1,392 research outputs found
Quantum Bit String Commitment
A bit string commitment protocol securely commits classical bits in such
a way that the recipient can extract only bits of information about the
string. Classical reasoning might suggest that bit string commitment implies
bit commitment and hence, given the Mayers-Lo-Chau theorem, that
non-relativistic quantum bit string commitment is impossible. Not so: there
exist non-relativistic quantum bit string commitment protocols, with security
parameters and , that allow to commit
bits to so that 's probability of successfully cheating when revealing
any bit and 's probability of extracting more than bits of
information about the bit string before revelation are both less than
. With a slightly weakened but still restrictive definition of
security against , can be taken to be for a positive
constant . I briefly discuss possible applications.Comment: Published version. (Refs updated.
Exponential quantum enhancement for distributed addition with local nonlinearity
We consider classical and entanglement-assisted versions of a distributed
computation scheme that computes nonlinear Boolean functions of a set of input
bits supplied by separated parties. Communication between the parties is
restricted to take place through a specific apparatus which enforces the
constraints that all nonlinear, nonlocal classical logic is performed by a
single receiver, and that all communication occurs through a limited number of
one-bit channels. In the entanglement-assisted version, the number of channels
required to compute a Boolean function of fixed nonlinearity can become
exponentially smaller than in the classical version. We demonstrate this
exponential enhancement for the problem of distributed integer addition.Comment: To appear in Quantum Information Processin
Quantum Computation of a Complex System : the Kicked Harper Model
The simulation of complex quantum systems on a quantum computer is studied,
taking the kicked Harper model as an example. This well-studied system has a
rich variety of dynamical behavior depending on parameters, displays
interesting phenomena such as fractal spectra, mixed phase space, dynamical
localization, anomalous diffusion, or partial delocalization, and can describe
electrons in a magnetic field. Three different quantum algorithms are presented
and analyzed, enabling to simulate efficiently the evolution operator of this
system with different precision using different resources. Depending on the
parameters chosen, the system is near-integrable, localized, or partially
delocalized. In each case we identify transport or spectral quantities which
can be obtained more efficiently on a quantum computer than on a classical one.
In most cases, a polynomial gain compared to classical algorithms is obtained,
which can be quadratic or less depending on the parameter regime. We also
present the effects of static imperfections on the quantities selected, and
show that depending on the regime of parameters, very different behaviors are
observed. Some quantities can be obtained reliably with moderate levels of
imperfection, whereas others are exponentially sensitive to imperfection
strength. In particular, the imperfection threshold for delocalization becomes
exponentially small in the partially delocalized regime. Our results show that
interesting behavior can be observed with as little as 7-8 qubits, and can be
reliably measured in presence of moderate levels of internal imperfections
Unconditionally Secure Bit Commitment
We describe a new classical bit commitment protocol based on cryptographic
constraints imposed by special relativity. The protocol is unconditionally
secure against classical or quantum attacks. It evades the no-go results of
Mayers, Lo and Chau by requiring from Alice a sequence of communications,
including a post-revelation verification, each of which is guaranteed to be
independent of its predecessor.Comment: Typos corrected. Reference details added. To appear in Phys. Rev.
Let
Multipartite Nonlocal Quantum Correlations Resistant to Imperfections
We use techniques for lower bounds on communication to derive necessary
conditions in terms of detector efficiency or amount of super-luminal
communication for being able to reproduce with classical local hidden-variable
theories the quantum correlations occurring in EPR-type experiments in the
presence of noise. We apply our method to an example involving n parties
sharing a GHZ-type state on which they carry out measurements and show that for
local-hidden variable theories, the amount of super-luminal classical
communication c and the detector efficiency eta are constrained by eta 2^(-c/n)
= O(n^(-1/6)) even for constant general error probability epsilon = O(1)
Why the Tsirelson bound?
