Format results
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Integrability in gauge/string dualities
Pedro Vieira Perimeter Institute for Theoretical Physics
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Qu-transitions. Phase transitions in the quantum era.
Piers Coleman Rutgers University
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Untangling entanglement: An observer-dependent perspective
Lorenza Viola Dartmouth College
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Inference and Questions
John Skilling Maximum Entropy Data Consultants Ltd.
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Fermi's lazy photon, the GEO600 anomaly, and the no-Riemann-no-pie theorem
Giovanni Amelino-Camelia Sapienza Università di Roma - Dipartimento di Fisica
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Landscape of holographic superconductors
Sean Hartnoll Stanford University
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Decoherence and Entanglement Dynamics of Coupled Qubits
Alioscia Hamma University of Naples Federico II
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Discovery and Identification of s-channel Resonances at the LHC
Stephen Godfrey Carleton University
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(Towards) the end of the cosmological constant problem!
Niayesh Afshordi University of Waterloo
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Where quantum meets logic, . . . in a world of pictures!
Bob Coecke Quantinuum
PIRSA:09040001Yes, that's indeed where it happens. These pictures are not ordinary pictures but come with category-theoretic algebraic semantics, support automated reasoning and design of protocols, and match perfectly the developments in important areas of mathematics such as representation theory, proof theory, TQFT & GR, knot theory etc. More concretely, we report on the progress in a research program that aims to capture logical structures within quantum phenomena and quantum informatic tasks in purely diagrammatic terms. These picture calculi are faithful representations of certain kinds of monoidal categories, and structures therein. However, the goal of this program is partly to `release' these intuitive languages (or calculi) from their category-theoretic underpinning, and conceiving these pictures as mathematical entities in their own right. In this new language one is able to model and reason about things such a complementary observables, phase data, quantum circuits and algorithms, a variety of different quantum computational models, hidden-variable models, aspects of non-locality, and reason about all of these in terms of intuitive diagram transformations. Some recent benchmarks are the diagraamatic computation of quantum Fourier transform due to Duncan and myself, a purely diagrammatic proof of the no-cloning theorem due to Abramsky, and a categorical characterisation of GHZ-type non-locality due to Edwards, Spekkens and myself. For informal introductions we refer to: [1] Kindergarten quantum mechanics. http://arxiv.org/abs/quant-ph/0510032 [2] Introducing categories to the practicing physicist. http://arxiv.org/abs/0808.1032 For recent more advanced developments we suggest: [3] Selinger: Dagger compact closed categories and completely positive maps QPL\'05 http://www.mathstat.dal.ca/~selinger/papers.html#dagger [4] Coecke, Pavlovic, Vicary: A new description of orthogonal bases. http://arxiv.org/abs/0810.0812 [5] Coecke, Paquette, Perdrix: Bases in diagrammatic quantum protocols http://arxiv.org/abs/0808.1029 [6] Coecke, Duncan: Interacting quantum observables. ICALP\'08. http://www.springerlink.com/content/y443214116h76122/ [7] Coecke, Edwards: Toy quantum categories. QPL\'08. http://arxiv.org/abs/0808.1037 -
Integrability in gauge/string dualities
Pedro Vieira Perimeter Institute for Theoretical Physics
Integrability in gauge/string dualities will be reviewed in a broad perspective with a particular emphasis on the recently proposed equations describing the full planar spectrum of anomalous dimensions in AdS/CFT [N.Gromov, V.Kazakov, PV]. These are a concise version of Thermodynamic Bethe equations, called Y-system, which generalize the asymptotic Bethe equations of Beisert and Staudacher (which yield the full spectrum of N=4 SYM for asymptotically long local operators) and incorporate the 4-loop results for the shortest twist two operators obtained by Bajnok and Janik from the dual string sigma model (thus reproducing perturbative gauge theory computations with thousands of diagrams). On the way, we will explain some of the interesting open problems in the field. -
Qu-transitions. Phase transitions in the quantum era.
