18,381 research outputs found
Lower critical field measurements in YBa2Cu3O(6+x) single crystals
The temperature dependence of the lower critical field in YBa2Cu3O(6+x) single crystals was determined by magnetization measurements with the applied field parallel and perpendicular to the c-axis. Results are compared with data from the literature and fitted to Ginzberg-Landau equations by assuming a linear dependence of the parameter kappa on temperature. A value of 7 plus or minus 2 kOe was estimated for the thermodynamic critical field at T = O by comparison of calculated H (sub c2) values with experimental data from the literature
Measurement of H(sub c1) in a single crystal of YBa2Cu3O7 with low pinning
The measurement of H(sub c1) in barium yttrium copper oxide (BYCO) is often ambiguous because the presence of large pinning forces makes it difficult to discern exactly where the first deviation from linearity occurs. In addition there are complications because demagnetizing factors are often not well known. By utilizing a single crystal of YBCO with a nearly cubic shape, the uncertainty in the demagnetizing factor was minimized. In addition, the crystal used exhibited a very small amount of pinning with H applied perpendicular to the c axis, and a sharp break in the initial magnetization vs. field curve could be observed over a wide range of temperature. This allowed a precise determination of H(sub c1). The measured values of H(sub c1) could be well described by the Abrikosov relation with a Ginzburg-Landau parameter which varied linearly with temperature
Method of convex rigid frames and applications in studies of multipartite quNit pure-states
In this Letter we suggest a method of convex rigid frames in the studies of
the multipartite quNit pure-states. We illustrate what are the convex rigid
frames and what is the method of convex rigid frames. As the applications we
use this method to solve some basic problems and give some new results (three
theorems): The problem of the partial separability of the multipartite quNit
pure-states and its geometric explanation; The problem of the classification of
the multipartite quNit pure-states, and give a perfect explanation of the local
unitary transformations; Thirdly, we discuss the invariants of classes and give
a possible physical explanation.Comment: 6 pages, no figur
Permutation asymmetry inducing entanglement between degrees of freedom in multiphoton states
We describe and examine entanglement between different degrees of freedom in
multiphoton states based on the permutation properties. From the state
description, the entanglement comes from the permutation asymmetry. According
to the different permutation properties, the multiphoton states can be divided
into several parts. It will help to deal with the multiphoton interference,
which can be used as the measurement of the entanglement.Comment: Final versio
Perfection of materials technology for producing improved Gunn-effect devices
Chemical vapor deposition system for improved Gunn effect devices using arsenic chloride 3 metho
Theoretical efficient high capacity Quantum Key Distribution Scheme
A theoretical quantum key distribution scheme using EPR pairs is presented.
This scheme is efficient in that it uses all EPR pairs in distributing the key
except those chosen for checking eavesdroppers. The high capacity is achieved
because each EPR pair carries 2 bits of key code.Comment: 3 pages and 1 figure, to appear in Physical Review
Information-disturbance tradeoff in estimating a maximally entangled state
We derive the amount of information retrieved by a quantum measurement in
estimating an unknown maximally entangled state, along with the pertaining
disturbance on the state itself. The optimal tradeoff between information and
disturbance is obtained, and a corresponding optimal measurement is provided.Comment: 4 pages. Accepted for publication on Physical Review Letter
A Search for Stellar Obscuration Events due to Dark Clouds
The recent detections of a large population of faint submillimetre sources,
an excess halo gamma-ray background, and the extreme scattering events observed
for extragalactic radio sources have been explained as being due to baryonic
dark matter in the form of small, dark, gas clouds. In this paper we present
the results of a search for the transient stellar obscurations such clouds are
expected to cause. We examine the Macho project light curves of 48 x 10^6 stars
toward the Galactic bulge, LMC and SMC for the presence of dark cloud
extinction events. We find no evidence for the existence of a population of
dark gas clouds with Av > 0.2 and masses between ~ 10^-4 and 10^-2 M_solar in
the Galactic disk or halo. However, it is possible that such dark cloud
populations could exist if they are clustered in regions away from the observed
lines of sight.Comment: 13 pages, 9 figures, submitted to Ap
Mixed State Entanglement and Quantum Error Correction
Entanglement purification protocols (EPP) and quantum error-correcting codes
(QECC) provide two ways of protecting quantum states from interaction with the
environment. In an EPP, perfectly entangled pure states are extracted, with
some yield D, from a mixed state M shared by two parties; with a QECC, an arbi-
trary quantum state can be transmitted at some rate Q through a
noisy channel without degradation. We prove that an EPP involving one-
way classical communication and acting on mixed state (obtained
by sharing halves of EPR pairs through a channel ) yields a QECC on
with rate , and vice versa. We compare the amount of entanglement
E(M) required to prepare a mixed state M by local actions with the amounts
and that can be locally distilled from it by EPPs using one-
and two-way classical communication respectively, and give an exact expression
for when is Bell-diagonal. While EPPs require classical communica-
tion, QECCs do not, and we prove Q is not increased by adding one-way classical
communication. However, both D and Q can be increased by adding two-way com-
munication. We show that certain noisy quantum channels, for example a 50%
depolarizing channel, can be used for reliable transmission of quantum states
if two-way communication is available, but cannot be used if only one-way com-
munication is available. We exhibit a family of codes based on universal hash-
ing able toachieve an asymptotic (or ) of 1-S for simple noise models,
where S is the error entropy. We also obtain a specific, simple 5-bit single-
error-correcting quantum block code. We prove that {\em iff} a QECC results in
high fidelity for the case of no error the QECC can be recast into a form where
the encoder is the matrix inverse of the decoder.Comment: Resubmission with various corrections and expansions. See also
http://vesta.physics.ucla.edu/~smolin/ for related papers and information. 82
pages latex including 19 postscript figures included using psfig macro
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