Scientific Series

Displaying 3205 - 3216 of 5012

Divergences in Spinfoam Quantum Gravity

Aldo Riello
Perimeter Institute for Theoretical Physics
November 28, 2013

PIRSA:13110063
Quantum Gravity
Collisions in AdS and the thermalisation of heavy-ion collisions

Wilke van der Schee
European Organization for Nuclear Research (CERN)
November 28, 2013

PIRSA:13110089
Strong Gravity
Homological Product Codes

Sergey Bravyi
IBM (United States)
November 27, 2013

PIRSA:13110091
Quantum Information
Insightful supersymmetry

Erich Poppitz
University of Toronto
November 27, 2013

PIRSA:13110061
Particle Physics
Bounding the Elliptope of Quantum Correlations & Proving Separability in Mixed States

Elie Wolfe
Perimeter Institute for Theoretical Physics
November 26, 2013

PIRSA:13110092
Quantum Foundations
Classical Space Times from S Matrices

Ira Rothstein
Carnegie Mellon University
November 26, 2013

PIRSA:13110053
Particle Physics
BiGravity: from Cosmological Solutions to Dual Galileons

Matteo Fasiello
University of Portsmouth
November 26, 2013

PIRSA:13110093
Cosmology
Dimensional reduction in the sky
November 21, 2013

PIRSA:13110059
Quantum Gravity
Charged particle motion in magnetized black holes

Valeri Frolov
University of Alberta
November 21, 2013

PIRSA:13110066
Strong Gravity
Quantum Adversary (Upper) Bound

Shelby Kimmel
Massachusetts Institute of Technology (MIT)
November 20, 2013

PIRSA:13110090
Quantum Information
Light and matter: towards macroscopic quantum systems

Jacob Taylor
Office of Science and Technology Policy
November 20, 2013

PIRSA:13110060
Quantum Information
The Amplitude Mode in Condensed Matter : Higgs Hunting on a Budget

Daniel Arovas
University of California, San Diego
November 19, 2013

PIRSA:13110054
Condensed Matter

Divergences in Spinfoam Quantum Gravity

Aldo Riello
Perimeter Institute for Theoretical Physics
November 28, 2013

PIRSA:13110063
Quantum Gravity
The most relevant evidences in favour of the Lorentzian EPRL-FK spinfoam model come from its capibility of reproducing the expected semiclassical limit in the large spin regime. The main examples of this are the large spin limit of the vertex amplitude, later extended to arbitrary triangulations, and that of the spinfoam graviton propagator, which was calculated on the simplest possible two complex. These results are very promising. Nonetheless, their relevance may be endangered by the effects associated to radiative corrections. In this seminar, I will focus on the role played by the simplest diverging graph, the so called 'melon graph', which is known to play a fundamental role in tensorial group field theories. In particular, I will discuss its most divergent part and its geometrical interpretation. I will finally comment on the result, with particular attention to its physical consequences, especially in relation with the semiclassical limit of the spinfoam graviton propagator.
Collisions in AdS and the thermalisation of heavy-ion collisions

Wilke van der Schee
European Organization for Nuclear Research (CERN)
November 28, 2013

PIRSA:13110089
Strong Gravity
The motivation of this seminar is to understand the thermalisation of heavy ion collisions using AdS/CFT. These collisions can be modelled as colliding planar gravitational shock waves. This gives rise to rich and interesting dynamics; wide shocks come to a full stop and expand hydrodynamically, as was previously found by Chesler and Yaffe. High energy collisions (corresponding to thin shocks) pass through each other, after which a plasma forms in the middle, within a proper time 1/T, with T the local temperature at that time. After this I will discuss recent results where we studied the influence of microscopic structure in the longitudinal direction of the shock waves, and thereby found a coherent regime. This has implications for both fluctuations in nucleus-nucleus collisions, and for recent proton-lead collisions at at LHC. The final part will cover a radially expanding calculation, where some simplifications allowed us to solve the model all the way till the final particle spectra, with an interesting comparison with experimental data.
Homological Product Codes

Sergey Bravyi
IBM (United States)
November 27, 2013

PIRSA:13110091
Quantum Information
Quantum codes with low-weight stabilizers known as LDPC codes have been actively studied recently due to their potential applications in fault-tolerant quantum computing. However, all families of quantum LDPC codes known to this date suffer from a poor distance scaling limited by the square-root of the code length. This is in a sharp contrast with the classical case where good families of LDPC codes are known that combine constant encoding rate and linear distance. Here we propose the first family of good quantum codes with low-weight stabilizers. The new codes have a constant encoding rate, linear distance, and stabilizers acting on at most square root of n qubits, where n is the code length. For comparison, all previously known families of good quantum codes have stabilizers of linear weight. Our proof combines two techniques: randomized constructions of good quantum codes and the homological product operation from algebraic topology. We conjecture that similar methods can produce good stabilizer codes with stabilizer weight n^a for any a>0. Finally, we apply the homological product to construct new small codes with low-weight stabilizers. This is a joint work with Matthew Hastings.
Insightful supersymmetry

