Quantum Physics

Displaying 1441 - 1452 of 2279

A histories perpective on bounding quantum correlations

Joe Henson BNP Paribas Asset Management London
March 18, 2014

PIRSA:14030111
Quantum Physics
Seeing is Believing: Direct Observation of a General Quantum State

Jeff Lundeen University of Ottawa
March 17, 2014

PIRSA:14030110
Quantum Physics
Maximal Privacy Without Coherence

Debbie Leung Institute for Quantum Computing (IQC)
March 12, 2014

PIRSA:14030105
Quantum Physics
Psi-epistemic models are exponentially bad at explaining the distinguishability of quantum states

Matthew Leifer Chapman University
February 18, 2014

PIRSA:14020145
Quantum Physics
Probabilistic protocols in quantum information

Joshua Combes University of Colorado Boulder
February 13, 2014

PIRSA:14020138
Quantum Physics
Applications of Information Theory in Direct Sum and Direct Product Problems
February 12, 2014

PIRSA:14020142
Quantum Physics
Bosonic particle-correlated states

Zhang Jiang University of New Mexico
February 06, 2014

PIRSA:14020135
Quantum Physics
Hardness of correcting errors on a stabilizer code

Pavithran Iyer
January 22, 2014

PIRSA:14010099
Quantum Physics
Does the Quantum Particle know its own Energy?

Rafael Sorkin Perimeter Institute for Theoretical Physics
January 21, 2014

PIRSA:14010106
Quantum Physics
Entanglement farming: Harnessing the properties of fixed points in quantum evolution

Eduardo Martin-Martinez University of Waterloo
January 15, 2014

PIRSA:14010104
Quantum Physics
Noncontextuality without determinism and admissible (in)compatibility relations: revisiting Specker's parable.

Ravi Kunjwal Funds for Scientific Research - FNRS
January 14, 2014

PIRSA:14010102
Quantum Physics
The computational power of matchgates and the XY interaction on arbitrary graphs

Daniel Brod Universidade Federal Fluminense
January 08, 2014

PIRSA:14010085
Quantum Physics

A histories perpective on bounding quantum correlations

Joe Henson BNP Paribas Asset Management London
March 18, 2014

PIRSA:14030111
Quantum Physics
There has recently been much interest in finding simple principles that explain the particular sets of experimental probabilities that are possible with quantum mechanics in Bell-type experiments. In the quantum gravity community, similar questions had been raised, about whether a certain generalisation of quantum mechanics allowed more than quantum mechanics in this regard. We now bring these two strands of work together to see what can be learned on both sides.
Seeing is Believing: Direct Observation of a General Quantum State

Jeff Lundeen University of Ottawa
March 17, 2014

PIRSA:14030110
Quantum Physics
Central to quantum theory, the wavefunction is a complex distribution associated with a quantum system. Despite its fundamental role, it is typically introduced as an abstract element of the theory with no explicit definition. Rather, physicists come to a working understanding of it through its use to calculate measurement outcome probabilities through the Born Rule. Tomographic methods can reconstruct the wavefunction from measured probabilities. In contrast, I present a method to directly measure the wavefunction so that its real and imaginary components appear straight on our measurement apparatus. I will also present new work extending this concept to mixed quantum states. This extension directly measures a little-known proposal by Dirac for a classical analog to a quantum operator. Furthermore, it reveals that our direct measurement is a rigorous example of a quasi-probability phase-space (i.e. x,p) distribution that is closely related to the Q, P, and Wigner functions. Our direct measurement method gives the quantum state a plain and general meaning in terms of a specific set of simple operations in the lab.
Maximal Privacy Without Coherence

