5,876 research outputs found
Hybrid Quantum Repeater Protocol With Fast Local Processing
We propose a hybrid quantum repeater protocol combining the advantages of
continuous and discrete variables. The repeater is based on the previous work
of Brask et al. [Phys. Rev. Lett. 105, 160501 (2010)] but we present two ways
of improving this protocol. In the previous protocol entangled single-photon
states are produced and grown into superpositions of coherent states, known as
two-mode cat states. The entanglement is then distributed using homodyne
detection. To improve the protocol, we replace the time-consuming non-local
growth of cat states with local growth of single-mode cat states, eliminating
the need for classical communication during growth. Entanglement is generated
in subsequent connection processes. Furthermore the growth procedure is
optimized. We review the main elements of the original protocol and present the
two modifications. Finally the two protocols are compared and the modified
protocol is shown to perform significantly better than the original protocol.Comment: 14 pages, 7 figure
The Generic, Incommensurate Transition in the two-dimensional Boson Hubbard Model
The generic transition in the boson Hubbard model, occurring at an
incommensurate chemical potential, is studied in the link-current
representation using the recently developed directed geometrical worm
algorithm. We find clear evidence for a multi-peak structure in the energy
distribution for finite lattices, usually indicative of a first order phase
transition. However, this multi-peak structure is shown to disappear in the
thermodynamic limit revealing that the true phase transition is second order.
These findings cast doubts over the conclusion drawn in a number of previous
works considering the relevance of disorder at this transition.Comment: 13 pages, 10 figure
Towards low-dimensional hole systems in Be-doped GaAs nanowires
GaAs was central to the development of quantum devices but is rarely used for
nanowire-based quantum devices with InAs, InSb and SiGe instead taking the
leading role. p-type GaAs nanowires offer a path to studying strongly-confined
0D and 1D hole systems with strong spin-orbit effects, motivating our
development of nanowire transistors featuring Be-doped p-type GaAs nanowires,
AuBe alloy contacts and patterned local gate electrodes towards making
nanowire-based quantum hole devices. We report on nanowire transistors with
traditional substrate back-gates and EBL-defined metal/oxide top-gates produced
using GaAs nanowires with three different Be-doping densities and various AuBe
contact processing recipes. We show that contact annealing only brings small
improvements for the moderately-doped devices under conditions of lower anneal
temperature and short anneal time. We only obtain good transistor performance
for moderate doping, with conduction freezing out at low temperature for
lowly-doped nanowires and inability to reach a clear off-state under gating for
the highly-doped nanowires. Our best devices give on-state conductivity 95 nS,
off-state conductivity 2 pS, on-off ratio ~, and sub-threshold slope 50
mV/dec at T = 4 K. Lastly, we made a device featuring a moderately-doped
nanowire with annealed contacts and multiple top-gates. Top-gate sweeps show a
plateau in the sub-threshold region that is reproducible in separate cool-downs
and indicative of possible conductance quantization highlighting the potential
for future quantum device studies in this material system
Quantum Numbers for Excitations of Bose-Einstein Condensates in 1D Optical Lattices
The excitation spectrum and the band structure of a Bose-Einstein condensate
in a periodic potential are investigated. Analyses within full 3D systems,
finite 1D systems, and ideal periodic 1D systems are compared. We find two
branches of excitations in the spectra of the finite 1D model. The band
structures for the first and (part of) the second band are compared between a
finite 1D and the fully periodic 1D systems, utilizing a new definition of a
effective wavenumber and a phase-slip number. The upper and lower edges of the
first gap coincide well between the two cases. The remaining difference is
explained by the existence of the two branches due to the finite-size effect.Comment: 5 pages, 9 figure
Multi-particle entanglement of hot trapped ions
We propose an efficient method to produce multi-particle entangled states of
ions in an ion trap for which a wide range of interesting effects and
applications have been suggested. Our preparation scheme exploits the
collective vibrational motion of the ions, but it works in such a way that this
motion need not be fully controlled in the experiment. The ions may, e.g., be
in thermal motion and exchange mechanical energy with a surrounding heat bath
without detrimental effects on the internal state preparation. Our scheme does
not require access to the individual ions in the trap.Comment: 4 pages, including 3 figures. To appear in Phys. Rev. Lett. This
paper previously appeared under the name "Schrodingers cat in a hot trap".
