3,188 research outputs found
Ising Dynamics with Damping
We show for the Ising model that is possible construct a discrete time
stochastic model analogous to the Langevin equation that incorporates an
arbitrary amount of damping. It is shown to give the correct equilibrium
statistics and is then used to investigate nonequilibrium phenomena, in
particular, magnetic avalanches. The value of damping can greatly alter the
shape of hysteresis loops, and for small damping and high disorder, the
morphology of large avalanches can be drastically effected. Small damping also
alters the size distribution of avalanches at criticality.Comment: 8 pages, 8 figures, 2 colum
The Likely Orbital Period of the Ultracompact Low-Mass X-Ray Binary 2S 0918-549
We report the discovery of the likely orbital period of the ultracompact
low-mass X-ray binary (LMXB) 2S 0918-549. Using time-resolved optical
photometry carried out with the 8-m Gemini South Telescope, we obtained a
2.4-hr long, Sloan r' light curve of 2S 0918-549 and found a periodic,
sinusoidal modulation at 17.4+/-0.1 min with a semiamplitude of 0.015+/-0.002
mag, which we identify as the binary period. In addition to 4U 0513-40 in the
globular cluster NGC 1851 and the Galactic disk source 4U 1543-624, 2S 0918-549
is the third member of the ultracompact LMXBs that have orbital periods around
18 min. Our result verifies the suggestion of 2S 0918-549 as an ultracompact
binary based on its X-ray and optical spectroscopic properties. Given that the
donor in 2S 0918-549 has been suggested to be either a C-O or He white dwarf,
its likely mass and radius are around 0.024--0.029 M_sun and 0.03--0.032 R_sun,
respectively, for the former case and 0.034--0.039 M_sun and 0.033--0.035 R_sun
for the latter case. If the optical modulation arises from X-ray heating of the
mass donor, its sinusoidal shape suggests that the binary has a low inclination
angle, probably around 10 deg.Comment: 5 pages, 5 figures, accepted for publication in Ap
Realisation of a programmable two-qubit quantum processor
The universal quantum computer is a device capable of simulating any physical
system and represents a major goal for the field of quantum information
science. Algorithms performed on such a device are predicted to offer
significant gains for some important computational tasks. In the context of
quantum information, "universal" refers to the ability to perform arbitrary
unitary transformations in the system's computational space. The combination of
arbitrary single-quantum-bit (qubit) gates with an entangling two-qubit gate is
a gate set capable of achieving universal control of any number of qubits,
provided that these gates can be performed repeatedly and between arbitrary
pairs of qubits. Although gate sets have been demonstrated in several
technologies, they have as yet been tailored toward specific tasks, forming a
small subset of all unitary operators. Here we demonstrate a programmable
quantum processor that realises arbitrary unitary transformations on two
qubits, which are stored in trapped atomic ions. Using quantum state and
process tomography, we characterise the fidelity of our implementation for 160
randomly chosen operations. This universal control is equivalent to simulating
any pairwise interaction between spin-1/2 systems. A programmable multi-qubit
register could form a core component of a large-scale quantum processor, and
the methods used here are suitable for such a device.Comment: 7 pages, 4 figure
Thermodynamic entropy of a many body energy eigenstate
It is argued that a typical many body energy eigenstate has a well defined
thermodynamic entropy and that individual eigenstates possess thermodynamic
characteristics analogous to those of generic isolated systems. We examine
large systems with eigenstate energies equivalent to finite temperatures. When
quasi-static evolution of a system is adiabatic (in the quantum mechanical
sense), two coupled subsystems can transfer heat from one subsystem to another
yet remain in an energy eigenstate. To explicitly construct the entropy from
the wave function, degrees of freedom are divided into two unequal parts. It is
argued that the entanglement entropy between these two subsystems is the
thermodynamic entropy per degree of freedom for the smaller subsystem. This is
done by tracing over the larger subsystem to obtain a density matrix, and
calculating the diagonal and off-diagonal contributions to the entanglement
entropy.Comment: 18 page
Quantum Computation with Coherent Spin States and the Close Hadamard Problem
We study a model of quantum computation based on the
continuously-parameterized yet finite-dimensional Hilbert space of a spin
system. We explore the computational powers of this model by analyzing a pilot
problem we refer to as the close Hadamard problem. We prove that the close
Hadamard problem can be solved in the spin system model with arbitrarily small
error probability in a constant number of oracle queries. We conclude that this
model of quantum computation is suitable for solving certain types of problems.
