Scientific Series

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Quantum criticality in SU(N) anti-ferromagnets: general strategies to circumvent the sign problem

Ribhu Kaul
University of Kentucky
April 22, 2014

PIRSA:14040083
Condensed Matter
Causality constraints in gravitational theories
April 22, 2014

PIRSA:14040116
Quantum Fields and Strings
Compact objects as dark-matter probes

Paolo Pani
Instituto Superior Tecnico - Departamento de Física
April 17, 2014

PIRSA:14040078
Strong Gravity
Universal problems

David Mattingly
University of New Hampshire
April 17, 2014

PIRSA:14040074
Quantum Gravity
Exact Classical Simulation of the Quantum-Mechanical GHZ Distribution

Gilles Brassard
Université de Montréal
April 16, 2014

PIRSA:14040060
Quantum Information
Categories of Convex Sets and C*-Algebras

Robert Furber
Radboud Universiteit Nijmegen
April 15, 2014

PIRSA:14040140
Quantum Foundations
Boson Tagging with Wavelets

William Shepherd
University of California System
April 15, 2014

PIRSA:14040063
Particle Physics
Groups and Clusters of Galaxies: Where Cosmology and Astrophysics Collide

Arif Babul
University of Victoria
April 15, 2014

PIRSA:14040133
Cosmology
New Exact Results in Calabi-Yau, D-branes, and Orientifolds

Sungjay Lee
University of Cambridge
April 15, 2014

PIRSA:14040081
Quantum Fields and Strings
The computational power of quantum walk

Andrew Childs
University of Waterloo

April 11, 2014

PIRSA:14040135
Quantum cosmology in Lorentz-violating gravity
April 10, 2014

PIRSA:14040079
Quantum Gravity
A non-commuting stabilizer formalism

Xiaotong Ni
Max Planck Institute for Gravitational Physics - Albert Einstein Institute (AEI)
April 09, 2014

PIRSA:14040136
Quantum Foundations

Quantum Gravity

Quantum criticality in SU(N) anti-ferromagnets: general strategies to circumvent the sign problem

Ribhu Kaul
University of Kentucky
April 22, 2014

PIRSA:14040083
Condensed Matter
In the first part of the seminar, I will describe a general approach to write down a large family of model SU(N) anti-ferromagnets that do not suffer from the sign problem. In the second half I will show how such Hamiltonians are useful for the study of deconfined quantum critical points and possibly other exotic physics.
Causality constraints in gravitational theories
April 22, 2014

PIRSA:14040116
Quantum Fields and Strings
Compact objects as dark-matter probes

Paolo Pani
Instituto Superior Tecnico - Departamento de Física
April 17, 2014

PIRSA:14040078
Strong Gravity
In the last few years the possibility of constraining dark matter with astrophysical observations of compact objects, such as white dwarfs, neutron stars and black holes, has been explored. The ultra-high density interior of neutron stars and the strong-curvature regions near massive black holes make these objects unique laboratories to test weakly-interacting particles. In the first part of this talk I will review some recent work on the potential of compact objects as dark-matter detectors, including the consequences of dark-matter accretion on compact stars and superradiant black-hole instabilities triggered by ultralight bosons. In the second part I will discus how observations of neutron stars can be used to constrain the dark-matter fraction in primordial black holes. In a close encounter with a neutron star, a primordial black hole can get gravitationally captured due to accretion, dynamical friction and tidal heating. Within a short time, the black hole is trapped inside the star and disrupts it by rapid accretion. The mere existence of old neutron stars limits the abundance of primordial black holes in the only mass range that was still unconstrained, thus suggesting that these objects cannot be the dominant dark matter constituent.
Universal problems

