Scientific Series

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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
General Relativity & Quantum Cosmology
A non-commuting stabilizer formalism

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

PIRSA:14040136
Quantum Physics

General Relativity & Quantum Cosmology
Almost quantum correlations

Antonio Acin Institute of Photonic Sciences (ICFO)

April 09, 2014

PIRSA:14040065
Beyond local causality: causation and correlation after Bell

Matthew Pusey University of York
April 08, 2014

PIRSA:14040130
Quantum Physics
Is the Milky Way Ringing?

Lawrence Widrow Queen's University
April 08, 2014

PIRSA:14040137
Astrophysics
The unitary Fermi gas across the superfluid transition

Olga Goulko University of Massachusetts Amherst
April 04, 2014

PIRSA:14040119
Condensed Matter
Quantum Gravity, Trees, and Polynomials

Fabien Vignes-Tourneret Université Claude Bernard Lyon 1
April 03, 2014

PIRSA:14040134
General Relativity & Quantum Cosmology
Quantum Computing with Noninteracting Particles

Alex Arkhipov Massachusetts Institute of Technology (MIT)
April 02, 2014

PIRSA:14040077
Quantum Physics
Scribble, Scribble

Isabella Stefanescu

April 02, 2014

PIRSA:14040117
Fibonacci Anyons From Abelian Bilayer Quantum Hall States

Abolhassan Vaezi Cornell University
April 01, 2014

PIRSA:14040120
Condensed Matter
Incompatibility of observables in quantum theory and other probabilistic theories
April 01, 2014

PIRSA:14040075
Quantum Physics

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
General Relativity & Quantum Cosmology
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 Physics

General Relativity & Quantum Cosmology
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.
Almost quantum correlations

Antonio Acin Institute of Photonic Sciences (ICFO)

April 09, 2014

PIRSA:14040065

Quantum theory is successfully tested in any experimental lab every day. Apart from its experimental validity, quantum theory also constitutes a robust theoretical framework: small variations of its formalism often lead to highly implausible consequences, such as violation of the no-signalling principle or a significant increase of the computational power. In fact, it has been argued that quantum theory may represent an island in theory space. We show that, at the level of correlations, quantum theory may not be as special as initially thought. In order to do so, we define the set of almost quantum correlations and prove that this set is (i) strictly larger than the set of quantum correlations but (ii) satisfies most of the information principles introduced to characterize quantum correlations, such as local orthogonality, macroscopic locality, no advantage for nonlocal computation or non-trivial communication complexity. We also provide numerical evidence that the set is compatible with information causality. Finally, we briefly discuss how the set of almost quantum correlations naturally emerges in the consistent histories approach to quantum physics.
Beyond local causality: causation and correlation after Bell

Matthew Pusey University of York
April 08, 2014

PIRSA:14040130
Quantum Physics
There is now a remarkable mathematical theory of causation. But applying this theory to a Bell scenario implies the Bell inequalities, which are violated in experiment. We alleviate this tension by translating the basic definitions of the theory into the framework of generalised probabilistic theories. We find that a surprising number of results carry over: the d-separation criterion for conditional independence (the no-signalling principle on steroids), and even certain quantitative limits on correlations. Finally, we begin a classification of the causal structures, such as the Bell scenarios, that are "interesting" from this perspective. Joint work with Joe Henson and Raymond Lal.
Is the Milky Way Ringing?

Lawrence Widrow Queen's University
April 08, 2014

PIRSA:14040137
Astrophysics
Recent observations from three different astronomical surveys have revealed evidence for asymmetries about the Galactic midplane in the kinematics of solar neighborhood stars. These asymmetries appear, in part, as compression-rarefaction modes in the bulk motions of stars perpendicular to the midplane. I will discuss the hypothesis that these motions were caused by the recent passage of a satellite galaxy or dark matter subhalo through the Galactic disk. In short, we may be witnessing the early stages of a disk-heating event during which the Galaxy's disk relaxes to a new state after interacting with substructure from its own halo.
The unitary Fermi gas across the superfluid transition

Olga Goulko University of Massachusetts Amherst
April 04, 2014

PIRSA:14040119
Condensed Matter
Quantum Monte Carlo is a versatile tool for studying strongly interacting theories in condensed matter physics from first principles. A prominent example is the unitary Fermi gas: a two-component system of fermions interacting with divergent scattering length. I will present numerical results for different properties of the homogeneous, spin-balanced unitary Fermi gas across the superfluid transition, such as the critical temperature, the equation of state and the temperature dependence of the contact density. Our values show good agreement with recent experimental data.
Quantum Gravity, Trees, and Polynomials

Fabien Vignes-Tourneret Université Claude Bernard Lyon 1
April 03, 2014

PIRSA:14040134
General Relativity & Quantum Cosmology
The perturbative series of colored group field theory are governed by a combinatorial 1/N-expansion. Controlling its coefficients is essential in order to understand the continuum limit. I will show how such a program is naturally related to higher-dimensional generalizations of trees in a colored Boulatov-Ooguri model, and present some partial results on the enumeration of such strucures in melonic graphs. This talk is mainly based on recent results by Baratin, Carrozza, Oriti, Ryan, and Smerlak ("Melonic phase transition in group field theory". arXiv: 1307.5026).
Quantum Computing with Noninteracting Particles