Wheeler's question 'why the quantum' has two aspects: why is the world
quantum and not classical, and why is it quantum rather than superquantum,
i.e., why the Tsirelson bound for quantum correlations? I discuss a remarkable
answer to this question proposed by Pawlowski et al (2009), who provide an
information-theoretic derivation of the Tsirelson bound from a principle they
call 'information causality.'Comment: 17 page
Quantum Algorithm for the Collision Problem
In this note, we give a quantum algorithm that finds collisions in arbitrary
r-to-one functions after only O((N/r)^(1/3)) expected evaluations of the
function. Assuming the function is given by a black box, this is more efficient
than the best possible classical algorithm, even allowing probabilism. We also
give a similar algorithm for finding claws in pairs of functions. Furthermore,
we exhibit a space-time tradeoff for our technique. Our approach uses Grover's
quantum searching algorithm in a novel way.Comment: 8 pages, LaTeX2
Low Cost and Compact Quantum Cryptography
We present the design of a novel free-space quantum cryptography system,
complete with purpose-built software, that can operate in daylight conditions.
The transmitter and receiver modules are built using inexpensive off-the-shelf
components. Both modules are compact allowing the generation of renewed shared
secrets on demand over a short range of a few metres. An analysis of the
software is shown as well as results of error rates and therefore shared secret
yields at varying background light levels. As the system is designed to
eventually work in short-range consumer applications, we also present a use
scenario where the consumer can regularly 'top up' a store of secrets for use
in a variety of one-time-pad and authentication protocols.Comment: 18 pages, 9 figures, to be published in New Journal of Physic
Radiative levitation: a likely explanation for pulsations in the unique hot O subdwarf star SDSS J160043.6+074802.9
Context. SDSS J160043.6+074802.9 (J1600+0748 for short) is the only hot sdO star for which unambiguous multiperiodic luminosity variations have been reported so far. These rapid variations, with periods in the range from ~60 s to ~120 s, are best qualitatively explained in terms of pulsational instabilities, but the exact nature of the driving mechanism has remained a puzzle.
Aims. Our primary goal is to examine quantitatively how pulsation modes can be excited in an object such as J1600+0748. Given the failure of uniform-metallicity models as well documented in the recent Ph.D. thesis of C. RodrĂguez-LĂłpez, we consider the effects of radiative levitation on iron as a means to boost the efficiency of the opacity-driving mechanism in models of J1600+0748.
Methods. We combine high sensitivity time-averaged optical spectroscopy and full nonadiabatic calculations to carry out our study. In the first instance, this is used to estimate the location of J1600+0748 in the log plane. Given this essential input, we pulsate stellar models consistent with these atmospheric parameters. We construct both uniform-metallicity models and structures in which the iron abundance is specified by the condition of diffusive equilibrium between gravitational settling and radiative levitation.
Results. On the basis of NTLE H/He synthetic spectra, we find that the target star has the following atmospheric parameters: log g = 5.93 0.11, = 71 070 2725 K, and log N(He)/N(H) = -0.85 0.08. This takes into account our deconvolution of the spectrum of J1600+0748 as it is polluted by the light of a main sequence companion. We confirm that uniform-metallicity stellar models with Z in the range from 0.02 to 0.10 cannot excite pulsation modes of the kind observed. On the other hand, we find that the inclusion of radiative levitation, as we implemented it, leads to pulsational instabilities in a period range that overlaps with, although it is narrower than, the observed range in J1600+0748. The excited modes correspond to low-order, low-degree p-modes.
Conclusions. We infer that radiative levitation is a likely essential ingredient in the excitation physics at work in J1600+0748
Necessary and sufficient detection efficiency for the Mermin inequalities
We prove that the threshold detection efficiency for a loophole-free Bell
experiment using an -qubit Greenberger-Horne-Zeilinger state and the
correlations appearing in the -partite Mermin inequality is . If
the detection efficiency is equal to or lower than this value, there are local
hidden variable models that can simulate all the quantum predictions. If the
detection efficiency is above this value, there is no local hidden variable
model that can simulate all the quantum predictions.Comment: REVTeX4, 5 pages, 1 figur
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