Erich Poppitz
University of Toronto
November 27, 2013

PIRSA:13110061
Particle Physics
It has recently been realized that some studies of supersymmetric gauge theories, when properly interpreted, lead to insights whose importance transcends supersymmetry. I will illustrate the insightful nature of supersymmetry by two examples having to do with the microscopic description of the thermal deconfinement transition, in non-supersymmetric pure Yang-Mills theory and in QCD with adjoint fermions. A host of strange ``topological" molecules will be seen to be the major players in the confinement-deconfinement dynamics. Interesting connections between topology, ``condensed-matter" gases of electric and magnetic charges, and attempts to interpret the divergent perturbation series will emerge. Much of the presentation will be aimed at non-experts.
Bounding the Elliptope of Quantum Correlations & Proving Separability in Mixed States

Elie Wolfe
Perimeter Institute for Theoretical Physics
November 26, 2013

PIRSA:13110092
Quantum Foundations
We present a method for determining the maximum possible violation of any linear Bell inequality per quantum mechanics. Essentially this amounts to a constrained optimization problem for an observable’s eigenvalues, but the problem can be reformulated so as to be analytically tractable. This opens the door for an arbitrarily precise characterization of quantum correlations, including allowing for non-random marginal expectation values. Such a characterization is critical when contrasting QM to superficially similar general probabilistic theories. We use such marginal-involving quantum bounds to estimate the volume of all possible quantum statistics in the complete 8-dimensional probability space of the Bell-CHSH scenario, measured relative to both local hidden variable models as well as general no-signaling theories. See arXiv:1106.2169. Time permitting, we’ll also discuss how one might go about trying to prove that a given mixed state is, in fact, not entangled. (The converse problem of certifying non-zero entanglement has received extensive treatment already.) Instead of directly asking if any separable representation exists for the state, we suggest simply checking to see if it “fits” some particular known-separable form. We demonstrate how a surprisingly valuable sufficient separability criterion follows merely from considering a highly-generic separable form. The criterion we generate for diagonally-symmetric mixed states is apparently completely tight, necessary and sufficient. We use integration to quantify the “volume” of states captured by our criterion, and show that it is as large as the volume of states associated with the PPT criterion; this simultaneously proves our criterion to be necessary as well as the PPT criterion to be sufficient, on this family of states. The utility of a sufficient separability criterion is evidenced by categorically rejecting Dicke-model superradiance for entanglement generation schema. See arXiv:1307.5779.
Classical Space Times from S Matrices

Ira Rothstein
Carnegie Mellon University
November 26, 2013

PIRSA:13110053
Particle Physics
Progress in calculating S matrix elements have shown that the malicious redundancies in non-linear gauge theories can be circumvented by utilizing unitarity methods in conjunction with BCFW recursion relations. When calculating in this fashion all of the interaction vertices beyond the three point function can be ignored. This simplification is especially useful in gravity which contains an infinite number of such non-linear interactions. It is natural to ask whether off-shell quantities, such as classical solutions, can also be generated using only the three point vertex. In this talk I will show that this is indeed the case by extracting classical solutions to GR from on-hell two to two scattering S-matrix elements. In so doing we will completely circumvent the action as well as the equations of motion. The only inputs will be Lorentz invariance, the existence of a massless spin-two particle and locality. Because of the double copy relation this implies there exists, a yet to be understood, connection between solutions to Yang-Mills theory and Gravity. I will also discuss how this technique can be used to simplify calculations of higher order post-Newtonian corrections to gravitational potentials relevant to the problem of binary inspirals.
BiGravity: from Cosmological Solutions to Dual Galileons

Matteo Fasiello
University of Portsmouth
November 26, 2013

PIRSA:13110093
Cosmology
I will present Cosmological FRW Solutions in BiGravity Theories and discuss their stability. After deriving the stability bound, one realizes that in Bigravity (in contradistinction to the FRW massive gravity case) the tension between requirements stemming from stability and those set by observations is resolved. The stability bound can also be derived in the decoupling limit of Bigravity. In this context an intriguing duality between Galilean interactions has emerged.
Dimensional reduction in the sky
November 21, 2013

PIRSA:13110059
Quantum Gravity
In several approaches to quantum-gravity, the spectral dimension of spacetime runs from the standard value of 4 in the infrared (IR) to a smaller value in the ultraviolet (UV). Describing this running in terms of deformed dispersion relations, I show that a striking cosmological implication is that that UV behavior leading to 2 spectral dimensions results in an exactly scale-invariant spectrum of vacuum scalar and tensor fluctuations. I discuss scenarios that break exact scale-invariance and show that the tensor to scalar ratio is fixed by the UV ratio between the speed of gravity and the speed of light. Cosmological perturbations in this framework display a wavelength-dependent speed of light, but by transforming to a suitable "rainbow frame" this feature can be removed, at the expense of modifying gravity. In particular it turns out that the following concepts are closely connected: scale-invariance of vacuum fluctuations, conformal invariance of the gravitational coupling, UV reduction to spectral dimension 2 in position space and UV reduction to Hausdorff dimension 2 in energy-momentum space.
Charged particle motion in magnetized black holes