Piers Coleman Rutgers University
PIRSA:09040013Physicists are often so awestruck by the lofty achievements of the past, we end up thinking all the big stuff is done, which blinds us to the revolutions ahead. We are still firmly in the throes of the quantum revolution that began a hundred years ago. Quantum gravity, quantum computers, qu-bits and quantum phase transitions, are manifestations of this ongoing revolution. Nowhere is this more so, than in the evolution of our understanding of the collective properties of quantum matter. Fifty years ago, physicists were profoundly shaken by the discovery of universal power-law correlations at classical second-order phase transitions. Today, interest has shifted to Quantum Phase Transitions: phase transitions at absolute zero driven by the violent jigglings of quantum zero-point motion. Quantum, or Qu-transitions have been observed in ferromagnets, helium-3, ferro-electrics, heavy electron and high temperature superconductors. Unlike its classical counterpart, a quantum critical point is a kind of 'black hole' in the materials phase diagram: a singularity at absolute zero that profoundly influences wide swaths of the material phase diagram at finite temperature. I'll talk about some of the novel ideas in this field including 'avoided criticality' - the idea that high temperature superconductivity nucleates about quantum critical points - and the growing indications that electron quasiparticles break up at a quantum critical point. -
Untangling entanglement: An observer-dependent perspective
Lorenza Viola Dartmouth College
Entanglement is one of the most fundamental and yet most elusive properties of quantum mechanics. Not only does entanglement play a central role in quantum information science, it also provides an increasingly prominent bridging notion across different subfields of Physics --- including quantum foundations, quantum gravity, quantum statistical mechanics, and beyond. Arguably, the property of a state being entangled or not is by no means unambiguously defined. Rather, it depends strongly on how we decide to regard the whole as composed of its part or, more generally, on the restricted ways in which we are able to observe and control the system at hand. Acknowledging the implications of such an operationally constrained point of view naturally has led to a notion of 'generalized entanglement,' which is directly based on quantum observables and offers added flexibility in a variety of contexts. In this talk, I will survey some of the main accomplishments of the generalized entanglement program to date, with an eye toward recent developments and open problems. -
Inference and Questions
John Skilling Maximum Entropy Data Consultants Ltd.
We know the mathematical laws of quantum mechanics, but as yet we are not so sure why those laws should be inevitable. In the simpler but related environment of classical inference, we also know the laws (of probability). With better understanding of quantum mechanics as the eventual goal, Kevin Knuth and I have been probing the foundations of inference. The world we wish to infer is a partially-ordered set ('poset') of states, which may as often supposed be exclusive, but need not be (e.g. A might be a requirement for B). In inference, a state of mind about the world degrades from perfect knowledge through logical OR, which allows for uncertain alternatives. We don't need AND, and we don't need NOT; we just need OR. This display of acceptable states of mind is [close to] a mathematical 'lattice'. We find that the OR structure by itself (!) forces the ordinary rules of probability calculus. No other rules are compatible with the structure of a lattice, so the ordinary rules are inevitable. The standard Shannon information/entropy is likewise inevitable. Taking this idea further, the OR of states of mind gives a lattice of 'Questions' that might be useful for automated learning. Disconcertingly, this lattice is very much larger (in class aleph-2), and the natural valuations on it exhibit large range. I will present this extension, and ask whether we can rationally foresee its use in practical application. -
Fermi's lazy photon, the GEO600 anomaly, and the no-Riemann-no-pie theorem
Giovanni Amelino-Camelia Sapienza Università di Roma - Dipartimento di Fisica
I comment on rather significant recent developments that are relevant for proposals I had presented in previous PI seminars. The Fermi/GLAST space telescope has reported observations that would naturally fit previous formalizations of Planck-scale-induced in-vacuo dispersion (but also quite a few other things). And the unexplained excess noise found at the GEO600 interferometer is just of the type that had been previously described in terms of phenomenological models of spacetime foam (but may well be caused by quite a few other things). On the pure-theory side I can finally keep my promise to show that spacetime noncommutativity is a valuable tool of exploration of nonclassicality of spacetime, allowing the derivation of discretized spectra of distance, area, volume, and also providing a completely new overall geometric picture, in which amusingly the number Pi looses some of its privileges. -
Making a Splash--Breaking a Neck, The Making of Complexity in Physical Systems