The paper has been revised according to Phys. Rev. policy on Schrodinger
cats. No cats were harmed during the production of this manuscrip
Bogoliubov theory of entanglement in a Bose-Einstein condensate
We consider a Bose-Einstein condensate which is illuminated by a short
resonant light pulse that coherently couples two internal states of the atoms.
We show that the subsequent time evolution prepares the atoms in an interesting
entangled state called a spin squeezed state. This evolution is analysed in
detail by developing a Bogoliubov theory which describes the entanglement of
the atoms. Our calculation is a consistent expansion in , where
is the number of particles in the condensate, and our theory predict that it is
possible to produce spin squeezing by at least a factor of . Within
the Bogoliubov approximation this result is independent of temperature.Comment: 14 pages, including 5 figures, minor changes in the presentatio
Generation and detection of a sub-Poissonian atom number distribution in a one-dimensional optical lattice
We demonstrate preparation and detection of an atom number distribution in a
one-dimensional atomic lattice with the variance dB below the Poissonian
noise level. A mesoscopic ensemble containing a few thousand atoms is trapped
in the evanescent field of a nanofiber. The atom number is measured through
dual-color homodyne interferometry with a pW-power shot noise limited probe.
Strong coupling of the evanescent probe guided by the nanofiber allows for a
real-time measurement with a precision of atoms on an ensemble of some
atoms in a one-dimensional trap. The method is very well suited for
generating collective atomic entangled or spin-squeezed states via a quantum
non-demolition measurement as well as for tomography of exotic atomic states in
a one-dimensional lattice
What Can Wireless Cellular Technologies Do about the Upcoming Smart Metering Traffic?
The introduction of smart electricity meters with cellular radio interface
puts an additional load on the wireless cellular networks. Currently, these
meters are designed for low duty cycle billing and occasional system check,
which generates a low-rate sporadic traffic. As the number of distributed
energy resources increases, the household power will become more variable and
thus unpredictable from the viewpoint of the Distribution System Operator
(DSO). It is therefore expected, in the near future, to have an increased
number of Wide Area Measurement System (WAMS) devices with Phasor Measurement
Unit (PMU)-like capabilities in the distribution grid, thus allowing the
utilities to monitor the low voltage grid quality while providing information
required for tighter grid control. From a communication standpoint, the traffic
profile will change drastically towards higher data volumes and higher rates
per device. In this paper, we characterize the current traffic generated by
smart electricity meters and supplement it with the potential traffic
requirements brought by introducing enhanced Smart Meters, i.e., meters with
PMU-like capabilities. Our study shows how GSM/GPRS and LTE cellular system
performance behaves with the current and next generation smart meters traffic,
where it is clearly seen that the PMU data will seriously challenge these
wireless systems. We conclude by highlighting the possible solutions for
upgrading the cellular standards, in order to cope with the upcoming smart
metering traffic.Comment: Submitted; change: corrected location of eSM box in Fig. 1; May 22,
2015: Major revision after review; v4: revised, accepted for publicatio
Patient–physician communication concerning participation in cancer chemotherapy trials
Cancer patients demand a high level of involvement in decisions concerning treatment. Many patients are informed about experimental trials, and especially the first consultation may be crucial for the future communication and treatment process. Patients with nonresectable non-small-cell lung cancer or colorectal cancer informed about experimental chemotherapy completed a questionnaire on satisfaction with the communication process, general attitude towards experimental treatments, the substance of information, and personal contact with the physician following their first consultation in a medical oncology unit. Physicians completed a questionnaire on their perception of the patients’ satisfaction. Among 68 physician–cancer patient pairs, 29 patients were informed on chemotherapy in randomised trials and 39 in nonrandomised studies. The general attitude towards experimental treatment was positive or very positive in 71% of patients. Information on the treatment was perceived as completely adequate in 93% of patients informed on randomised and in 67% informed on nonrandomised trials. Physicians underestimated the patients’ satisfaction with the overall communication process, the personal contact, the patients’ perceived sufficiency of the specific treatment information and their ability to decide on study entry. In conclusion, considerable differences were observed between patients informed about experimental chemotherapy in randomised and nonrandomised trials, both with respect to their perception of how adequate the information on the specific treatments were, and whether it was sufficient for decisions on study entry. This study type effect should be accounted for in future evaluations of communication and patient satisfaction. The data also support the fact that cancer patients have a desire for and ability to understand rather detailed and comprehensive treatment information
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