418 research outputs found
Dynamical instabilities of a resonator driven by a superconducting single-electron transistor
We investigate the dynamical instabilities of a resonator coupled to a
superconducting single-electron transistor (SSET) tuned to the Josephson
quasiparticle (JQP) resonance. Starting from the quantum master equation of the
system, we use a standard semiclassical approximation to derive a closed set of
mean field equations which describe the average dynamics of the resonator and
SSET charge. Using amplitude and phase coordinates for the resonator and
assuming that the amplitude changes much more slowly than the phase, we explore
the instabilities which arise in the resonator dynamics as a function of
coupling to the SSET, detuning from the JQP resonance and the resonator
frequency. We find that the locations (in parameter space) and sizes of the
limit cycle states predicted by the mean field equations agree well with
numerical solutions of the full master equation for sufficiently weak
SSET-resonator coupling. The mean field equations also give a good qualitative
description of the set of dynamical transitions in the resonator state that
occur as the coupling is progressively increased.Comment: 23 pages, 6 Figures, Accepted for NJ
Current noise of a superconducting single electron transistor coupled to a resonator
We analyze the current and zero-frequency current noise properties of a
superconducting single electron resonator (SSET) coupled to a resonator,
focusing on the regime where the SSET is operated in the vicinity of the
Josephson quasiparticle resonance. We consider a range of coupling strengths
and resonator frequencies to reflect the fact that in practice the system can
be tuned to quite a high degree with the resonator formed either by a
nanomechanical oscillator or a superconducting stripline fabricated in close
proximity to the SSET. For very weak couplings the SSET acts on the resonator
like an effective thermal bath. In this regime the current characteristics of
the SSET are only weakly modified by the resonator. Using a mean field
approach, we show that the current noise is nevertheless very sensitive to the
correlations between the resonator and the SSET charge. For stronger couplings,
the SSET can drive the resonator into limit cycle states where self-sustained
oscillation occurs and we find that regions of well-defined bistability exist.
Dynamical transitions into and out of the limit cycle state are marked by
strong fluctuations in the resonator energy, but these fluctuations are
suppressed within the limit cycle state. We find that the current noise of the
SSET is strongly influenced by the fluctuations in the resonator energy and
hence should provide a useful indicator of the resonator's dynamics.Comment: Reduced quality figures for arXiv version; v2 minor correction
Entanglement and decoherence of a micromechanical resonator via coupling to a Cooper box
We analyse the quantum dynamics of a micromechanical resonator capacitively
coupled to a Cooper box. With appropriate quantum state control of the Cooper
box, the resonator can be driven into a superposition of spatially separated
states. The Cooper box can also be used to probe the environmentally-induced
decoherence of the resonator superposition state.Comment: 4 pages, 3 figure
Quantum master equation descriptions of a nanomechanical resonator coupled to a single-electron transistor
We analyse the quantum dynamics of a nanomechanical resonator coupled to a
normal-state single-electron transistor (SET). Starting from a microscopic
description of the system, we derive a master equation for the SET island
charge and resonator which is valid in the limit of weak electro-mechanical
coupling. Using this master equation we show that, apart from brief transients,
the resonator always behaves like a damped harmonic oscillator with a shifted
frequency and relaxes into a thermal-like steady state. Although the behaviour
remains qualitatively the same, we find that the magnitude of the resonator
damping rate and frequency shift depend very sensitively on the relative
magnitudes of the resonator period and the electron tunnelling time. Maximum
damping occurs when the electrical and mechanical time-scales are the same, but
the frequency shift is greatest when the resonator moves much more slowly than
the island charge. We then derive reduced master equations which describe just
the resonator dynamics. By making slightly different approximations, we obtain
two different reduced master equations for the resonator. Apart from minor
differences, the two reduced master equations give rise to a consistent picture
of the resonator dynamics which matches that obtained from the master equation
including the SET island charge.Comment: 22 pages, 4 figure
Variational calculations for the hydrogen-antihydrogen system with a mass-scaled Born-Oppenheimer potential
The problem of proton-antiproton motion in the --
system is investigated by means of the variational method. We introduce a
modified nuclear interaction through mass-scaling of the Born-Oppenheimer
potential. This improved treatment of the interaction includes the nondivergent
part of the otherwise divergent adiabatic correction and shows the correct
threshold behavior.
Using this potential we calculate the vibrational energy levels with angular
momentum 0 and 1 and the corresponding nuclear wave functions, as well as the
S-wave scattering length. We obtain a full set of all bound states together
with a large number of discretized continuum states that might be utilized in
variational four-body calculations. The results of our calculations gives an
indication of resonance states in the hydrogen-antihydrogen system
Pulsar Searches with the SKA
The Square Kilometre Array will be an amazing instrument for pulsar
astronomy. While the full SKA will be sensitive enough to detect all pulsars in
the Galaxy visible from Earth, already with SKA1, pulsar searches will discover
enough pulsars to increase the currently known population by a factor of four,
no doubt including a range of amazing unknown sources. Real time processing is
needed to deal with the 60 PB of pulsar search data collected per day, using a
signal processing pipeline required to perform more than 10 POps. Here we
present the suggested design of the pulsar search engine for the SKA and
discuss challenges and solutions to the pulsar search venture.Comment: 4 pages, 1 figure. To be published in Proceedings of IAU Symposium
337: Pulsar Astrophysics - The Next 50 Year
1987: Abilene Christian College Bible Lectures - Full Text
THE MIND OF CHRIST
Being the Abilene Christian University Annual Bible Lectures 1987
Published by A.C.U. Press
1634 Campus Court Abilene, Texas 7960
Low frequency current noise of the single-electron shuttle
Coupling between electronic and mechanical degrees of freedom in a single
electron shuttle system can cause a mechanical instability leading to shuttle
transport of electrons between external leads. We predict that the resulting
low frequency current noise can be enhanced due to amplitude fluctuations of
the shuttle oscillations. Moreover, at the onset of mechanical instability a
pronounced peak in the low frequency noise is expected.Comment: 14 pages, 3 figures, 1 tabl
Generic entanglement generation, quantum statistics and complementarity
A general and an arbitrarily efficient scheme for entangling the spins (or
any spin-like degree of freedom) of two independent uncorrelated identical
particles by a combination of two particle interferometry and which way
detection is formulated. It is shown that the same setup could be used to
identify the quantum statistics of the incident particles from either the sign
or the magnitude of measured spin correlations. Our setup also exhibits a
curious complementarity between particle distinguishability and the amount of
generated entanglement.Comment: To appear in Phys. Rev. Let
Non-Abelian dynamics and heavy multiquarks, Steiner-tree confinement in hadron spectroscopy
A brief review is first presented of attempts to predict stable multiquark
states within current models of hadron spectroscopy. Then a model combining
flip-flop and connected Steiner trees is introduced and shown to lead to stable
multiquarks, in particular for some configurations involving several heavy
quarks and bearing exotic quantum numbers.Comment: 8 pages, 5 figures, Invited talk at the 21st European Conference on
Few-Body Problems in Physics, Salamanca, Spain, August 29th--September 3rd,
2010, to appear in the Proceedings, ed.~A.~Valcarce et al., to appear in
Few-Body Syste
- …