4,282 research outputs found
Device independent quantum key distribution secure against coherent attacks with memoryless measurement devices
Device independent quantum key distribution aims to provide a higher degree
of security than traditional QKD schemes by reducing the number of assumptions
that need to be made about the physical devices used. The previous proof of
security by Pironio et al. applies only to collective attacks where the state
is identical and independent and the measurement devices operate identically
for each trial in the protocol. We extend this result to a more general class
of attacks where the state is arbitrary and the measurement devices have no
memory. We accomplish this by a reduction of arbitrary adversary strategies to
qubit strategies and a proof of security for qubit strategies based on the
previous proof by Pironio et al. and techniques adapted from Renner.Comment: 13 pages. Expanded main proofs with more detail, miscellaneous edits
for clarit
On the universality of small scale turbulence
The proposed universality of small scale turbulence is investigated for a set
of measurements in a cryogenic free jet with a variation of the Reynolds number
(Re) from 8500 to 10^6. The traditional analysis of the statistics of velocity
increments by means of structure functions or probability density functions is
replaced by a new method which is based on the theory of stochastic Markovian
processes. It gives access to a more complete characterization by means of
joint probabilities of finding velocity increments at several scales. Based on
this more precise method our results call in question the concept of
universality.Comment: 4 pages, 4 figure
Classification of Reductive Monoid Spaces Over an Arbitrary Field
In this semi-expository paper we review the notion of a spherical space. In
particular we present some recent results of Wedhorn on the classification of
spherical spaces over arbitrary fields. As an application, we introduce and
classify reductive monoid spaces over an arbitrary field.Comment: This is the final versio
Secure certification of mixed quantum states with application to two-party randomness generation
We investigate sampling procedures that certify that an arbitrary quantum
state on subsystems is close to an ideal mixed state
for a given reference state , up to errors on a few positions. This
task makes no sense classically: it would correspond to certifying that a given
bitstring was generated according to some desired probability distribution.
However, in the quantum case, this is possible if one has access to a prover
who can supply a purification of the mixed state.
In this work, we introduce the concept of mixed-state certification, and we
show that a natural sampling protocol offers secure certification in the
presence of a possibly dishonest prover: if the verifier accepts then he can be
almost certain that the state in question has been correctly prepared, up to a
small number of errors.
We then apply this result to two-party quantum coin-tossing. Given that
strong coin tossing is impossible, it is natural to ask "how close can we get".
This question has been well studied and is nowadays well understood from the
perspective of the bias of individual coin tosses. We approach and answer this
question from a different---and somewhat orthogonal---perspective, where we do
not look at individual coin tosses but at the global entropy instead. We show
how two distrusting parties can produce a common high-entropy source, where the
entropy is an arbitrarily small fraction below the maximum (except with
negligible probability)
Discrimination between the superconducting gap and the pseudo-gap in Bi2212 from intrinsic tunneling spectroscopy in magnetic field
Intrinsic tunneling spectroscopy in high magnetic field () is used for a
direct test of superconducting features in a quasiparticle density of states of
high- superconductors. We were able to distinguish with a great clarity
two co-existing gaps: (i) the superconducting gap, which closes as and , and (ii) the -axis pseudo-gap, which does not
change neither with , nor . Strikingly different magnetic field
dependencies, together with previously observed different temperature
dependencies of the two gaps ~\cite{Krasnov}, speak against the superconducting
origin of the pseudo-gap.Comment: 4 pages, 4 eps figure
Barriers to Physical Activity: Physical Activity in Military Employees; Barrier impact on Perceived Health
Much of the world faces a growing obesity epidemic. (pp92-97) The consequences of this serious problem are well established and potentially devastating. Conditions associated with an elevated body mass include diabetes, hypertension, coronary heart disease, and certain malignancies.(pp97-100), For many, obesity and elevated body mass are a consequence of decreased physical activity; industrial advancements (eg, convenient transportation, technological advancements, and decreased need for manual labor) have contributed to an overall decrease in physical activity worldwide
Parameter-free expression for superconducting Tc in cuprates
A parameter-free expression for the superconducting critical temperature of
layered cuprates is derived which allows us to express Tc in terms of
experimentally measured parameters. It yields Tc values observed in about 30
lanthanum, yttrium and mercury-based samples for different levels of doping.
This remarkable agreement with the experiment as well as the unusual critical
behaviour and the normal-state gap indicate that many cuprates are close to the
Bose-Einstein condensation regime.Comment: 5 pages, 2 figures. Will be published in Physical Review
Search for Magnetic Field Induced Gap in a High-Tc Superconductor
Break junctions made of the optimally doped high temperature superconductor
Bi2Sr2Ca2CuO8 with Tc of 90 K has been investigated in magnetic fields up to 12
T, at temperatures from 4.2 K to Tc. The junction resistance varied between
1kOhm and 300kOhm. The differential conductance at low biases did not exhibit a
significant magnetic field dependence, indicating that a magnetic-field-induced
gap (Krishana et al., Science 277 83 (1997)), if exists, must be smaller than
0.25 meV.Comment: 3 pages, 2 figure
Effect of bilayer coupling on tunneling conductance of double-layer high T_c cuprates
Physical effects of bilayer coupling on the tunneling spectroscopy of high
T cuprates are investigated. The bilayer coupling separates the bonding
and antibonding bands and leads to a splitting of the coherence peaks in the
tunneling differential conductance. However, the coherence peak of the bonding
band is strongly suppressed and broadened by the particle-hole asymmetry in the
density of states and finite quasiparticle life-time, and is difficult to
resolve by experiments. This gives a qualitative account why the bilayer
splitting of the coherence peaks was not clearly observed in tunneling
measurements of double-layer high-T oxides.Comment: 4 pages, 3 figures, to be published in PR
Determination of the Coherence Length and the Cooper-Pair Size in Unconventional Superconductors by Tunnelling Spectroscopy
The main purpose of the paper is to discuss a possibility of the
determination of the values of the coherence length and the Cooper-pair size in
unconventional superconductors by using tunnelling spectroscopy. In the mixed
state of type-II superconductors, an applied magnetic field penetrates the
superconductor in the form of vortices which form a regular lattice. In
unconventional superconductors, the inner structure of a vortex core has a
complex structure which is determined by the order parameter of the
superconducting state and by the pairing wavefunction of the Cooper pairs. In
clean superconductors, the spatial variations of the order parameter and the
pairing wavefunction occur over the distances of the order of the coherence
length and the Cooper-pair size, respectively. Therefore, by performing
tunnelling spectroscopy along a line passing through a vortex core, one is
able, in principle, to estimate the values of the coherent length and the
Cooper-pair size.Comment: 13 pages, including 17 figure
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