PIRSA:09030002The fundamental laws of physics are very simple. The world about us is very complex. Living things are very complex indeed. This complexity has led some thinkers to suggest that living things are not the outcome of physical law but instead the creation of a designer. Here I examine how complexity is produced naturally in fluids. -
Landscape of holographic superconductors
Sean Hartnoll Stanford University
Holographic superconductors provide tractable models for the onset of superconductivity in strongly coupled theories. They have some features in common with experimentally studied nonconventional superconductors. I will review the physics of holographic superconductors and go on to show that many such models are to be found in the string landscape of AdS_4 vacua. -
Decoherence and Entanglement Dynamics of Coupled Qubits
Alioscia Hamma University of Naples Federico II
We study the entanglement dynamics and relaxation properties of a system of two interacting qubits in the two cases (I) two independent bosonic baths and (II) one common bath, at temperature $T$. The entanglement dynamics is studied in terms of the concurrence C(t) between the two spins and of the von Neumann entropy S(t) with respect to the bath, as a function of time. We prove that the system does thermalize. In the case (II) of a single bath, the existence of a decoherence-free (DFS) subspace makes entanglement dynamics very rich. We show that when the system is initially in a state with a component in the DFS the relaxation time is surprisingly long, showing the existence of semi-decoherence free subspaces. The equilibrium state in this case is not the Gibbs state. The entanglement dynamics for the single bath case is also studied as a function of temperature, coupling strength with the environment and strength of tunneling coupling. The case of the mixed state is finally shown and discussed. -
Discovery and Identification of s-channel Resonances at the LHC
Stephen Godfrey Carleton University
s-channel resonances are predicted by many models of Physics Beyond the Standard Model and it is quite possible that such an object will be discovered in the early years of the LHC program. If this occurs, the task will be to understand its origins. A brief survey of models that predict s-channel resonances will be given, concentrating mainly on extra neutral gauge bosons (Z' 's) arising from extended gauge theories. This will be followed by a description of how to search for a Z' and the resulting Z' discovery reach of the LHC. I will describe various diagnostic measurements to study Z' 's and describe some new observables we have proposed that can distinguish between models that take advantage of the ability to tag 3rd generation fermions. -
Acceleration in our past, present, and future
Leonardo Senatore ETH Zurich
The great advances in observational cosmology in the last few years have delivered us an unprecedented amount of new data. They begin to indicate with confidence that in the past our universe underwent a phase of acceleration, called inflation, and that it is currently undergoing a similar phase, usually thought of as a consequence of a cosmological constant. I will show how inflation can be probed, using to this purpose a very general effective field theory description. In particular, I will concentrate on the new and powerful signal of the non-gaussianity of the primordial density perturbations, explaining its theoretical motivation, the techniques to look for it in the data, and the current constraints from the WMAP experiment. This signature is very important not only to identify the precise mechanism that drove inflation, but also to shed light on possible alternatives, such as the recently proposed bouncing cosmology. I will describe how these alternative theories can be consistently formulated and be predictive, and how similar theories may have interesting implications for the current acceleration of the universe. If inflation happened in our past, it might actually have been eternal. The presence of such a phase offers a new way to address the problem of the cosmological constant and of the current acceleration of the universe. This will lead us to explain in precise terms what eternal inflation is. -
(Towards) the end of the cosmological constant problem!
Niayesh Afshordi University of Waterloo
The cosmological constant problem is arguably the deepest gap in our understanding of modern physics. The discovery of cosmic acceleration in the past decade and its surprising coincidence with cosmic structure formation has added an extra layer of complexity to the problem. I will describe how revisiting/revising some standard assumptions in the theory of gravity can decouple the quantum vacuum from geometry, which can potentially solve the cosmological constant problem. I will then argue that a possible fascinating outcome of such a theory is to relate black hole formation to cosmic acceleration, providing a possible solution to the cosmic coincidence. A diverse range of experimental/observational probes over the next decade will tell us whether we are close to the end of this century-old mystery, which in turn could shed light on the nature of quantum gravity and black holes.