The model is effective for problems where symmetries between the structure of
the information associated with the problem and the structure of the unitary
operators employed in the quantum algorithm can be exploited.Comment: RevTeX4, 13 pages with 8 figures. Accepted for publication in Quantum
Information Processing. Article number: s11128-015-1229-
Postponement of dark-count effects in practical quantum key-distribution by two-way post-processing
The influence of imperfections on achievable secret-key generation rates of
quantum key distribution protocols is investigated. As examples of relevant
imperfections, we consider tagging of Alice's qubits and dark counts at Bob's
detectors, while we focus on a powerful eavesdropping strategy which takes full
advantage of tagged signals. It is demonstrated that error correction and
privacy amplification based on a combination of a two-way classical
communication protocol and asymmetric Calderbank-Shor-Steane codes may
significantly postpone the disastrous influence of dark counts. As a result,
the distances are increased considerably over which a secret key can be
distributed in optical fibres reliably. Results are presented for the
four-state, the six-state, and the decoy-state protocols.Comment: Fully revised version (12 pages and 8 figures). Improved figures and
discussion added. To appear in Eur. Phys. J.
Accurate first-principle equation of state for the One-Component Plasma
Accurate "first-principle" expressions for the excess free energy
and internal energy of the classical one-component plasma (OCP) are
obtained. We use the Hubbard-Schofield transformation that maps the OCP
Hamiltonian onto the Ising-like Hamiltonian, with coefficients expressed in
terms of equilibrium correlation functions of a reference system. We use the
ideal gas as a reference system for which all the correlation functions are
known. Explicit calculations are performed with the high-order terms in the
Ising-like Hamiltonian omitted. For small values of the plasma parameter
the Debye-Huckel result for and is recovered. For
large these depend linearly on in accordance with the Monte
Carlo findings for the OCP. The MC data for the internal energy are reproduced
fairly well by the obtained analytical expression.Comment: 15 pages, 2 figures, to appear in Contrib. Plasma Phys., v.38 N4,
(1998
A note on entropic uncertainty relations of position and momentum
We consider two entropic uncertainty relations of position and momentum
recently discussed in literature. By a suitable rescaling of one of them, we
obtain a smooth interpolation of both for high-resolution and low-resolution
measurements respectively. Because our interpolation has never been mentioned
in literature before, we propose it as a candidate for an improved entropic
uncertainty relation of position and momentum. Up to now, the author has
neither been able to falsify nor prove the new inequality. In our opinion it is
a challenge to do either one.Comment: 2 pages, 2 figures, 2 references adde
The SuperCOSMOS Sky Survey. Paper III: Astrometry
In this, the third in a series of three papers concerning the SuperCOSMOS Sky
Survey, we describe the astrometric properties of the database. We describe the
algorithms employed in the derivation of the astrometric parameters of the
data, and demonstrate their accuracies by comparison with external datasets
using the first release of data, the South Galactic Cap survey. We show that
the celestial coordinates, which are tied to the International Celestial
Reference Frame via the Tycho-2 reference catalogue, are accurate to better
than +/- 0.2 arcsec at J,R=19,18 rising to +/- 0.3 arcsec at J,R=22,21 with
positional dependent systematic effects from bright to faint magnitudes at the
+/- 0.1 arcsec level. The proper motion measurements are shown to be accurate
to typically +/- 10 mas/yr at J,R=19,18 rising to +/- 50 mas/yr at J,R=22,21
and are tied to zero using the extragalactic reference frame. We show that the
zeropoint errors in the proper motions are 17 and are no
larger than 10 mas/yr for R < 17 mas/yr.Comment: 15 pages, 12 figures; accepted for publication in MNRA
- âŠ