15,606 research outputs found
Ultralow threshold graded-index separate-confinement heterostructure single quantum well (Al,Ga)As lasers
Broad area graded‐index separate‐confinement heterostructure single quantum well lasers grown by molecular‐beam epitaxy (MBE) with threshold current density as low as 93 A/cm^2 (520 μm long) have been fabricated. Buried lasers formed from similarly structured MBE material with liquid phase epitaxy regrowth had threshold currents at submilliampere levels when high reflectivity coatings were applied to the end facets. A cw threshold current of 0.55 mA was obtained for a laser with facet reflectivities of ∼80%, a cavity length of 120 μm, and an active region stripe width of 1 μm. These devices driven directly with logic level signals have switch‐on delays <50 ps without any current prebias. Such lasers permit fully on–off switching while at the same time obviating the need for bias monitoring and feedback control
Surgery and the Spectrum of the Dirac Operator
We show that for generic Riemannian metrics on a simply-connected closed spin
manifold of dimension at least 5 the dimension of the space of harmonic spinors
is no larger than it must be by the index theorem. The same result holds for
periodic fundamental groups of odd order.
The proof is based on a surgery theorem for the Dirac spectrum which says
that if one performs surgery of codimension at least 3 on a closed Riemannian
spin manifold, then the Dirac spectrum changes arbitrarily little provided the
metric on the manifold after surgery is chosen properly.Comment: 23 pages, 4 figures, to appear in J. Reine Angew. Mat
Finitely Many Dirac-Delta Interactions on Riemannian Manifolds
This work is intended as an attempt to study the non-perturbative
renormalization of bound state problem of finitely many Dirac-delta
interactions on Riemannian manifolds, S^2, H^2 and H^3. We formulate the
problem in terms of a finite dimensional matrix, called the characteristic
matrix. The bound state energies can be found from the characteristic equation.
The characteristic matrix can be found after a regularization and
renormalization by using a sharp cut-off in the eigenvalue spectrum of the
Laplacian, as it is done in the flat space, or using the heat kernel method.
These two approaches are equivalent in the case of compact manifolds. The heat
kernel method has a general advantage to find lower bounds on the spectrum even
for compact manifolds as shown in the case of S^2. The heat kernels for H^2 and
H^3 are known explicitly, thus we can calculate the characteristic matrix.
Using the result, we give lower bound estimates of the discrete spectrum.Comment: To be published in JM
APPARATUS FOR THE STUDY OF FISSION-GAS RELEASE FROM NEUTRON-ACTIVATED FUELED GRAPHITE
A simple laboratory apparatus for the study of fissiongas release from neutron-activated fueled graphite was developed. Xenon-133 released from a heated specimen is carried in a helium sweep gas to a charcoal trap, where the accumulated activity is monitored continuously by a scintillation detector, ratemeter, and pen recorder. The maximum specimen temperature (2500 deg F) is achieved in 10 min with an induction heater. All instrumentation is commercially available. Data for several neutron-activated fueled-graphite specimens heated in the range from 800 to 2500 deg F are presented to illustrate the typical results obtained with the apparatus. (auth
High rate, long-distance quantum key distribution over 250km of ultra low loss fibres
We present a fully automated quantum key distribution prototype running at
625 MHz clock rate. Taking advantage of ultra low loss fibres and low-noise
superconducting detectors, we can distribute 6,000 secret bits per second over
100 km and 15 bits per second over 250km
The Frequency and Content of Discussions About Alcohol Use in Primary Care and Application of the Chief Medical Officer’s Low-Risk Drinking Guidelines: A Cross-Sectional Survey of General Practitioners and Practice Nurses in the UK
ABSTRACT
Aims: To examine how often General Practitioners (GPs) and Practice Nurses (PNs) working in primary care discuss alcohol with patients, what factors prompt discussions, how they approach patient discussions, and whether the Chief Medical Officer’s (CMO’s) revised low-risk drinking guidelines are appropriately advised.
Methods: Cross-sectional survey with GPs and PNs working in primary care in the UK, conducted January-March 2017 (n=2,020). A vignette exercise examined what factors would prompt a discussion about alcohol, whether they would discuss before or after a patient reported exceeded the revised CMO guidelines (14 units per week), and whether the CMO’s drinking guidelines were appropriately advised. For all patients, participants were asked how often they discussed alcohol and how they approached the discussion (e.g. used screening tool).
