3,382 research outputs found
Analytic approximations to the phase diagram of the Jaynes-Cummings-Hubbard model with application to ion chains
We discuss analytic approximations to the ground state phase diagram of the
homogeneous Jaynes-Cummings-Hubbard (JCH) Hamiltonian with general short-range
hopping. The JCH model describes e.g. radial phonon excitations of a linear
chain of ions coupled to an external laser field tuned to the red motional
sideband with Coulomb mediated hopping or an array of high- coupled cavities
containing a two-level atom and photons. Specifically we consider the cases of
a linear array of coupled cavities and a linear ion chain. We derive
approximate analytic expressions for the boundaries between Mott-insulating and
superfluid phases and give explicit expressions for the critical value of the
hopping amplitude within the different approximation schemes. In the case of an
array of cavities, which is represented by the standard JCH model we compare
both approximations to numerical data from density-matrix renormalization group
(DMRG) calculations.Comment: 9 pages, 5 figures, extended and corrected second versio
Coin Tossing is Strictly Weaker Than Bit Commitment
We define cryptographic assumptions applicable to two mistrustful parties who
each control two or more separate secure sites between which special relativity
guarantees a time lapse in communication. We show that, under these
assumptions, unconditionally secure coin tossing can be carried out by
exchanges of classical information. We show also, following Mayers, Lo and
Chau, that unconditionally secure bit commitment cannot be carried out by
finitely many exchanges of classical or quantum information. Finally we show
that, under standard cryptographic assumptions, coin tossing is strictly weaker
than bit commitment. That is, no secure classical or quantum bit commitment
protocol can be built from a finite number of invocations of a secure coin
tossing black box together with finitely many additional information exchanges.Comment: Final version; to appear in Phys. Rev. Let
Correlation length in cuprates deduced from the impurity-induced magnetization
We report a new multi-nuclei based NMR method which allows us to image the
staggered polarization induced by nonmagnetic Li impurities in underdoped O6.6
and slightly overdoped O7 YBa2Cu3O6+y above T_C. The spatial extension of the
polarization xi_imp approximately follows a Curie law, increasing up to six
lattice constants at T=80K at O6.6 in the pseudogap regime. Near optimal
doping, the staggered magnetization has the same shape, with xi_imp reduced by
a factor 2. xi_imp is argued to reveal the intrinsic magnetic correlation
length of the pure system. It is found to display a smooth evolution through
the pseudogap regime.Comment: 8 latex pages + 8 figures, to appear in Physical Review B, this
resubmitted version is twice longer than the previous one : we detail here
our method to determine the impurity-induced magnetizatio
A quantum protocol for cheat-sensitive weak coin flipping
We present a quantum protocol for the task of weak coin flipping. We find
that, for one choice of parameters in the protocol, the maximum probability of
a dishonest party winning the coin flip if the other party is honest is
1/sqrt(2). We also show that if parties restrict themselves to strategies
wherein they cannot be caught cheating, their maximum probability of winning
can be even smaller. As such, the protocol offers additional security in the
form of cheat sensitivity.Comment: 4 pages RevTex. Differs from the journal version only in that the
sentences: "The ordering of the authors on this paper was chosen by a coin
flip implemented by a trusted third party. TR lost." have not been remove
Theory of anomalous magnon softening in ferromagnetic manganites
In metallic manganites with low Curie temperatures, a peculiar softening of
the magnon spectrum close to the magnetic zone boundary has experimentally been
observed. Here we present a theory of the renormalization of the magnetic
excitation spectrum in colossal magnetoresistance compounds. The theory is
based on the modulation of magnetic exchange bonds by the orbital degree of
freedom of double-degenerate e_g electrons. The model considered is an
orbitally degenerate double-exchange system coupled to Jahn-Teller active
phonons which we treat in the limit of strong onsite repulsions. Charge and
coupled orbital-lattice fluctuations are identified as the main origin of the
unusual softening of the magnetic spectrum
Unconditionally secure quantum bit commitment is impossible
The claim of quantum cryptography has always been that it can provide
protocols that are unconditionally secure, that is, for which the security does
not depend on any restriction on the time, space or technology available to the
cheaters. We show that this claim does not hold for any quantum bit commitment
protocol. Since many cryptographic tasks use bit commitment as a basic
primitive, this result implies a severe setback for quantum cryptography. The
model used encompasses all reasonable implementations of quantum bit commitment
protocols in which the participants have not met before, including those that
make use of the theory of special relativity.Comment: 4 pages, revtex. Journal version replacing the version published in
the proceedings of PhysComp96. This is a significantly improved version which
emphasis the generality of the resul
Single Nucleotide Polymorphisms (SNPs) reveal sibship among founders of a Bangladeshi rohu (Labeo rohita) breeding population
Rohu (Labeo rohita) is a significant freshwater aquaculture species with approximately 1.8 Mt produced annually. Fin clips obtained from the founders of a newly established Bangladesh-based breeding population (~140 fish from each of the Halda, Jamuna and Padma rivers) were used to identify 9157 SNPs and 14 411 silicoDArT markers using the Diversity Arrays Technology (DArT) genotyping-by-sequencing platform known as using the DArTseqTM platform. After quality control, 1985 SNPs were retained and used to examine population structure within and among river systems. Examination of genomic relationships revealed evidence of full- and half-sibling relationships among founders. Accordingly, sibship and dummy parents were assigned within each river population using a maximum likelihood approach with COLONY software. Founders that had no dummy parents in common were then identified for population genetic analyses. Only 40 unique dummy parents and 17 founders with no common dummy parents were identified from the Halda river, compared with 206 (96) from the Jamuna and 184 (83) from the Padma. Overall pairwise FST estimates among rivers were low (< 0.005) and the optimum number of clusters using unsupervised K-means clustering was one, indicating little genetic divergence among the river populations in our SNPs. These results suggest that observed sibship among founders should be accounted for in future pedigree-based analyses and it cannot be assumed that fertilised spawn collections are representative samples of river populations
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
1 The Prosodic Marking of Phrase Boundaries: Expectations and Results
ABSTRACT Using sentence templates and a stochastic context-free grammar a large corpus (10,000 sentences) has been created, where prosodic phrase boundaries are labeled in the sentences automatically during sentence generation. With perception experiments on a subset of 500 utterances we verified that 92 % of the automatically marked boundaries were perceived as prosodically marked. In initial automatic classification experiments for three levels of boundaries recognition rates up to 81 % could be achieved. 1.1 Introduction and Material A successful automatic detection of phrase boundaries can be of great help for parsing a word hypotheses graph in an automatic speech understanding (ASU) system. Our recognition paradigm lies within the statistical approach; we therefore need a large training database, i.e. a corpus with reference labels for prosodically marked phrase boundaries. In this paper we wil
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