David Mattingly
University of New Hampshire
April 17, 2014

PIRSA:14040074
Quantum Gravity
Arguments that gravity cannot be a local renormalizable quantum field theory come from both field theory lore and black hole physics. Two current approaches to quantum gravity, asymptotic safety and Horava-Lifshitz gravity, both of which treat quantum gravity as a local renormalizable QFT, are explicitly constructed to counter field theory arguments about the non-renormalizability of gravity. However, any proposed renormalizable theory of quantum gravity must also answer black hole physics based counter-arguments. Formulating these arguments concretely requires understanding black hole solutions and thermodynamics in these theories. For Horava-Lifshitz gravity this entails understanding the thermodynamics of universal horizons. I describe the current status of universal horizon physics and which aspects are/are not still in tension with the fundamental premise of renormalizability.
Exact Classical Simulation of the Quantum-Mechanical GHZ Distribution

Gilles Brassard
Université de Montréal
April 16, 2014

PIRSA:14040060
Quantum Information
John Bell has shown that the correlations entailed by quantum mechanics cannot be reproduced by a classical process involving non-communicating parties. But can they be simulated with the help of bounded communication? This problem has been studied for more than twenty years and it is now well understood in the case of bipartite entanglement. However, the issue was still widely open for multipartite entanglement, even for the simplest case, which is the tripartite Greenberger-Horne-Zeilinger (GHZ) state. We give an exact simulation of arbitrary independent von Neumann measurements on general n-partite GHZ states. Our protocol requires O(n^2) bits of expected communication between the parties, and O(n log n) expected time is sufficient to carry it out in parallel. Furthermore, we need only an expectation of O(n) independent unbiased random bits, with no need for the generation of continuous real random variables nor prior shared random variables. In the case of equatorial measurements, we improve earlier results with a protocol that needs only O(n log n) bits of communication and O(log^2 n) parallel time. At the cost of a slight increase in the number of bits communicated, these tasks can be accomplished with a constant expected number of rounds.
Categories of Convex Sets and C*-Algebras

Robert Furber
Radboud Universiteit Nijmegen
April 15, 2014

PIRSA:14040140
Quantum Foundations
The start of the talk will be an outline how the ordinary notions of quantum theory translate into the category of C*-algebras, where there are several possible choices of morphisms. The second half will relate this to a category of convex sets used as state spaces. Alfsen and Shultz have characterized the convex sets arising from state spaces C*-algebras and this result can be applied to get a categorical equivalence between C*-algebras and state spaces of C*-algebras which is a generalization of the equivalence between the Schroedinger and Heisenberg pictures.
Boson Tagging with Wavelets

William Shepherd
University of California System
April 15, 2014

PIRSA:14040063
Particle Physics
I'll present a proof-of-concept new technique for tagging boosted objects which decay into two colored particles based on the wavelet transform. It is able to moderately improve the sensitivity of searches for such particles by 6-7%. I will also discuss future directions of applicability.
Groups and Clusters of Galaxies: Where Cosmology and Astrophysics Collide

Arif Babul
University of Victoria
April 15, 2014

PIRSA:14040133
Cosmology
Groups and clusters of galaxies are the most massive gravitationally bound objects in the Universe. They are also the most recent cosmic objects to form. In the currently accepted models of cosmic structure formation, the number density distribution of the most massive of these systems, and how this has been changing with time, depend sensitively to the parameters describing the large-scale geometry and the expansion history of the universe. However, to exploit galaxy clusters as cosmological probes, we must be able to relate their observable properties to their total mass. I will discuss our ongoing effort to calibrate the X-ray/SZ observations to the total mass for the 50 clusters comprising the Canadian Cluster Comparison Project (CCCP) sample. Resulting scaling relations, the associated scatter, and even how these change with time, invariably depend on "astrophysics" unfolding in these systems. The precision with which the cosmological parameters can be determined depends critically on how well we understand why groups and clusters manifest the observed properties that they do. This, however, has proven challenging. I will provide an update of the current state of affairs and highlight some of the key gaps in our understanding of the underlying physics.
New Exact Results in Calabi-Yau, D-branes, and Orientifolds