Results: The most common prompts to discuss alcohol in the vignette exercise were physical cues (44.7% of participants) or alcohol-related symptoms (23.8%). Most practitioners (70.1%) said they would wait until a patient was exceeding CMO guidelines before instigating discussion. Two-fifths (38.1%) appropriately advised the CMO guidelines in the vignette exercise, with PNs less likely to do so than GPs (OR=0.77, p=0.03). Less than half (44.7%) reportedly asked about alcohol always/often with all patients, with PNs more likely to ask always/often than GPs (OR=2.22, p<0.001). Almost three-quarters said they would enquire by asking about units (70.3%), compared to using screening tools.
Conclusion: Further research is required to identify mechanisms to increase the frequency of discussions about alcohol and appropriate recommendation of the CMO drinking guidelines to patients.This research was supported by funding from Cancer Research UK. JMB is supported by the Medical Research Council (MRC) (Grant MC_UU_12015/4)
Flipping quantum coins
Coin flipping is a cryptographic primitive in which two distrustful parties
wish to generate a random bit in order to choose between two alternatives. This
task is impossible to realize when it relies solely on the asynchronous
exchange of classical bits: one dishonest player has complete control over the
final outcome. It is only when coin flipping is supplemented with quantum
communication that this problem can be alleviated, although partial bias
remains. Unfortunately, practical systems are subject to loss of quantum data,
which restores complete or nearly complete bias in previous protocols. We
report herein on the first implementation of a quantum coin-flipping protocol
that is impervious to loss. Moreover, in the presence of unavoidable
experimental noise, we propose to use this protocol sequentially to implement
many coin flips, which guarantees that a cheater unwillingly reveals
asymptotically, through an increased error rate, how many outcomes have been
fixed. Hence, we demonstrate for the first time the possibility of flipping
coins in a realistic setting. Flipping quantum coins thereby joins quantum key
distribution as one of the few currently practical applications of quantum
communication. We anticipate our findings to be useful for various
cryptographic protocols and other applications, such as an online casino, in
which a possibly unlimited number of coin flips has to be performed and where
each player is free to decide at any time whether to continue playing or not.Comment: 17 pages, 3 figure
Thermodynamic Relations in Correlated Systems
Several useful thermodynamic relations are derived for metal-insulator
transitions, as generalizations of the Clausius-Clapeyron and Eherenfest
theorems. These relations hold in any spatial dimensions and at any
temperatures. First, they relate several thermodynamic quantities to the slope
of the metal-insulator phase boundary drawn in the plane of the chemical
potential and the Coulomb interaction in the phase diagram of the Hubbard
model. The relations impose constraints on the critical properties of the Mott
transition. These thermodynamic relations are indeed confirmed to be satisfied
in the cases of the one- and two-dimensional Hubbard models. One of these
relations yields that at the continuous Mott transition with a diverging charge
compressibility, the doublon susceptibility also diverges. The constraints on
the shapes of the phase boundary containing a first-order metal-insulator
transition at finite temperatures are clarified based on the thermodynamic
relations. For example, the first-order phase boundary is parallel to the
temperature axis asymptotically in the zero temperature limit. The
applicability of the thermodynamic relations are not restricted only to the
metal-insulator transition of the Hubbard model, but also hold in correlated
systems with any types of phases in general. We demonstrate such examples in an
extended Hubbard model with intersite Coulomb repulsion containing the charge
order phase.Comment: 10 pages, 9 figure
Practical long-distance quantum key distribution system using decoy levels
Quantum key distribution (QKD) has the potential for widespread real-world
applications. To date no secure long-distance experiment has demonstrated the
truly practical operation needed to move QKD from the laboratory to the real
world due largely to limitations in synchronization and poor detector
performance. Here we report results obtained using a fully automated, robust
QKD system based on the Bennett Brassard 1984 protocol (BB84) with low-noise
superconducting nanowire single-photon detectors (SNSPDs) and decoy levels.
Secret key is produced with unconditional security over a record 144.3 km of
optical fibre, an increase of more than a factor of five compared to the
previous record for unconditionally secure key generation in a practical QKD
system.Comment: 9 page
Perturbation study on the spin and charge susceptibilities of the two-dimensional Hubbard model
We investigate the spin and charge susceptibilities of the two-dimensional
Hubbard model based upon the perturbative calculation in the strength of
correlation . For comparable to a bare bandwidth, the charge
susceptibility decreases near the half-filling as hole-doping approaches zero.
This behavior suggesting the precursor of the Mott-Hubbard gap formation cannot
be obtained without the vertex corrections beyond the random phase
approximation. In the low-temperature region, the spin susceptibility deviates
from the Curie-Weiss-like law and finally turns to decrease with the decrease
of temperature. This spin-gap-like behavior is originating from the van Hove
singularity in the density of states.Comment: Revtex file + 11 figures, to appear in Phys. Rev.
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