337 research outputs found
Multiparticle entanglement purification for two-colorable graph states
We investigate multiparticle entanglement purification schemes which allow
one to purify all two colorable graph states, a class of states which includes
e.g. cluster states, GHZ states and codewords of various error correction
codes. The schemes include both recurrence protocols and hashing protocols. We
analyze these schemes under realistic conditions and observe for a generic
error model that the threshold value for imperfect local operations depends on
the structure of the corresponding interaction graph, but is otherwise
independent of the number of parties. The qualitative behavior can be
understood from an analytically solvable model which deals only with a
restricted class of errors. We compare direct multiparticle entanglement
purification protocols with schemes based on bipartite entanglement
purification and show that the direct multiparticle entanglement purification
is more efficient and the achievable fidelity of the purified states is larger.
We also show that the purification protocol allows one to produce private
entanglement, an important aspect when using the produced entangled states for
secure applications. Finally we discuss an experimental realization of a
multiparty purification protocol in optical lattices which is issued to improve
the fidelity of cluster states created in such systems.Comment: 22 pages, 8 figures; replaced with published versio
Entanglement purification of multi-mode quantum states
An iterative random procedure is considered allowing an entanglement
purification of a class of multi-mode quantum states. In certain cases, a
complete purification may be achieved using only a single signal state
preparation. A physical implementation based on beam splitter arrays and
non-linear elements is suggested. The influence of loss is analyzed in the
example of a purification of entangled N-mode coherent states.Comment: 6 pages, 3 eps-figures, using revtex
Quantum Communication and Decoherence
In this contribution we will give a brief overview on the methods used to
overcome decoherence in quantum communication protocols. We give an
introduction to quantum error correction, entanglement purification and quantum
cryptography. It is shown that entanglement purification can be used to create
``private entanglement'', which makes it a useful tool for cryptographic
protocols.Comment: 31 pages, 10 figures, LaTeX, book chapter to appear in ``Coherent
Evolution in Noisy Environments'', Lecture Notes in Physics, (Springer
Verlag, Berlin-Heidelberg-New York). Minor typos correcte
A security proof of quantum cryptography based entirely on entanglement purification
We give a proof that entanglement purification, even with noisy apparatus, is
sufficient to disentangle an eavesdropper (Eve) from the communication channel.
In the security regime, the purification process factorises the overall initial
state into a tensor-product state of Alice and Bob, on one side, and Eve on the
other side, thus establishing a completely private, albeit noisy, quantum
communication channel between Alice and Bob. The security regime is found to
coincide for all practical purposes with the purification regime of a two-way
recurrence protocol. This makes two-way entanglement purification protocols,
which constitute an important element in the quantum repeater, an efficient
tool for secure long-distance quantum cryptography.Comment: Follow-up paper to quant-ph/0108060, submitted to PRA; 24 pages,
revex
Absence of string order in the anisotropic S=2 Heisenberg antiferromagnet
We study an AFM Heisenberg S=2 quantum spin chain at T=0 with both
interaction and on-site anisotropy, H = \sum_{i}
{1/2}(S^{+}_{i}S^{-}_{i+1}+S^{-}_{i}S^{+}_{i+1})
+J^{z}S^{z}_{i}S^{z}_{i+1}+D(S^{z}_{i})^{2}. Contradictory scenarios exist for
the S=2 anisotropic phase diagram, implying different mechanisms of the
emergence of the classical limit. One main AKLT-based scenario predicts the
emergence of a cascade of phase transitions not seen in the S=1 case. Another
scenario is in favor of an almost classical phase diagram for S=2; the S=1 case
then is very special with its dominant quantum effects. Numerical studies have
not been conclusive. Using the DMRG, the existence of hidden topological order
in the anisotropic S=2 chain is examined, as it distinguishes between the
proposed scenarios. We show that the topological order is zero in the
thermodynamical limit in all disordered phases, in particular in the new phase
interposed between the Haldane and large- phases. This excludes the
AKLT-model based scenario in favor of an almost classical phase diagram for the
spin chains.Comment: 9 pages, 9 eps figures, uses RevTeX, submitted to PR
Improved high-temperature expansion and critical equation of state of three-dimensional Ising-like systems
High-temperature series are computed for a generalized Ising model with
arbitrary potential. Two specific ``improved'' potentials (suppressing leading
scaling corrections) are selected by Monte Carlo computation. Critical
exponents are extracted from high-temperature series specialized to improved
potentials, achieving high accuracy; our best estimates are:
, , , ,
. By the same technique, the coefficients of the small-field
expansion for the effective potential (Helmholtz free energy) are computed.