Debbie Leung Institute for Quantum Computing (IQC)
March 12, 2014

PIRSA:14030105
Quantum Physics
Privacy and coherence have long been considered closely related properties of a quantum state. Indeed, a coherently transmitted quantum state is inherently private. Surprisingly, coherent quantum communication is not always required for privacy: there are quantum channels that are too noisy to transmit quantum information but it can send private classical information. Here, we ask how different the private classical and the quantum capacities can be. We present a class of channels N_d with input dimension d^2, quantum capacity Q(N_d) <= 1, and private classical capacity P(N_d) = log d. These channels asymptotically saturate an interesting inequality P(N) <= (log d_A + Q(N))/2 for any channel N with input dimension d_A, and capture the essence of privacy stripped of the confounding influence of coherence.
Psi-epistemic models are exponentially bad at explaining the distinguishability of quantum states

Matthew Leifer Chapman University
February 18, 2014

PIRSA:14020145
Quantum Physics
The status of the quantum state is perhaps the most controversial issue in the foundations of quantum theory. Is it an epistemic state (representing knowledge, information, or belief) or an ontic state (a direct reflection of reality)? In the ontological models framework, quantum states correspond to probability measures over more fundamental states of reality. The quantum state is then ontic if every pair of pure states corresponds to a pair of measures that do not overlap, and is otherwise epistemic. Recently, several authors have derived theorems that aim to show that the quantum state must be ontic in this framework. Each of these theorems involve auxiliary assumptions of varying degrees of plausibility. Without such assumptions, it has been shown that models exist in which the quantum state is epistemic. However, the definition of an epistemic quantum state used in these works is extremely permissive. Only two quantum states need correspond to overlapping measures and furthermore the amount of overlap may be arbitrarily small. In order to provide an explanation of quantum phenomena such as no-cloning and the indistinguishability of pure states, the amount of overlap should be comparable to the inner product of the quantum states. In this talk, I show, without making auxiliary assumptions, that the ratio of overlap to inner product must go to zero exponentially in Hilbert space dimension for some families of states. This is done by connecting the overlap to Kochen-Specker noncontextuality, from which we infer that any contextuality inequality gives a bound on the ratio of overlap to inner product.
Probabilistic protocols in quantum information

Joshua Combes University of Colorado Boulder
February 13, 2014

PIRSA:14020138
Quantum Physics
Probabilistic protocols in quantum information are an attempt to improve performance by occasionally reporting a better result than could be expected from a deterministic protocol. Here we show that probabilistic protocols can never improve performance beyond the quantum limits on the corresponding deterministic protocol. To illustrate this result we examine three common probabilistic protocols: probabilistic amplification, weak value amplification, and probabilistic metrology. In each of these protocols we show explicitly that the optimal deterministic protocol is better than the corresponding probabilistic protocol when the probabilistic nature of the protocol is correctly accounted for.
Applications of Information Theory in Direct Sum and Direct Product Problems
February 12, 2014

PIRSA:14020142
Quantum Physics
A fundamental question in complexity theory is how much resource is needed to solve k independent instances of a problem compared to the resource required to solve one instance. Suppose solving one instance of a problem with probability of correctness p, we require c units of some resource in a given model of computation. A direct sum theorem states that in order to compute k independent instances of a problem, it requires k times units of the resource needed to compute one instance. A strong direct product theorem states that, with o(k • c) units of the resource, one can only compute all the k instances correctly with probability exponentially small in k. In this talk, I am going to present some of recent progress on direct sum and direct product theorems in the model of communication complexity and two-prover one-round games with information-theoretic approach. The talk is based on parts of my doctoral work.
Bosonic particle-correlated states

