33,662 research outputs found
Quantum cryptography protocols robust against photon number splitting attacks for weak laser pulses implementations
We introduce a new class of quantum quantum key distribution protocols,
tailored to be robust against photon number splitting (PNS) attacks. We study
one of these protocols, which differs from the BB84 only in the classical
sifting procedure. This protocol is provably better than BB84 against PNS
attacks at zero error.Comment: 4 pages, 2 figure
Scaling Laws for Non-Intercommuting Cosmic String Networks
We study the evolution of non-interacting and entangled cosmic string
networks in the context of the velocity-dependent one-scale model. Such
networks may be formed in several contexts, including brane inflation. We show
that the frozen network solution , although generic, is only a
transient one, and that the asymptotic solution is still as in the
case of ordinary (intercommuting) strings, although in the present context the
universe will usually be string-dominated. Thus the behaviour of two strings
when they cross does not seem to affect their scaling laws, but only their
densities relative to the background.Comment: Phys. Rev. D (in press); v2: final published version (references
added, typos corrected
On Multipartite Pure-State Entanglement
We show that pure states of multipartite quantum systems are multiseparable
(i.e. give separable density matrices on tracing any party) if and only if they
have a generalized Schmidt decomposition. Implications of this result for the
quantification of multipartite pure-state entanglement are discussed. Further,
as an application of the techniques used here, we show that any purification of
a bipartite PPT bound entangled state is tri-inseparable, i.e. has none of its
three bipartite partial traces separable.Comment: 8 Pages ReVTeX, 4 figures (eps); v2: Revised terminology, added two
references and other minor changes; v3: Minor changes, added two references,
added author's middle initial; v4: One footnote remove
Quantum key distribution with 2-bit quantum codes
We propose a prepare-and-measure scheme for quantum key distribution with
2-bit quantum codes. The protocol is unconditionally secure under whatever type
of intercept-and-resend attack. Given the symmetric and independent errors to
the transmitted qubits, our scheme can tolerate a bit error rate up to 26% in
4-state protocol and 30% in 6-state protocol, respectively. These values are
higher than all currently known threshold values for prepare-and-measure
protocols. A specific realization with linear optics is given.Comment: Approved for publication in Physical Review Letter
The Impact of Line Misidentification on Cosmological Constraints from Euclid and other Spectroscopic Galaxy Surveys
We perform forecasts for how baryon acoustic oscillation (BAO) scale and
redshift-space distortion (RSD) measurements from future spectroscopic emission
line galaxy (ELG) surveys such as Euclid are degraded in the presence of
spectral line misidentification. Using analytic calculations verified with mock
galaxy catalogs from log-normal simulations we find that constraints are
degraded in two ways, even when the interloper power spectrum is modeled
correctly in the likelihood. Firstly, there is a loss of signal-to-noise ratio
for the power spectrum of the target galaxies, which propagates to all
cosmological constraints and increases with contamination fraction, .
Secondly, degeneracies can open up between and cosmological parameters.
In our calculations this typically increases BAO scale uncertainties at the
10-20% level when marginalizing over parameters determining the broadband power
spectrum shape. External constraints on , or parameters determining the
shape of the power spectrum, for example from cosmic microwave background (CMB)
measurements, can remove this effect. There is a near-perfect degeneracy
between and the power spectrum amplitude for low values, where
is not well determined from the contaminated sample alone. This has the
potential to strongly degrade RSD constraints. The degeneracy can be broken
with an external constraint on , for example from cross-correlation with a
separate galaxy sample containing the misidentified line, or deeper
sub-surveys.Comment: 18 pages, 7 figures, updated to match version accepted by ApJ (extra
paragraph added at the end of Section 4.3, minor text edits
Quantum computers can search arbitrarily large databases by a single query
This paper shows that a quantum mechanical algorithm that can query
information relating to multiple items of the database, can search a database
in a single query (a query is defined as any question to the database to which
the database has to return a (YES/NO) answer). A classical algorithm will be
limited to the information theoretic bound of at least O(log N) queries (which
it would achieve by using a binary search).Comment: Several enhancements to the original pape
Supercatalysis
We show that entanglement-assisted transformations of bipartite entangled
states can be more efficient than catalysis [D. Jonathan and M. B. Plenio,
Phys. Rev. Lett. 83, 3566 (1999)}, i.e., given two incomparable bipartite
states not only can the transformation be enabled by performing collective
operations with an auxiliary entangled state, but the entanglement of the
auxiliary state itself can be enhanced. We refer to this phenomenon as
supercatalysis. We provide results on the properties of supercatalysis and its
relationship with catalysis. In particular, we obtain a useful necessary and
sufficient condition for catalysis, provide several sufficient conditions for
supercatalysis and study the extent to which entanglement of the auxiliary
state can be enhanced via supercatalysis.Comment: Latex, 5 page
On the origin of noisy states whose teleportation fidelity can be enhanced through dissipation
Recently Badziag \emph{et al.} \cite{badziag} obtained a class of noisy
states whose teleportation fidelity can be enhanced by subjecting one of the
qubits to dissipative interaction with the environment via amplitude damping
channel (ADC). We show that such noisy states result while sharing the states
(| \Phi ^{\pm}> =\frac{1}{\sqrt{2}}(| 00> \pm | 11>)) across ADC. We also show
that under similar dissipative interactions different Bell states give rise to
noisy entangled states that are qualitatively very different from each other in
the sense, only the noisy entangled states constructed from the Bell states (|
\Phi ^{\pm}>) can \emph{}be made better sometimes by subjecting the unaffected
qubit to a dissipative interaction with the environment. Importantly if the
noisy state is non teleporting then it can always be made teleporting with this
prescription. We derive the most general restrictions on improvement of such
noisy states assuming that the damping parameters being different for both the
qubits. However this curious prescription does not work for the noisy entangled
states generated from (| \Psi ^{\pm}> =\frac{1}{\sqrt{2}}(| 01> \pm | 10>)).
This shows that an apriori knowledge of the noisy channel might be helpful to
decide which Bell state needs to be shared between Alice and Bob. \emph{}Comment: Latex, 18 pages: Revised version with a new result. Submitted to PR
Calculation of transonic steady and oscillatory pressures on a low aspect ratio model and comparison with experiment
Pressure data measured by the British Royal Aircraft Establishment for the AGARD SMP tailplane are compared with results calculated using the transonic small perturbation code XTRAN3S. A brief description of the analysis is given and a recently developed finite difference grid is described. Results are presented for five steady and nine harmonically oscillating cases near zero angle of attack and for a range of subsonic and transonic Mach numbers
Semantic categories underlying the meaning of ‘place’
This paper analyses the semantics of natural language expressions that are associated with the intuitive notion of ‘place’. We note that the nature of such terms is highly contested, and suggest that this arises from two main considerations: 1) there are a number of logically
distinct categories of place expression, which are not always clearly distinguished in discourse about ‘place’; 2) the many non-substantive place count nouns (such as ‘place’, ‘region’, ‘area’, etc.) employed in natural
language are highly ambiguous. With respect to consideration 1), we propose that place-related expressions
should be classified into the following distinct logical types: a) ‘place-like’ count nouns (further subdivided into abstract, spatial and substantive varieties), b) proper names of ‘place-like’ objects, c) locative property phrases, and d) definite descriptions of ‘place-like’ objects. We outline possible formal representations for each of these. To address consideration 2), we examine meanings, connotations and ambiguities of the English vocabulary of abstract and generic place count nouns, and identify underlying elements of meaning, which explain both
similarities and differences in the sense and usage of the various terms
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