Quantum Information

Quantum mechanics redefines information and its fundamental properties. Researchers at Perimeter Institute work to understand the properties of quantum information and study which information processing tasks are feasible, and which are infeasible or impossible. This includes research in quantum cryptography, which studies the trade-off between information extraction and disturbance, and its applications. It also includes research in quantum error correction, which involves the study of methods for protecting information against decoherence. Another important side of the field is studying the application of quantum information techniques and insights to other areas of physics, including quantum foundations and condensed matter.

Displaying 877 - 888 of 1359

Quantum technologies as tools to deepen our understanding of quantum theory and relativity

Ivette Fuentes University of Southampton
June 05, 2014

PIRSA:14060006
Quantum Information
The quest for self-correcting quantum memory

Olivier Landon-Cardinal California Institute of Technology
May 12, 2014

PIRSA:14050028
Quantum Information
Exact Classical Simulation of the Quantum-Mechanical GHZ Distribution

Gilles Brassard Université de Montréal
April 16, 2014

PIRSA:14040060
Quantum Information
Quantum Computing with Noninteracting Particles

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

PIRSA:14040077
Quantum Information
Universal topological quantum computation from a superconductor/Abelian quantum Hall heterostructure

Roger Mong University of Pittsburgh
March 19, 2014

PIRSA:14030118
Quantum Information
Maximal Privacy Without Coherence

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

PIRSA:14030105
Quantum Information
Probabilistic protocols in quantum information

Joshua Combes University of Colorado Boulder
February 13, 2014

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

PIRSA:14020142
Quantum Information
Bosonic particle-correlated states

Zhang Jiang University of New Mexico
February 06, 2014

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

Pavithran Iyer
January 22, 2014

PIRSA:14010099
Quantum Information
Entanglement farming: Harnessing the properties of fixed points in quantum evolution

Eduardo Martin-Martinez University of Waterloo
January 15, 2014

PIRSA:14010104
Quantum Information
The computational power of matchgates and the XY interaction on arbitrary graphs

Daniel Brod Universidade Federal Fluminense
January 08, 2014

PIRSA:14010085
Quantum Information

Quantum technologies as tools to deepen our understanding of quantum theory and relativity

Ivette Fuentes University of Southampton
June 05, 2014

PIRSA:14060006
Quantum Information
Quantum information and quantum metrology can be used to study gravitational effects such as gravitational waves and the universality of the equivalence principle. On one hand, the possibility of carrying out experiments to probe gravity using quantum systems opens an avenue to deepen our understanding of the overlap of these theories. On the other hand, incorporating relativity in quantum technologies promises the development of a new generation of relativistic quantum applications of relevance in Earth-based and space-based setups. In this talk, I will introduce a framework for the application of quantum information and quantum metrology techniques to relativistic quantum fields. I will show how, using this framework, we have been able to develop an accelerometer and a gravitational wave detector which exploit both quantum and relativistic effects. Moreover, our framework can be used to estimate with high precision spacetime parameters such as the Earth's Schwarzchild radius and the gravitational constant.
The quest for self-correcting quantum memory

Olivier Landon-Cardinal California Institute of Technology
May 12, 2014

PIRSA:14050028
Quantum Information
A self-correcting quantum memory is a physical system whose quantum state can be preserved over a long period of time without the need for any external intervention. The most promising candidates are topological quantum systems which would protect information encoded in their degenerate groundspace while interacting with a thermal environment. Many models have been suggested but several approaches have been shown to fail due to no-go results of increasingly general scope. In a nutshell, 2D topological models and many 3D topological models have point-like excitations which propagate freely and change the groundstate at any non-zero temperature. A recent suggestion is to introduce effective long-range interactions between those point-like excitations. In this presentation, I will first explain the desiderata for self-correction, review the recent advances and no-go results, and describe the current endeavours to define a self-correcting system in 2D and 3D. Time permitting, I will briefly present our recent work on the thermal instability of models which aim to introduce effective long-range interactions between point-like excitations (joint work with Beni Yoshida, John Preskill and David Poulin).
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.
Quantum Computing with Noninteracting Particles

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

PIRSA:14040077
Quantum Information
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.
Universal topological quantum computation from a superconductor/Abelian quantum Hall heterostructure

Roger Mong University of Pittsburgh
March 19, 2014

PIRSA:14030118
Quantum Information
Non-Abelian anyons promise to reveal spectacular features of quantum mechanics that could ultimately provide the foundation for a decoherence-free quantum computer. The Moore-Read quantum Hall state and a (relatively simple) two-dimensional p+ip superconductor both support Ising non-Abelian anyons, also referred to as Majorana zero modes. Here we construct a novel two-dimensional superconductor in which charge-2e Cooper pairs are built from fractionalized quasiparticles, and like the Z3 Read-Rezayi state, harbors Fibonacci anyons that--unlike Ising anyons--allow for universal topological quantum computation solely through braiding.
Maximal Privacy Without Coherence

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

PIRSA:14030105
Quantum Information
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.
Probabilistic protocols in quantum information

Joshua Combes University of Colorado Boulder
February 13, 2014

PIRSA:14020138
Quantum Information
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 Information
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 Information
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 Information
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.
Entanglement farming: Harnessing the properties of fixed points in quantum evolution

Eduardo Martin-Martinez University of Waterloo
January 15, 2014

PIRSA:14010104
Quantum Information
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.
The computational power of matchgates and the XY interaction on arbitrary graphs

Daniel Brod Universidade Federal Fluminense
January 08, 2014

PIRSA:14010085
Quantum Information
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
Quantum technologies as tools to deepen our understanding of quantum theory and relativity	Ivette Fuentes	2014-06-05	Scientific Series	View details
The quest for self-correcting quantum memory	Olivier Landon-Cardinal	2014-05-12	Scientific Series	View details
Exact Classical Simulation of the Quantum-Mechanical GHZ Distribution	Gilles Brassard	2014-04-16	Scientific Series	View details
Quantum Computing with Noninteracting Particles	Alex Arkhipov	2014-04-02	Scientific Series	View details
Universal topological quantum computation from a superconductor/Abelian quantum Hall heterostructure	Roger Mong	2014-03-19	Scientific Series	View details
Maximal Privacy Without Coherence	Debbie Leung	2014-03-12	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
Entanglement farming: Harnessing the properties of fixed points in quantum evolution	Eduardo Martin-Martinez	2014-01-15	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

Quantum Information

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