Zhang Jiang University of New Mexico
February 06, 2014

PIRSA:14020135
Quantum Physics
Quantum many-body problems are notorious hard. This is partly because the Hilbert space becomes exponentially big with the particle number N. While exact solutions are often considered intractable, numerous approaches have been proposed using approximations. A common trait of these approaches is to use an ansatz such that the number of parameters either does not depend on N or is proportional to N, e.g., the matrix-product state for spin lattices, the BCS wave function for superconductivity, the Laughlin wave function for fractional quantum Hall effects, and the Gross-Pitaecskii theory for BECs. Among them the product ansatz for BECs has precisely predicted many useful properties of Bose gases at ultra-low temperature. As particle-particle correlation becomes important, however, it begins to fail. To capture the quantum correlations, we propose a new
set of states, which constitute a natural generalization of the product-state ansatz. Our state of N=d& times;n identical particles is derived by symmetrizing the n-fold product of a d-particle quantum state. For fixed d, the parameter space of our state does not grow with N. Numerically, we show that our ansatz gives the right description for the ground state and time evolution of the two-site Bose-Hubbard model.
Hardness of correcting errors on a stabilizer code

Pavithran Iyer
January 22, 2014

PIRSA:14010099
Quantum Physics
Problems in computer science are often classified based on the scaling of the runtimes for algorithms that can solve the problem. Easy problems are efficiently solvable but often in physics we encounter problems that take too long to be solved on a classical computer. Here we look at one such problem in the context of quantum error correction. We will further show that no efficient algorithm for this problem is likely to exist. We will address the computational hardness of a decoding problem, pertaining to quantum stabilizer codes considering independent X and Z errors on each qubit. Much like classical linear codes, errors are detected by measuring certain check operators which yield an error syndrome, and the decoding problem consists of determining the most likely recovery given the syndrome. The corresponding classical problem is known to be NP-Complete, and a similar decoding problem for quantum codes is known to be NP-Complete too. However, this decoding strategy is not optimal in the quantum setting as it does not take into account error degeneracy, which causes distinct errors to have the same effect on the code. Here, we show that optimal decoding of stabilizer codes is computationally much harder than optimal decoding of classical linear codes, it is #P-Complete.
Does the Quantum Particle know its own Energy?

Rafael Sorkin Perimeter Institute for Theoretical Physics
January 21, 2014

PIRSA:14010106
Quantum Physics
If a wave function does not describe microscopic reality then what does? Reformulating quantum mechanics in path-integral terms leads to a notion of ``precluded event" and thence to the proposal that quantal reality differs from classical reality in the same way as a set of worldlines differs from a single worldline. One can then ask, for example, which sets of electron trajectories correspond to a Hydrogen atom in its ground state and how they differ from those of an excited state. We address the analogous questions for simple model that replaces the electron by a particle hopping (in discrete time) on a circular lattice.
Entanglement farming: Harnessing the properties of fixed points in quantum evolution

Eduardo Martin-Martinez University of Waterloo
January 15, 2014

PIRSA:14010104
Quantum Physics
We show that in certain generic circumstances the state of light of an optical cavity traversed by beams of atoms is naturally driven towards a non-thermal metastable state. This state can be such that successive pairs of unentangled particles sent through the cavity will reliably emerge significantly entangled thus providing a renewable source of quantum entanglement. Significant for possible experimental realizations is the fact that this entangling fixed point state of the cavity can be reached largely independently of the initial state in which the cavity was prepared. Our results suggest that reliable entanglement farming on the basis of such a fixed point state should be possible also in various other experimental settings, namely with the to-be-entangled particles replaced by arbitrary qudits and with the cavity replaced by a suitable reservoir system.
Noncontextuality without determinism and admissible (in)compatibility relations: revisiting Specker's parable.

Ravi Kunjwal Funds for Scientific Research - FNRS
January 14, 2014

PIRSA:14010102
Quantum Physics
The purpose of this talk is twofold: First, following Spekkens, to motivate noncontextuality as a natural principle one might expect to hold in nature and introduce operational noncontextuality inequalities motivated by a contextuality scenario first considered by Ernst Specker. These inequalities do not rely on the assumption of outcome-determinism which is implicit in the usual Kochen-Specker (KS) inequalities. We argue that they are the appropriate generalization of KS inequalities, serving as a test for the possibility of noncontextual explanations of experimental data. This is very much in the spirit of Bell inequalities, which provide theory-independent tests for local hidden variable explanations of experimental data without relying on the assumption of outcome-determinism. The second purpose is to point out a curious feature of quantum theory, motivated by the connections between (in)compatibility and (non)contextuality: namely, that it admits all conceivable (in)compatibility relations between observables.
The computational power of matchgates and the XY interaction on arbitrary graphs

