3,557 research outputs found
Quantum non-malleability and authentication
In encryption, non-malleability is a highly desirable property: it ensures
that adversaries cannot manipulate the plaintext by acting on the ciphertext.
Ambainis, Bouda and Winter gave a definition of non-malleability for the
encryption of quantum data. In this work, we show that this definition is too
weak, as it allows adversaries to "inject" plaintexts of their choice into the
ciphertext. We give a new definition of quantum non-malleability which resolves
this problem. Our definition is expressed in terms of entropic quantities,
considers stronger adversaries, and does not assume secrecy. Rather, we prove
that quantum non-malleability implies secrecy; this is in stark contrast to the
classical setting, where the two properties are completely independent. For
unitary schemes, our notion of non-malleability is equivalent to encryption
with a two-design (and hence also to the definition of Ambainis et al.). Our
techniques also yield new results regarding the closely-related task of quantum
authentication. We show that "total authentication" (a notion recently proposed
by Garg, Yuen and Zhandry) can be satisfied with two-designs, a significant
improvement over the eight-design construction of Garg et al. We also show
that, under a mild adaptation of the rejection procedure, both total
authentication and our notion of non-malleability yield quantum authentication
as defined by Dupuis, Nielsen and Salvail.Comment: 20+13 pages, one figure. v2: published version plus extra material.
v3: references added and update
Generalized Entropies
We study an entropy measure for quantum systems that generalizes the von
Neumann entropy as well as its classical counterpart, the Gibbs or Shannon
entropy. The entropy measure is based on hypothesis testing and has an elegant
formulation as a semidefinite program, a type of convex optimization. After
establishing a few basic properties, we prove upper and lower bounds in terms
of the smooth entropies, a family of entropy measures that is used to
characterize a wide range of operational quantities. From the formulation as a
semidefinite program, we also prove a result on decomposition of hypothesis
tests, which leads to a chain rule for the entropy.Comment: 21 page
Spin Anisotropy and Slow Dynamics in Spin Glasses
We report on an extensive study of the influence of spin anisotropy on spin
glass aging dynamics. New temperature cycle experiments allow us to compare
quantitatively the memory effect in four Heisenberg spin glasses with various
degrees of random anisotropy and one Ising spin glass. The sharpness of the
memory effect appears to decrease continuously with the spin anisotropy.
Besides, the spin glass coherence length is determined by magnetic field change
experiments for the first time in the Ising sample. For three representative
samples, from Heisenberg to Ising spin glasses, we can consistently account for
both sets of experiments (temperature cycle and magnetic field change) using a
single expression for the growth of the coherence length with time.Comment: 4 pages and 4 figures - Service de Physique de l'Etat Condense CNRS
URA 2464), DSM/DRECAM, CEA Saclay, Franc
Implementation of barycentric resampling for continuous wave searches in gravitational wave data
We describe an efficient implementation of a coherent statistic for
continuous gravitational wave searches from neutron stars. The algorithm works
by transforming the data taken by a gravitational wave detector from a moving
Earth bound frame to one that sits at the Solar System barycenter. Many
practical difficulties arise in the implementation of this algorithm, some of
which have not been discussed previously. These difficulties include
constraints of small computer memory, discreteness of the data, losses due to
interpolation and gaps in real data. This implementation is considerably more
efficient than previous implementations of these kinds of searches on Laser
Interferometer Gravitational Wave (LIGO) detector data.Comment: 10 pages, 3 figure
Coupling Lattice Boltzmann and Molecular Dynamics models for dense fluids
We propose a hybrid model, coupling Lattice Boltzmann and Molecular Dynamics
models, for the simulation of dense fluids. Time and length scales are
decoupled by using an iterative Schwarz domain decomposition algorithm. The MD
and LB formulations communicate via the exchange of velocities and velocity
gradients at the interface. We validate the present LB-MD model in simulations
of flows of liquid argon past and through a carbon nanotube. Comparisons with
existing hybrid algorithms and with reference MD solutions demonstrate the
validity of the present approach.Comment: 14 pages, 5 figure
Holomorphic Simplicity Constraints for 4d Riemannian Spinfoam Models
Starting from the reformulation of the classical phase space of Loop Quantum
Gravity in terms of spinor variables and spinor networks, we build coherent
spin network states and show how to use them to write the spinfoam path
integral for topological BF theory in terms of Gaussian integrals in the
spinors. Finally, we use this framework to revisit the simplicity constraints
reducing topological BF theory to 4d Riemannian gravity. These holomorphic
simplicity constraints lead us to a new spinfoam model for quantum gravity
whose amplitudes are defined as the evaluation of the coherent spin networks.Comment: 4 pages. Proceedings of Loops'11, Madrid. To appear in Journal of
Physics: Conference Series (JPCS
Operator product expansion coefficients from the nonperturbative functional renormalization group
Using the nonperturbative functional renormalization group (FRG) within the Blaizot-M\'endez-Galain-Wschebor approximation, we compute the operator product expansion (OPE) coefficient associated with the operators and in the three-dimensional universality class and in the Ising universality class () in dimensions . When available, exact results and estimates from the conformal bootstrap and Monte-Carlo simulations compare extremely well to our results, while FRG is able to provide values across the whole range of and considered
Decoupling with unitary approximate two-designs
Consider a bipartite system, of which one subsystem, A, undergoes a physical
evolution separated from the other subsystem, R. One may ask under which
conditions this evolution destroys all initial correlations between the
subsystems A and R, i.e. decouples the subsystems. A quantitative answer to
this question is provided by decoupling theorems, which have been developed
recently in the area of quantum information theory. This paper builds on
preceding work, which shows that decoupling is achieved if the evolution on A
consists of a typical unitary, chosen with respect to the Haar measure,
followed by a process that adds sufficient decoherence. Here, we prove a
generalized decoupling theorem for the case where the unitary is chosen from an
approximate two-design. A main implication of this result is that decoupling is
physical, in the sense that it occurs already for short sequences of random
two-body interactions, which can be modeled as efficient circuits. Our
decoupling result is independent of the dimension of the R system, which shows
that approximate 2-designs are appropriate for decoupling even if the dimension
of this system is large.Comment: Published versio
Renyi generalizations of the conditional quantum mutual information
The conditional quantum mutual information of a tripartite state
is an information quantity which lies at the center of many
problems in quantum information theory. Three of its main properties are that
it is non-negative for any tripartite state, that it decreases under local
operations applied to systems and , and that it obeys the duality
relation for a four-party pure state on systems . The
conditional mutual information also underlies the squashed entanglement, an
entanglement measure that satisfies all of the axioms desired for an
entanglement measure. As such, it has been an open question to find R\'enyi
generalizations of the conditional mutual information, that would allow for a
deeper understanding of the original quantity and find applications beyond the
traditional memoryless setting of quantum information theory. The present paper
addresses this question, by defining different -R\'enyi generalizations
of the conditional mutual information, some of which we can
prove converge to the conditional mutual information in the limit
. Furthermore, we prove that many of these generalizations
satisfy non-negativity, duality, and monotonicity with respect to local
operations on one of the systems or (with it being left as an open
question to prove that monotoniticity holds with respect to local operations on
both systems). The quantities defined here should find applications in quantum
information theory and perhaps even in other areas of physics, but we leave
this for future work. We also state a conjecture regarding the monotonicity of
the R\'enyi conditional mutual informations defined here with respect to the
R\'enyi parameter . We prove that this conjecture is true in some
special cases and when is in a neighborhood of one.Comment: v6: 53 pages, final published versio
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