportation scheme. I'll also discuss sufficient conditions for 'remote steering' of ensembles using entanglement, rendering insecure bit commitment protocols of the form shown to be secure in the unentangled case. Connections to the category-theoretic approach of Coecke and Abramsky, Selinger, Baez, and collaborators may be touched on if time permits. Joint work with various groups of collaborators including Jonathan Barrett, Matthew Leifer, Alexander Wilce, Oscar Dahlsten, and Ben Toner. Howard Barnum http://streamer.perimeterinstitute.ca/mp3/9b5ba30b-6b7a-4443-88e7-6c05f644c002.mp3 Science http://streamer.perimeterinstitute.ca/mp3/9b5ba30b-6b7a-4443-88e7-6c05f644c002.mp3 Wed, 15 Oct 2008 16:00:00 -0400 We study the possibilit