904 research outputs found
A quantum de Finetti theorem in phase space representation
The quantum versions of de Finetti's theorem derived so far express the
convergence of n-partite symmetric states, i.e., states that are invariant
under permutations of their n parties, towards probabilistic mixtures of
independent and identically distributed (i.i.d.) states. Unfortunately, these
theorems only hold in finite-dimensional Hilbert spaces, and their direct
generalization to infinite-dimensional Hilbert spaces is known to fail. Here,
we address this problem by considering invariance under orthogonal
transformations in phase space instead of permutations in state space, which
leads to a new type of quantum de Finetti's theorem that is particularly
relevant to continuous-variable systems. Specifically, an n-mode bosonic state
that is invariant with respect to this continuous symmetry in phase space is
proven to converge towards a probabilistic mixture of i.i.d. Gaussian states
(actually, n identical thermal states).Comment: 5 page
Multipartite Classical and Quantum Secrecy Monotones
In order to study multipartite quantum cryptography, we introduce quantities
which vanish on product probability distributions, and which can only decrease
if the parties carry out local operations or carry out public classical
communication. These ``secrecy monotones'' therefore measure how much secret
correlations are shared by the parties. In the bipartite case we show that the
mutual information is a secrecy monotone. In the multipartite case we describe
two different generalisations of the mutual information, both of which are
secrecy monotones. The existence of two distinct secrecy monotones allows us to
show that in multipartite quantum cryptography the parties must make
irreversible choices about which multipartite correlations they want to obtain.
Secrecy monotones can be extended to the quantum domain and are then defined on
density matrices. We illustrate this generalisation by considering tri-partite
quantum cryptography based on the Greenberger-Horne-Zeilinger (GHZ) state. We
show that before carrying out measurements on the state, the parties must make
an irreversible decision about what probability distribution they want to
obtain
Reversibility of continuous-variable quantum cloning
We analyze a reversibility of optimal Gaussian quantum cloning of a
coherent state using only local operations on the clones and classical
communication between them and propose a feasible experimental test of this
feature. Performing Bell-type homodyne measurement on one clone and anti-clone,
an arbitrary unknown input state (not only a coherent state) can be restored in
the other clone by applying appropriate local unitary displacement operation.
We generalize this concept to a partial LOCC reversal of the cloning and we
show that this procedure converts the symmetric cloner to an asymmetric cloner.
Further, we discuss a distributed LOCC reversal in optimal Gaussian
cloning of coherent states which transforms it to optimal cloning for
. Assuming the quantum cloning as a possible eavesdropping attack on
quantum communication link, the reversibility can be utilized to improve the
security of the link even after the attack.Comment: 7 pages, 5 figure
Information-theoretic interpretation of quantum error-correcting codes
Quantum error-correcting codes are analyzed from an information-theoretic
perspective centered on quantum conditional and mutual entropies. This approach
parallels the description of classical error correction in Shannon theory,
while clarifying the differences between classical and quantum codes. More
specifically, it is shown how quantum information theory accounts for the fact
that "redundant" information can be distributed over quantum bits even though
this does not violate the quantum "no-cloning" theorem. Such a remarkable
feature, which has no counterpart for classical codes, is related to the
property that the ternary mutual entropy vanishes for a tripartite system in a
pure state. This information-theoretic description of quantum coding is used to
derive the quantum analogue of the Singleton bound on the number of logical
bits that can be preserved by a code of fixed length which can recover a given
number of errors.Comment: 14 pages RevTeX, 8 Postscript figures. Added appendix. To appear in
Phys. Rev.
Entanglement enhanced classical capacity of quantum communication channels with correlated noise in arbitrary dimensions
We study the capacity of d-dimensional quantum channels with memory modeled
by correlated noise. We show that, in agreement with previous results on Pauli
qubit channels, there are situations where maximally entangled input states
achieve higher values of mutual information than product states. Moreover, a
strong dependence of this effect on the nature of the noise correlations as
well as on the parity of the space dimension is found. We conjecture that when
entanglement gives an advantage in terms of mutual information, maximally
entangled states saturate the channel capacity.Comment: 10 pages, 5 figure
Security of Quantum Key Distribution with entangled quNits
We consider a generalisation of Ekert's entanglement-based quantum
cryptographic protocol where qubits are replaced by quits (i.e.,
N-dimensional systems). In order to study its robustness against optimal
incoherent attacks, we derive the information gained by a potential
eavesdropper during a cloning-based individual attack. In doing so, we
generalize Cerf's formalism for cloning machines and establish the form of the
most general cloning machine that respects all the symmetries of the problem.
We obtain an upper bound on the error rate that guarantees the confidentiality
of quNit generalisations of the Ekert's protocol for qubits.Comment: 15 pages, equation 15 and conclusions corrected the 14th of April
2003, new results adde
The Use of Blended Data to Improve Public Assistance Programs: Results from a Partnership between the U.S. Census Bureau, USDA, and State Program Agencies
The Census Bureau is partnering with state public assistance agencies to acquire program participation data and estimate new statistics that deepen a state’s understanding of program participants and improve outreach efforts to those who are eligible but do not participate. In collaboration with the Economic Research Service and the Food and Nutrition Service within the United States Department of Agriculture, the Census Bureau obtains individual-level program participation administrative records (AR) data for three state programs, the Supplemental Nutrition Assistance Program (SNAP), Temporary Aid for Needy Families (TANF), and the Special Supplemental Nutrition Program for Women, Infants and Children (WIC). The Census Bureau constructs a unique data set for each state program by linking the AR data to survey response data for the same individuals. These linked data enable the Census Bureau to model which survey respondents are eligible for program participation and also to observe which eligible individuals participate in the program. The Census Bureau then estimates eligibility and participation rates by a variety of demographic and economic characteristics and by county. The individual-level data also enable the Census Bureau to construct a statistical profile of eligible individuals and families that do not participate to assist state agencies with their outreach programs. All statistical results provided back to state agencies in table reports and data visualizations are reviewed to insure that individual identities are protected and not disclosed. This paper will present results for several state programs that have partnered the Census Bureau
Coherent pulse implementations of quantum cryptography protocols resistant to photon number splitting attacks
A new class of quantum cryptography (QC) protocols that are robust against
the most general photon number splitting attacks in a weak coherent pulse
implementation has been recently proposed. In this article we give a quite
exhaustive analysis of several eavesdropping attacks on these schemes. The
eavesdropper (Eve) is supposed to have unlimited technological power while the
honest parties (Alice and Bob) use present day technology, in particular an
attenuated laser as an approximation of a single-photon source. They exploit
the nonorthogonality of quantum states for decreasing the information
accessible to Eve in the multi-photon pulses accidentally produced by the
imperfect source. An implementation of some of these protocols using present
day technology allow for a secure key distribution up to distances of
150 km. We also show that strong-pulse implementations, where a strong pulse is
included as a reference, allow for key distribution robust against photon
number splitting attacks.Comment: 16 pages, 11 figure
An Estimate of the Vibrational Frequencies of Spherical Virus Particles
The possible normal modes of vibration of a nearly spherical virus particle
are discussed. Two simple models for the particle are treated, a liquid drop
model and an elastic sphere model. Some estimates for the lowest vibrational
frequency are given for each model. It is concluded that this frequency is
likely to be of the order of a few GHz for particles with a radius of the order
of 50 nm.Comment: 6 pages, 1 figur
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