Sungjay Lee
University of Cambridge
April 15, 2014

PIRSA:14040081
Quantum Fields and Strings
We compute the exact two-sphere, disk and real projective plane partition functions of two-dimensional supersymmetric theories using the localization technique. From these new results, we will attack old and new important problems in the string theory on Calabi-Yau spaces, and D-branes and Orientifold planes therein.
The computational power of quantum walk

Andrew Childs
University of Waterloo

April 11, 2014

PIRSA:14040135

Quantum computers have the potential to solve certain problems dramatically faster than classical computers. One of the main quantum algorithmic tools is the notion of quantum walk, a quantum mechanical analog of random walk. I will describe quantum algorithms based on this idea, including an optimal algorithm for evaluating Boolean formulas and one of the best known algorithms for simulating quantum dynamics. I will also show how quantum walk can be viewed as a universal model of quantum computation.
Quantum cosmology in Lorentz-violating gravity
April 10, 2014

PIRSA:14040079
Quantum Gravity
Lorentz invariance is considered a fundamental symmetry of physical theories. However, while Lorentz violations are strongly constrained in the matter sector, constraints in the gravitational sector are weaker, allowing to contemplate the idea of Lorentz-violating gravity theories. In this talk I will discuss the effects of Lorentz violations in the quantum cosmology scenario by analyzing the properties of a simple anisotropic model in the framework of Horava-Lifshitz gravity and, if time permitting, some partial results on the viability of this class of theories.
A non-commuting stabilizer formalism

Xiaotong Ni
Max Planck Institute for Gravitational Physics - Albert Einstein Institute (AEI)
April 09, 2014

PIRSA:14040136
Quantum Foundations

Quantum Gravity
We propose a non-commutative extension of the Pauli stabilizer formalism. The aim is to describe a class of many-body quantum states which is richer than the standard Pauli stabilizer states. In our framework, stabilizer operators are tensor products of single-qubit operators drawn from the group {\alpha I, X,S}, where \alpha=e^{i\pi/4} and S=diag(1,i). We provide techniques to efficiently compute various properties, related to e.g. bipartite entanglement, expectation values of local observables, preparation by means of quantum circuits, parent Hamiltonians etc. We also highlight significant differences compared to the Pauli stabilizer formalism. In particular we give examples of states in our formalism which cannot arise in the Pauli stabilizer formalism, such as topological models that supports non-abelian anyons. This is a joint work with O. Buerschaper and M. van den Nest.

Title	Speaker(s)	Date	Talk Type	Info link
Quantum criticality in SU(N) anti-ferromagnets: general strategies to circumvent the sign problem	Ribhu Kaul	2014-04-22	Scientific Series	View details
Causality constraints in gravitational theories		2014-04-22	Scientific Series	View details
Compact objects as dark-matter probes	Paolo Pani	2014-04-17	Scientific Series	View details
Universal problems	David Mattingly	2014-04-17	Scientific Series	View details
Exact Classical Simulation of the Quantum-Mechanical GHZ Distribution	Gilles Brassard	2014-04-16	Scientific Series	View details
Categories of Convex Sets and C*-Algebras	Robert Furber	2014-04-15	Scientific Series	View details
Boson Tagging with Wavelets	William Shepherd	2014-04-15	Scientific Series	View details
Groups and Clusters of Galaxies: Where Cosmology and Astrophysics Collide	Arif Babul	2014-04-15	Scientific Series	View details
New Exact Results in Calabi-Yau, D-branes, and Orientifolds	Sungjay Lee	2014-04-15	Scientific Series	View details
The computational power of quantum walk	Andrew Childs	2014-04-11	Scientific Series	View details
Quantum cosmology in Lorentz-violating gravity		2014-04-10	Scientific Series	View details
A non-commuting stabilizer formalism	Xiaotong Ni	2014-04-09	Scientific Series	View details

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