815 research outputs found
Quantum Nondemolition Measurement of a Kicked Qubit
We propose a quantum nondemolition measurement using a kicked two-state
system (qubit). By tuning the waiting time between kicks to be the qubit
oscillation period, the kicking apparatus performs a nondemolition measurement.
While dephasing is unavoidable, the nondemolition measurement can (1) slow
relaxation of diagonal density matrix elements, (2) avoid detector back-action,
and (3) allow for a large signal-to-noise ratio. Deviations from the ideal
behavior are studied by allowing for detuning of the waiting time, as well as
finite-time, noisy pulses. The scheme is illustrated with a double-dot qubit
measured by a gate-pulsed quantum point contact.Comment: 7 pages, 1 figur
Time-Dependent Current Partition in Mesoscopic Conductors
The currents at the terminals of a mesoscopic conductor are evaluated in the
presence of slowly oscillating potentials applied to the contacts of the
sample. The need to find a charge and current conserving solution to this
dynamic current partition problem is emphasized. We present results for the
electro-chemical admittance describing the long range Coulomb interaction in a
Hartree approach. For multiply connected samples we discuss the symmetry of the
admittance under reversal of an Aharonov-Bohm flux.Comment: 22 pages, 3 figures upon request, IBM RC 1971
Low frequency admittance of a quantum point contact
We present a current and charge conserving theory for the low frequency
admittance of a quantum point contact. We derive expressions for the
electrochemical capacitance and the displacement current. The latter is
determined by the {\em emittance} which equals the capacitance only in the
limit of vanishing transmission. With the opening of channels the capacitance
and the emittance decrease in a step-like manner in synchronism with the
conductance steps. For vanishing reflection, the capacitance vanishes and the
emittance is negative.Comment: 11 pages, revtex file, 2 ps figure
Theory of conductance and noise additivity in parallel mesoscopic conductors
We present a theory of conductance and noise in generic mesoscopic conductors
connected in parallel, and we demonstrate that the additivity of conductance
and of shot noise arises as a sole property of the junctions connecting the two
(or more) conductors in parallel. Consequences on the functionality of devices
based on the Aharonov-Bohm effect are also drawn.Comment: 4 pages, 2 figure
Effect of incoherent scattering on shot noise correlations in the quantum Hall regime
We investigate the effect of incoherent scattering in a Hanbury Brown and
Twiss situation with electrons in edge states of a three-terminal conductor
submitted to a strong perpendicular magnetic field. The modelization of
incoherent scattering is performed by introducing an additional voltage probe
through which the current is kept equal to zero which causes voltage
fluctuations at this probe. It is shown that inelastic scattering can lead in
this framework to positive correlations, whereas correlations remain always
negative for quasi-elastic scattering.Comment: 5 pages latex, 5 eps figure
Charge fluctuations in a quantum point contact attached to a superconducting lead
We show how to calculate the charge noise spectrum in a normal mesoscopic
conductor, which is capacitively coupled to a macroscopic gate, when this
conductor is attached to L normal leads and M superconducting leads, the only
restriction being that the superconducting leads must be at the same chemical
potential. We then proceed to examine results for a quantum point contact (QPC)
in a normal lead connecting to a superconductor. Of interest is the fluctuating
current in a gate capacitively coupled to a QPC. The results are compared with
the case when all leads are normal. We find a doubling of the equilibrium
charge fluctuations and a large enhancement (>2) in the current noise spectrum
to first order in |eV|, when a channel in the QPC is opening.Comment: 4 pages, 3 figure
Charge fluctuations in open chaotic cavities
We present a discussion of the charge response and the charge fluctuations of
mesoscopic chaotic cavities in terms of a generalized Wigner-Smith matrix. The
Wigner-Smith matrix is well known in investigations of time-delay of quantum
scattering. It is expressed in terms of the scattering matrix and its
derivatives with energy. We consider a similar matrix but instead of an energy
derivative we investigate the derivative with regard to the electric potential.
The resulting matrix is then the operator of charge. If this charge operator is
combined with a self-consistent treatment of Coulomb interaction, the charge
operator determines the capacitance of the system, the non-dissipative
ac-linear response, the RC-time with a novel charge relaxation resistance, and
in the presence of transport a resistance that governs the displacement
currents induced into a nearby conductor. In particular these capacitances and
resistances determine the relaxation rate and dephasing rate of a nearby qubit
(a double quantum dot). We discuss the role of screening of mesoscopic chaotic
detectors. Coulomb interaction effects in quantum pumping and in photon
assisted electron-hole shot noise are treated similarly. For the latter we
present novel results for chaotic cavities with non-ideal leads.Comment: 29 pages, 13 figures;v.2--minor changes; contribution for the special
issue of J. Phys. A on "Trends in Quantum Chaotic Scattering
Magnetic-field asymmetry of electron wave packet transmission in bent channels capacitively coupled to a metal gate
We study the electron wave packet moving through a bent channel. We
demonstrate that the packet transmission probability becomes an uneven function
of the magnetic field when the electron packet is capacitively coupled to a
metal plate. The coupling occurs through a non-linear potential which
translates a different kinetics of the transport for opposite magnetic field
orientations into a different potential felt by the scattered electron
Charge densities and charge noise in mesoscopic conductors
We introduce a hierarchy of density of states to characterize the charge
distribution in a mesoscopic conductor. At the bottom of this hierarchy are the
partial density of states which represent the contribution to the local density
of states if both the incident and the out-going scattering channel is
prescribed. The partial density of states play a prominent role in measurements
with a scanning tunneling microscope on multiprobe conductors in the presence
of current flow. The partial density of states determine the degree of
dephasing generated by a weakly coupled voltage probe. In addition the partial
density of states determine the frequency-dependent response of mesoscopic
conductors in the presence of slowly oscillating voltages applied to the
contacts of the sample. The partial density of states permit the formulation of
a Friedel sum rule which can be applied locally. We introduce the off-diagonal
elements of the partial density of states matrix to describe charge fluctuation
processes. This generalization leads to a local Wigner-Smith life-time matrix.Comment: 10 pages, 2 figure
Effect of Interactions on the Admittance of Ballistic Wires
A self-consistent theory of the admittance of a perfect ballistic, locally
charge neutral wire is proposed. Compared to a non-interacting theory,
screening effects drastically change the frequency behavior of the conductance.
In the single-channel case the frequency dependence of the admittance is
monotonic, while for two or more channels collective interchannel excitations
lead to resonant structures in the admittance. The imaginary part of the
admittance is typically positive, but can become negative near resonances.Comment: Presentation considerably modified; the results are unchanged. 4
pages, 2 figures .eps-format include
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