18,013 research outputs found
Optimized design of universal two-qubit gates
We construct optimized implementations of the CNOT and other universal
two-qubit gates that, unlike many of the previously proposed protocols, are
carried out in a single step. The new protocols require tunable inter-qubit
couplings but, in return, show a significant improvements in the quality of
gate operations. Our optimization procedure can be further extended to the
combinations of elementary two-qubit as well as irreducible many-qubit gates.Comment: 6 pages, 2 figure
Violation of the Leggett-Garg Inequality in Neutrino Oscillations
The Leggett-Garg inequality, an analogue of Bell's inequality involving
correlations of measurements on a system at different times, stands as one of
the hallmark tests of quantum mechanics against classical predictions. The
phenomenon of neutrino oscillations should adhere to quantum-mechanical
predictions and provide an observable violation of the Leggett-Garg inequality.
We demonstrate how oscillation phenomena can be used to test for violations of
the classical bound by performing measurements on an ensemble of neutrinos at
distinct energies, as opposed to a single neutrino at distinct times. A study
of the MINOS experiment's data shows a greater than violation over
a distance of 735 km, representing the longest distance over which either the
Leggett-Garg inequality or Bell's inequality has been tested.Comment: Updated to match published version. 6 pages, 2 figure
Mesoscopic Spin-Boson Models of Trapped Ions
Trapped ions arranged in Coulomb crystals provide us with the elements to
study the physics of a single spin coupled to a boson bath. In this work we
show that optical forces allow us to realize a variety of spin-boson models,
depending on the crystal geometry and the laser configuration. We study in
detail the Ohmic case, which can be implemented by illuminating a single ion
with a travelling wave. The mesoscopic character of the phonon bath in trapped
ions induces new effects like the appearance of quantum revivals in the spin
evolution.Comment: 4.4 pages, 5 figure
Sources and budgets for CO and O-3 in the northeastern Pacific during the spring of 2001: Results from the PHOBEA-II Experiment
Abstract. Ground and airborne measurements of CO, ozone, and aerosols were obtained in th
The twist-2 Compton operator and its hidden Wandzura-Wilczek and Callan-Gross relations
Power corrections for virtual Compton scattering at leading twist are
etermined at operator level. From the complete off-cone representation of the
twist-2 Compton operator integral representations for the trace, antisymmetric
and symmetric part of that operator are derived. The operator valued invariant
functions are written in terms of iterated operators and may lead to
interrelations. For matrix elements they go over into relations for generalized
parton distributions. -- Reducing to the s-channel relevant part one gets
operator pre-forms of the Wandzura-Wilczek and the (target mass corrected)
Callan-Gross relations whose structure is exactly the same as known from the
case of deep inelastic scattering; taking non-forward matrix elements one
reproduces earlier results [B. Geyer, D. Robaschik and J. Eilers, Nucl. Phys. B
704 (2005) 279] for the absorptive part of the virtual Compton amplitude. --
All these relations, obtained without any approximation or using equations of
motion, are determined solely by the twist-2 structure of the underlying
operator and, therefore, are purely of geometric origin.Comment: 13 pages, Latex 2e, Introduction shortend, Section Prerequisites
added, more obvious formulations used, some formulas rewritten as well as
added, conclusions extended, references added. Final version as appearing in
PR
Bose Hubbard model in the presence of Ohmic dissipation
We study the zero temperature mean-field phase diagram of the Bose-Hubbard
model in the presence of local coupling between the bosons and an external
bath. We consider a coupling that conserves the on-site occupation number,
preserving the robustness of the Mott and superfluid phases. We show that the
coupling to the bath renormalizes the chemical potential and the interaction
between the bosons and reduces the size of the superfluid regions between the
insulating lobes. For strong enough coupling, a finite value of hopping is
required to obtain superfluidity around the degeneracy points where Mott phases
with different occupation numbers coexist. We discuss the role that such a bath
coupling may play in experiments that probe the formation of the
insulator-superfluid shell structure in systems of trapped atoms.Comment: 5 pages, 2 figures. Error found in v1, now corrected, leads to
qualitative changes in result
Coherent control of population transfer between communicating defects
Population transfer between two identical, communicating defects in a
one-dimensional tight-binding lattice can be systematically controlled by
external time-periodic forcing. Employing a force with slowly changing
amplitude, the time it takes to transfer a particle from one defect to the
other can be altered over several orders of magnitude. An analytical expression
is derived which shows how the forcing effectively changes the energy splitting
between the defect states, and numerical model calculations illustrate the
possibility of coherent control of the transfer.Comment: 7 pages, 6 figures, to appear in Phys. Rev.
Non-adiabatic Josephson Dynamics in Junctions with in-Gap Quasiparticles
Conventional models of Josephson junction dynamics rely on the absence of low
energy quasiparticle states due to a large superconducting gap. With this
assumption the quasiparticle degrees of freedom become "frozen out" and the
phase difference becomes the only free variable, acting as a fictitious
particle in a local in time Josephson potential related to the adiabatic and
non-dissipative supercurrent across the junction. In this article we develop a
general framework to incorporate the effects of low energy quasiparticles
interacting non-adiabatically with the phase degree of freedom. Such
quasiparticle states exist generically in constriction type junctions with high
transparency channels or resonant states, as well as in junctions of
unconventional superconductors. Furthermore, recent experiments have revealed
the existence of spurious low energy in-gap states in tunnel junctions of
conventional superconductors - a system for which the adiabatic assumption
typically is assumed to hold. We show that the resonant interaction with such
low energy states rather than the Josephson potential defines nonlinear
Josephson dynamics at small amplitudes.Comment: 9 pages, 1 figur
Theory of Nonlinear Dispersive Waves and Selection of the Ground State
A theory of time dependent nonlinear dispersive equations of the Schroedinger
/ Gross-Pitaevskii and Hartree type is developed. The short, intermediate and
large time behavior is found, by deriving nonlinear Master equations (NLME),
governing the evolution of the mode powers, and by a novel multi-time scale
analysis of these equations. The scattering theory is developed and coherent
resonance phenomena and associated lifetimes are derived. Applications include
BEC large time dynamics and nonlinear optical systems. The theory reveals a
nonlinear transition phenomenon, ``selection of the ground state'', and NLME
predicts the decay of excited state, with half its energy transferred to the
ground state and half to radiation modes. Our results predict the recent
experimental observations of Mandelik et. al. in nonlinear optical waveguides
The possibility of a metal insulator transition in antidot arrays induced by an external driving
It is shown that a family of models associated with the kicked Harper model
is relevant for cyclotron resonance experiments in an antidot array. For this
purpose a simplified model for electronic motion in a related model system in
presence of a magnetic field and an AC electric field is developed. In the
limit of strong magnetic field it reduces to a model similar to the kicked
Harper model. This model is studied numerically and is found to be extremely
sensitive to the strength of the electric field. In particular, as the strength
of the electric field is varied a metal -- insulator transition may be found.
The experimental conditions required for this transition are discussed.Comment: 6 files: kharp.tex, fig1.ps fig2.ps fi3.ps fig4.ps fig5.p
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