Alex Arkhipov Massachusetts Institute of Technology (MIT)
April 02, 2014

PIRSA:14040077
Quantum Physics
We introduce an abstract model of computation corresponding to an experiment in which identical, non-interacting bosons are sent through a non-adaptive linear circuit before being measured. We show that despite the very limited nature of the model, an exact classical simulation would imply a collapse of the polynomial hierarchy. Moreover, under plausible conjectures, a "noisy" approximate simulation would do the same. This gives evidence that quantum computers can sample a distribution that classical computers cannot even approximate, even when restricted to use no entanglement except that arising from particles being identical. We briefly discuss experimental prospects for realizing this model. This talk is based on The Computational Complexity of Linear Optics [STOC '11], which is joint work with Scott Aaronson.
Scribble, Scribble

Isabella Stefanescu

April 02, 2014

PIRSA:14040117

The live performance of a drawing contains information, expression and meaning that a finished drawing does not. Part performance, part demonstration, Isabella Stefanescu’s talk will explore the artistic challenges in creating performance pieces with the Euphonopen, an assemblage of hardware and software designed to map the characteristic mark making gestures of an artist to sound. Various stages of the Euphonopen project have been presented at SIGGRAPH (2010), Concordia University Live Electroacoustic Colloquium (2011), McGill University Time Forms Conference (2013).
Fibonacci Anyons From Abelian Bilayer Quantum Hall States

Abolhassan Vaezi Cornell University
April 01, 2014

PIRSA:14040120
Condensed Matter
The possibility of realizing non-Abelian statistics and utilizing it for topological quantum computation (TQC) has generated widespread interest. However, the non-Abelian statistics that can be realized in most accessible proposals is not powerful enough for universal TQC. In this talk, I consider a simple bilayer fractional quantum Hall (FQH) system with the 1/3 Laughlin state in each layer, in the presence of interlayer tunneling. I show that interlayer tunneling can drive a continuous phase transition to an exotic non-Abelian state that contains the famous `Fibonacci' anyon, whose non-Abelian statistics is powerful enough for universal TQC. The analysis that I will present towards this result rests on startling agreements from a variety of distinct methods, including thin torus limits, effective field theories, and coupled wire constructions.

Next, I discuss the experimental aspects of our proposal and potential probes for the Fibonacci phase. I show that the charge gap remains open at the phase transition while the neutral gap closes. This raises the question of whether these exotic phases may have already been realized at nu=2/3 in bilayers, as past experiments may not have definitively ruled them out. Finally, I will discuss about the generalizations to multi-layer states as well as the duality between the interlayer pairing and interlayer tunneling problems.

Reference: arXiv:1403.3383
Incompatibility of observables in quantum theory and other probabilistic theories
April 01, 2014

PIRSA:14040075
Quantum Physics
We introduce a new way of quantifying the degrees of incompatibility of two observables in a probabilistic physical theory and, based on this, a global measure of the degree of incompatibility inherent in such theories. This opens up a flexible way of comparing probabilistic theories with respect to the nonclassical feature of incompatibility. We show that quantum theory contains observables that are as incompatible as any probabilistic physical theory can have. In particular, we prove that two of the most common pairs of complementary observables (position and momentum; number and phase) are maximally incompatible. However, if one adopts a more refined measure of the degree of incompatibility, for instance, by restricting the comparison to binary observables, it turns out that there are probabilistic theories whose inherent degree of incompatibility is greater than that of quantum theory. Finally, we analyze the noise tolerance of the incompatibility of a pair of observables in a CHSH-Bell experiment.

Title	Speaker(s)	Date	Talk Type	Info link
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
Almost quantum correlations	Antonio Acin	2014-04-09	Scientific Series	View details
Beyond local causality: causation and correlation after Bell	Matthew Pusey	2014-04-08	Scientific Series	View details
Is the Milky Way Ringing?	Lawrence Widrow	2014-04-08	Scientific Series	View details
The unitary Fermi gas across the superfluid transition	Olga Goulko	2014-04-04	Scientific Series	View details
Quantum Gravity, Trees, and Polynomials	Fabien Vignes-Tourneret	2014-04-03	Scientific Series	View details
Quantum Computing with Noninteracting Particles	Alex Arkhipov	2014-04-02	Scientific Series	View details
Scribble, Scribble	Isabella Stefanescu	2014-04-02	Scientific Series	View details
Fibonacci Anyons From Abelian Bilayer Quantum Hall States	Abolhassan Vaezi	2014-04-01	Scientific Series	View details
Incompatibility of observables in quantum theory and other probabilistic theories		2014-04-01	Scientific Series	View details

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Format results

The computational power of quantum walk

Quantum cosmology in Lorentz-violating gravity

A non-commuting stabilizer formalism

Almost quantum correlations

Beyond local causality: causation and correlation after Bell

Is the Milky Way Ringing?

The unitary Fermi gas across the superfluid transition

Quantum Gravity, Trees, and Polynomials

Quantum Computing with Noninteracting Particles

Scribble, Scribble

Fibonacci Anyons From Abelian Bilayer Quantum Hall States

Incompatibility of observables in quantum theory and other probabilistic theories

The computational power of quantum walk

Quantum cosmology in Lorentz-violating gravity

A non-commuting stabilizer formalism

Almost quantum correlations

Beyond local causality: causation and correlation after Bell

Is the Milky Way Ringing?

The unitary Fermi gas across the superfluid transition

Quantum Gravity, Trees, and Polynomials

Quantum Computing with Noninteracting Particles

Scribble, Scribble

Fibonacci Anyons From Abelian Bilayer Quantum Hall States

Incompatibility of observables in quantum theory and other probabilistic theories