Daniel Brod Universidade Federal Fluminense
January 08, 2014

PIRSA:14010085
Quantum Physics
Matchgates are a restricted set of two-qubit gates known to be classically simulable when acting on nearest-neighbor qubits on a path, but universal for quantum computation when the gates can also act on more distant qubits. In this talk, I will address the power of matchgates when they can act on pairs of qubits according to the edges of arbitrary graphs. Specifically, we show that matchgates are universal on any connected graph other than a path or a cycle, and that they are classically simulable on a cycle. We also prove that the same dichotomy holds for the XY interaction, a proper subset of matchgates that arises naturally in some implementations of quantum computing. This is based on a joint work with Ernesto Galvão and another with Andrew Childs.

Title	Speaker(s)	Date	Talk Type	Info link
A histories perpective on bounding quantum correlations	Joe Henson	2014-03-18	Scientific Series	View details
Seeing is Believing: Direct Observation of a General Quantum State	Jeff Lundeen	2014-03-17	Scientific Series	View details
Maximal Privacy Without Coherence	Debbie Leung	2014-03-12	Scientific Series	View details
Psi-epistemic models are exponentially bad at explaining the distinguishability of quantum states	Matthew Leifer	2014-02-18	Scientific Series	View details
Probabilistic protocols in quantum information	Joshua Combes	2014-02-13	Scientific Series	View details
Applications of Information Theory in Direct Sum and Direct Product Problems		2014-02-12	Scientific Series	View details
Bosonic particle-correlated states	Zhang Jiang	2014-02-06	Scientific Series	View details
Hardness of correcting errors on a stabilizer code	Pavithran Iyer	2014-01-22	Scientific Series	View details
Does the Quantum Particle know its own Energy?	Rafael Sorkin	2014-01-21	Scientific Series	View details
Entanglement farming: Harnessing the properties of fixed points in quantum evolution	Eduardo Martin-Martinez	2014-01-15	Scientific Series	View details
Noncontextuality without determinism and admissible (in)compatibility relations: revisiting Specker's parable.	Ravi Kunjwal	2014-01-14	Scientific Series	View details
The computational power of matchgates and the XY interaction on arbitrary graphs	Daniel Brod	2014-01-08	Scientific Series	View details

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Format results

A histories perpective on bounding quantum correlations

Seeing is Believing: Direct Observation of a General Quantum State

Maximal Privacy Without Coherence

Psi-epistemic models are exponentially bad at explaining the distinguishability of quantum states

Probabilistic protocols in quantum information

Applications of Information Theory in Direct Sum and Direct Product Problems

Bosonic particle-correlated states

Hardness of correcting errors on a stabilizer code

Does the Quantum Particle know its own Energy?

Entanglement farming: Harnessing the properties of fixed points in quantum evolution

Noncontextuality without determinism and admissible (in)compatibility relations: revisiting Specker's parable.

The computational power of matchgates and the XY interaction on arbitrary graphs

A histories perpective on bounding quantum correlations

Seeing is Believing: Direct Observation of a General Quantum State

Maximal Privacy Without Coherence

Psi-epistemic models are exponentially bad at explaining the distinguishability of quantum states

Probabilistic protocols in quantum information

Applications of Information Theory in Direct Sum and Direct Product Problems

Bosonic particle-correlated states

Hardness of correcting errors on a stabilizer code

Does the Quantum Particle know its own Energy?

Entanglement farming: Harnessing the properties of fixed points in quantum evolution

Noncontextuality without determinism and admissible (in)compatibility relations: revisiting Specker's parable.

The computational power of matchgates and the XY interaction on arbitrary graphs