These results are applied to the construction of parametric representations of
the critical equation of state. A systematic approximation scheme, based on a
global stationarity condition, is introduced (the lowest-order approximation
reproduces the linear parametric model). This scheme is used for an accurate
determination of universal ratios of amplitudes. A comparison with other
theoretical and experimental determinations of universal quantities is
presented.Comment: 65 pages, 1 figure, revtex. New Monte Carlo data by Hasenbusch
enabled us to improve the determination of the critical exponents and of the
equation of state. The discussion of several topics was improved and the
bibliography was update
Combined analysis of 635 patients confirms an age-related association of the serotonin 2A receptor gene with tardive dyskinesia and specificity for the non-orofacial subtype
Tardive dyskinesia (TD) is an important limiting factor in the use of typical antipsychotic drugs. Genetic variability in the serotonin 2A (5-HT2A) receptor may influence risk for TD but the results of prior studies are not confirmatory. The objective of this study was to determine association of T102C and His452Tyr polymorphisms in the 5-HT2A receptor gene (HTR2A) with TD in a large, multicentre patient sample. The design employed case-control analysis controlling for possible confounders using pooled, original data from published and available unpublished samples and employing logistic regression, analysis of variance and meta-analysis. The study sample consisted of 635 patients with schizophrenia or schizoaffective disorder (256 with TD and 379 without TD) drawn from five research centres, divided into six groups based on population origin. The main outcome measure was association of a categorical diagnosis of TD based on the Research Diagnostic Criteria for TD with HTR2A T102C and His452Tyr genotypes and haplotypes. The findings indicate significant association of TD with HTR2A T102C genotype (p = 0.002) over and above the effect of population group, also when controlling for age and gender (p = 0.0008), but not with His452Tyr genotype. The T102C genotype was significantly associated with TD in older (> median age 47 yr, p = 0.002) but not younger patients and in patients with non-orofacial (limb-truncal) (p=0.001) but not orofacial TD. By meta-analysis the Mantel-Haenszel (M-H) pooled odds ratio (OR) across all the available data was 1.64. A T102C-His452Tyr haplotype was significantly associated with TD (p = 0.0008). These findings confirm that genetic variability in HTR2A contributes a small but significant degree of risk for the expression of TD, particularly in older patients and specifically for the non-orofacial (limb-truncal) type. Together with other genetic variants associated with TD the findings could be used to assess risk in patients who are candidates for treatment with typical antipsychotic medications
Unit Root and Cointegration Tests for Cross-sectionally Correlated Panels. Estimating Regional Production Functions
Trust, control and knowledge transfer in small business networks
The ability to transfer knowledge effectively in the networks of small and medium-sized firms (SMEs) is paramount for supporting firm competitiveness. Our research is the first one that explores the joint effect of trust and control mechanisms on knowledge transfer in the case of networks of SMEs. We use a multiple case study approach based on six Italian networks of SMEs. We analyse the joint impact of different ethical based trustworthiness factors—namely benevolence and integrity—and the levers of control (LOCs)—namely, belief, boundary, diagnostic and interactive LOCs—on knowledge transfer between SMEs in networks. We find that trust substitutes for the implementation of boundary, diagnostic, and belief tools, while it works jointly with interactive tools in order to support knowledge transfer. These insights not only provide a rich foundation for follow-up research, but also inform SME managers about how to increase the effectiveness and efficiency of knowledge transfer with their network partners
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