Valeri Frolov
University of Alberta
November 21, 2013

PIRSA:13110066
Strong Gravity
There exist evidences that magnetic field in the vicinity of astrophysical black holes plays an important role. In particular it is required for explanation of such phenomenon as jet formation. Study of such problems in all their complexity requires 3D numerical simulations of the magnetohydrodynamics in a strong gravitational field. Quite often when dealing with such a complicated problem it is instructive to consider first its simplifications, which can be treated either analytically, or by integrating ordinary differential equations. Motion of a charged particle in a weakly magnetized black hole is an important example. We consider a non-rotating black hole in the weak magnetic field which is homogeneous at infinity. In the talk I discuss the following problems: How does such a magnetic field affect charged particle motion in the equatorial plane? How does it change the radius of the innermost stable circular orbits (ISCO) and period of rotation? I shall demonstratethat the magnetic field increases the efficiency of the energy extraction from the black hole and that magnetized black holes can be used as "particle accelerators". Finally, I shall discuss out-of-equatorial-plane motion and demonstrate that it is chaotic. Possible applications of these results to astrophysics are briefly discussed.
Quantum Adversary (Upper) Bound

Shelby Kimmel
Massachusetts Institute of Technology (MIT)
November 20, 2013

PIRSA:13110090
Quantum Information
I discuss a technique - the quantum adversary upper bound - that uses the structure of quantum algorithms to gain insight into the quantum query complexity of Boolean functions. Using this bound, I show that there must exist an algorithm for a certain Boolean formula that uses a constant number of queries. Since the method is non-constructive, it does not give information about the form of the algorithm. After describing the technique and applying it to a class of functions, I will outline quantum algorithms that match the non-constructive bound.
Light and matter: towards macroscopic quantum systems

Jacob Taylor
Office of Science and Technology Policy
November 20, 2013

PIRSA:13110060
Quantum Information
Advances in quantum engineering and material science are enabling new approaches for building systems that behave quantum mechanically on long time scales and large length scales. I will discuss how microwave and optical technologies in particular are leading to new domains of many-body physics, both classical and quantum, using photons and phonons as the constituent particles. Furthermore, I will highlight practical consequences of these advances, including improved force and acceleration sensing, efficient signal transduction, and topologically robust photonic circuits. Finally, I will consider how such large quantum systems may help us measure and constrain theories of quantum gravity and gravity-induced decoherence.
The Amplitude Mode in Condensed Matter : Higgs Hunting on a Budget

Daniel Arovas
University of California, San Diego
November 19, 2013

PIRSA:13110054
Condensed Matter
The amplitude mode is a ubiquitous phenomenon in systems with broken continuous symmetry and effective relativistic dynamics, and has been observed in magnets, charge density waves, cold atom systems, and superconductors. It is a simple analog of the Higgs boson of particle physics. I will discuss the properties of the amplitude mode and its somewhat surprising visibility in two-dimensional systems, recently confirmed in cold atom experiments. The behavior in the vicinity of a quantum critical point will be stressed, comparing theoretical, numerical, and experimental results.

Title	Speaker(s)	Date	Talk Type	Info link
Divergences in Spinfoam Quantum Gravity	Aldo Riello	2013-11-28	Scientific Series	View details
Collisions in AdS and the thermalisation of heavy-ion collisions	Wilke van der Schee	2013-11-28	Scientific Series	View details
Homological Product Codes	Sergey Bravyi	2013-11-27	Scientific Series	View details
Insightful supersymmetry	Erich Poppitz	2013-11-27	Scientific Series	View details
Bounding the Elliptope of Quantum Correlations & Proving Separability in Mixed States	Elie Wolfe	2013-11-26	Scientific Series	View details
Classical Space Times from S Matrices	Ira Rothstein	2013-11-26	Scientific Series	View details
BiGravity: from Cosmological Solutions to Dual Galileons	Matteo Fasiello	2013-11-26	Scientific Series	View details
Dimensional reduction in the sky		2013-11-21	Scientific Series	View details
Charged particle motion in magnetized black holes	Valeri Frolov	2013-11-21	Scientific Series	View details
Quantum Adversary (Upper) Bound	Shelby Kimmel	2013-11-20	Scientific Series	View details
Light and matter: towards macroscopic quantum systems	Jacob Taylor	2013-11-20	Scientific Series	View details
The Amplitude Mode in Condensed Matter : Higgs Hunting on a Budget	Daniel Arovas	2013-11-19	Scientific Series	View details

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