209 research outputs found
Unusual decoherence in qubit measurements with a Bose-Einstein condensate
We consider an electrostatic qubit located near a Bose-Einstein condensate
(BEC) of noninteracting bosons in a double-well potential, which is used for
qubit measurements. Tracing out the BEC variables we obtain a simple analytical
expression for the qubit's density-matrix. The qubit's evolution exhibits a
slow () damping of the qubit's coherence term, which however
turns to be a Gaussian one in the case of static qubit. This stays in contrast
to the exponential damping produced by most classical detectors. The
decoherence is, in general, incomplete and strongly depends on the initial
state of the qubit.Comment: 5 pages, additional explanations related to experimental realization
are added, typos corrected, Phys. Rev. A, in pres
Rate equations for quantum transport in multi-dot systems
Starting with the many-body Schr\"odinger equation we derive new rate
equations for resonant transport in quantum dots linked by ballistic channels
with high density of states. The charging and the Pauli exclusion principle
effects were taken into account. It is shown that the current in such a system
displays quantum coherence effects, even if the dots are away one from another.
A comparative analysis of quantum coherence effects in coupled and separated
dots is presented. The rate equations are extended for description of coherent
and incoherent transport in arbitrary multi-dot systems. It is demonstrated
that new rate equations constitute a generalization of the well-known optical
Bloch equations.Comment: Results are presented in more transparent way. Additional
explanations and figures are included. To appear in Phys. Rev.
Relaxation and Zeno effect in qubit measurements
We consider a qubit interacting with its environment and continuously
monitored by a detector represented by a point contact. Bloch-type equations
describing the entire system of the qubit, the environment and the detector are
derived. Using these equations we evaluate the detector current and its noise
spectrum in terms of the decoherence and relaxation rates of the qubit. Simple
expressions are obtained that show how these quantities can be accurately
measured. We demonstrate that due to interaction with the environment, the
measurement can never localize a qubit even for infinite decoherence rate.Comment: some clarifications added, to appear in Phys. Rev. Let
Resonant scattering on impurities in the Quantum Hall Effect
We develop a new approach to carrier transport between the edge states via
resonant scattering on impurities, which is applicable both for short and long
range impurities. A detailed analysis of resonant scattering on a single
impurity is performed. The results are used for study of the inter-edge
transport by multiple resonant hopping via different impurities' sites. It is
shown that the total conductance can be found from an effective Schroedinger
equation with constant diagonal matrix elements in the Hamiltonian, where the
complex non-diagonal matrix elements are the amplitudes of a carrier hopping
between different impurities. It is explicitly demonstrated how the complex
phase leads to Aharonov-Bohm oscillations in the total conductance. Neglecting
the contribution of self-crossing resonant-percolation trajectories, one finds
that the inter-edge carrier transport is similar to propagation in
one-dimensional system with off-diagonal disorder. We demonstrated that each
Landau band has an extended state , while all other states are
localized. The localization length behaves as .Comment: RevTex 41 pages; 3 Postscript figure on request; Final version
accepted for publication in Phys. Rev. B. A new section added contained a
comparison with other method
Effect of the measurement on the decay rate of a quantum system
We investigated the electron tunneling out of a quantum dot in the presence
of a continuous monitoring by a detector. It is shown that the Schr\"odinger
equation for the whole system can be reduced to new Bloch-type rate equations
describing the time-development of the detector and the measured system at
once. Using these equations we find that the continuous measurement of the
unstable system does not affect its exponential decay, ,
contrary to expectations based on the Quantum Zeno effect . However, the width
of the energy distribution of the tunneling electron is no more , but
increases due to the decoherence, generated by the detector.Comment: Additional explanations are added. Accepted for publications in Phys.
Rev. Let
Quantum master equation approach to quantum transport through mesoscopic systems
For quantum transport through mesoscopic system, a quantum master equation
approach is developed in terms of compact expressions for the transport current
and the reduced density matrix of the system. The present work is an extension
of Gurvitz's approach for quantum transport and quantum measurement, namely, to
finite temperature and arbitrary bias voltage. Our derivation starts from a
second-order cummulant expansion of the tunneling Hamiltonian, then follows
conditional average over the electrode reservoir states. As a consequence, in
the usual weak tunneling regime, the established formalism is applicable for a
wide range of transport problems. The validity of the formalism and its
convenience in application are well illustrated by a number of examples.Comment: 8 pages, 1 figure; with considerable extension of the previous
version submitted in September 2004; to appear in Phys. Rev.
Relativistic approaches to structure functions of nuclei
We employ a propagator technique to derive a new relativistic 1/\qq
expansion of the structure function of a nucleus, composed of point-nucleons.
We exploit non-relativistic features of low-momentum nucleons in the target and
only treat relativistically the nucleon after absorption of a high-momentum
virtual photon. The new series permits a 3-dimensional reduction of each term
and a formal summation of all Final State Interaction terms. We then show that
a relativistic structure function can be obtained from its non-relativistic
analog by a mere change of a scaling variable and an addition of an energy
shift. We compare the obtained result with an ad hoc generalized
Gersch-Rodriguez-Smith theory, previously used in computations of nuclear
structure functions.Comment: Comparison with data is included, to be published in PRC, Feb. 200
Influence of measurement on the life-time and the line-width of unstable systems
We investigate the quantum Zeno effect in the case of electron tunneling out
of a quantum dot in the presence of continuous monitoring by a detector. It is
shown that the Schr\"odinger equation for the whole system can be reduced to
Bloch-type rate equations describing the combined time-development of the
detector and the measured system. Using these equations we find that continuous
measurement of the unstable system does not affect its exponential decay to a
reservoir with a constant density of states. The width of the energy
distribution of the tunneling electron, however, is not equal to the inverse
life-time -- it increases due to the decoherence generated by the detector. We
extend the analysis to the case of a reservoir described by an energy dependent
density of states, and we show that continuous measurement of such quantum
systems affects both the exponential decay rate and the energy distribution.
The decay does not always slow down, but might be accelerated. The energy
distribution of the tunneling electron may reveal the lines invisible before
the measurement.Comment: 13 pages, 8 figures, comments and references added; to appear in
Phys. Rev.
- âŠ