1,411 research outputs found
Bounds to unitary evolution
Upper and lower bounds are established for the survival probability
of a quantum state, in terms of the energy moments
. Introducing a cut-off in the energy generally
enables considerable improvement in these bounds and allows the method to be
used where the exact energy moments do not exist.Comment: 5 pages, 8 figure
Intrinsic leakage of the Josephson flux qubit and breakdown of the two-level approximation for strong driving
Solid state devices for quantum bit computation (qubits) are not perfect
isolated two-level systems, since additional higher energy levels always exist.
One example is the Josephson flux qubit, which consists on a mesoscopic SQUID
loop with three Josephson junctions operated at or near a magnetic flux of half
quantum. We study intrinsic leakage effects, i.e., direct transitions from the
allowed qubit states to higher excited states of the system during the
application of pulses for quantum computation operations. The system is started
in the ground state and rf- magnetic field pulses are applied at the qubit
resonant frequency with pulse intensity . A perturbative calculation of
the average leakage for small is performed for this case, obtaining that
the leakage is quadratic in , and that it depends mainly on the matrix
elements of the supercurrent. Numerical simulations of the time dependent
Schr\"odinger equation corresponding to the full Hamiltonian of this device
were also performed. From the simulations we obtain the value of above
which the two-level approximation breaks down, and we estimate the maximum Rabi
frequency that can be achieved. We study the leakage as a function of the ratio
among the Josephson energies of the junctions of the device, obtaining
the best value for minimum leakage (). The effects of flux
noise on the leakage are also discussed.Comment: Final improved version. Some figures have changed with new results
added. To be published in Phys. Rev.
The quantum Gaussian well
Different features of a potential in the form of a Gaussian well have been
discussed extensively. Although the details of the calculation are involved,
the general approach uses a variational method and WKB approximation,
techniques which should be familiar to advanced undergraduates. A numerical
solution of the Schr\"odinger equation through diagonalization has been
developed in a self-contained way, and physical applications of the potential
are mentioned.Comment: 11 pages, 4 figures, To be published in American Journal of Physic
Quantum Langevin model for exoergic ion-molecule reactions and inelastic processes
We presents a fully quantal version of the Langevin model for the total rate
of exoergic ion-molecule reactions or inelastic processes. The model, which is
derived from a rigorous multichannel quantum-defect formulation of bimolecular
processes, agrees with the classical Langevin model at sufficiently high
temperatures. It also gives the first analytic description of ion-molecule
reactions and inelastic processes in the ultracold regime where the quantum
nature of the relative motion between the reactants becomes important.Comment: 5 pages, 3 figure
The deffect effect on electronic conductance in binomially tailored quantum wire
The paper considers the effect of the defects on the electronic transmission
properties in binomially tailored waveguide quantum wires, in which each Dirac
delta function potential strength have been weight on the binomial distribution
law. We have assumed that a single free-electron channel is incident on the
structure and the scattering of electrons is solely from the geometric nature
of the problem. We have used the transfer matrix method to study the electron
transmission. We found this novel structure has a good defect tolerance. We
found the structure tolerate up to in strength defect and in position defect
for the central Dirac delta function in the binomial distribution. Also, we
found this structure can tolerate both defect up to in strength and in position
dislocationComment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
Singular solutions to the Seiberg-Witten and Freund equations on flat space from an iterative method
Although it is well known that the Seiberg-Witten equations do not admit
nontrivial solutions in flat space, singular solutions to them have been
previously exhibited -- either in or in the dimensionally reduced spaces
and -- which have physical interest. In this work, we employ an
extension of the Hopf fibration to obtain an iterative procedure to generate
particular singular solutions to the Seiberg-Witten and Freund equations on
flat space. Examples of solutions obtained by such method are presented and
briefly discussed.Comment: 7 pages, minor changes. To appear in J. Math. Phy
Forbidden activation levels in a non-stationary tunneling process
Tunneling in the presence of an opaque barrier, part of which varies in time,
is investigated numerically and analytically in one dimension. Clearly, due to
the varying barrier a tunneling particle experiences spectral widening.
However, in the case of strong perturbations, the particles' activation to
certain energies is avoided. We show that this effect occurs only when the
perturbation decays faster than 1/t^2.Comment: 4pages,2 figures (Revtex
Generating Schr\"{o}dinger-cat states in momentum and internal-state space from Bose-Einstein condensates with repulsive interactions
Resonant Raman coupling between internal levels induced by continuous
illumination of non-collinear laser beams can create double-well momentum-space
potentials for multi-level ``periodically-dressed'' atoms. We develop an
approximate many-body formalism for a weakly interacting, trapped
periodically-dressed Bose gas which illustrates how a tunable exchange
interaction yields correlated many-body ground states. In contrast to the case
of a position-space double well, the ground state of stable
periodically-dressed Bose gases with repulsive interactions tends toward a
Schr\"{o}dinger cat state in the regime where interactions dominate the
momentum-space tunnelling induced by the external trapping potential. The
dependence of the momentum-space tunnelling and exchange interaction on
experimental parameters is derived. We discuss how real-time control of
experimental parameters can be used to create Schr\"{o}dinger cat states either
between momentum or internal states, and how these states could be dynamically
controlled towards highly sensitive interferometry and frequency metrology.Comment: 7 pages, 3 figures. Submitted to PR
A fast and robust approach to long-distance quantum communication with atomic ensembles
Quantum repeaters create long-distance entanglement between quantum systems
while overcoming difficulties such as the attenuation of single photons in a
fiber. Recently, an implementation of a repeater protocol based on single
qubits in atomic ensembles and linear optics has been proposed [Nature 414, 413
(2001)]. Motivated by rapid experimental progress towards implementing that
protocol, here we develop a more efficient scheme compatible with active
purification of arbitrary errors. Using similar resources as the earlier
protocol, our approach intrinsically purifies leakage out of the logical
subspace and all errors within the logical subspace, leading to greatly
improved performance in the presence of experimental inefficiencies. Our
analysis indicates that our scheme could generate approximately one pair per 3
minutes over 1280 km distance with fidelity (F>78%) sufficient to violate
Bell's inequality.Comment: 10 pages, 4 figures, 5 tables (Two appendixes are added to justify
two claims used in the maintext.
Gauge dependence of calculations in relativistic Coulomb excitation
Before a quantum-mechanical calculation involving electromagnetic
interactions is performed, a choice must be made of the gauge to be used in
expressing the potentials. If the calculation is done exactly, the observable
results it predicts will be independent of the choice of gauge. However, in
most practical calculations approximations are made, which can destroy the
gauge invariance of the predictions. We compare here the results of
coupled-channel time-dependent relativistic Coulomb excitation calculations, as
performed in either Lorentz or Coulomb gauges. We find significant differences
when the bombarding energy per nucleon is 2 GeV, which indicates that
the common practice of relying completely on the Lorentz gauge can be
dangerous.Comment: 23 pages, 3 figure
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