157 research outputs found
Charge-Relaxation and Dwell Time in the fluctuating Admittance of a Chaotic Cavity
We consider the admittance of a chaotic quantum dot, capacitively coupled to
a gate and connected to two electron reservoirs by multichannel ballistic point
contacts. For a dot in the regime of weak-localization and universal
conductance fluctuations, we calculate the average and variance of the
admittance using random-matrix theory. We find that the admittance is governed
by two time-scales: the classical admittance depends on the RC-time of the
quantum dot, but the relevant time scale for the weak-localization correction
and the admittance fluctuations is the dwell time. An extension of the circular
ensemble is used for a statistical description of the energy dependence of the
scattering matrix.Comment: 7 pages, RevTeX, 1 figur
Parity meter for charge qubits: an efficient quantum entangler
We propose a realization of a charge parity meter based on two double quantum
dots alongside a quantum point contact. Such a device is a specific example of
the general class of mesoscopic quadratic quantum measurement detectors
previously investigated by Mao et al. [Phys. Rev. Lett. 93, 056803 (2004)]. Our
setup accomplishes entangled state preparation by a current measurement alone,
and allows the qubits to be effectively decoupled by pinching off the parity
meter. Two applications of the parity meter are discussed: the measurement of
Bell's inequality in charge qubits and the realization of a controlled NOT
gate.Comment: 8 pages, 4 figures; v2: discussion of measurement time, references
adde
Distribution of parametric conductance derivatives of a quantum dot
The conductance G of a quantum dot with single-mode ballistic point contacts
depends sensitively on external parameters X, such as gate voltage and magnetic
field. We calculate the joint distribution of G and dG/dX by relating it to the
distribution of the Wigner-Smith time-delay matrix of a chaotic system. The
distribution of dG/dX has a singularity at zero and algebraic tails. While G
and dG/dX are correlated, the ratio of dG/dX and is independent
of G. Coulomb interactions change the distribution of dG/dX, by inducing a
transition from the grand-canonical to the canonical ensemble. All these
predictions can be tested in semiconductor microstructures or microwave
cavities.Comment: 4 pages, RevTeX, 3 figure
Transmission time of wave packets through tunneling barriers
The transmission of wave packets through tunneling barriers is studied in
detail by the method of quantum molecular dynamics. The distribution function
of the times describing the arrival of a tunneling packet in front of and
behind a barrier and the momentum distribution function of the packet are
calculated. The behavior of the average coordinate of a packet, the average
momentum, and their variances is investigated. It is found that under the
barrier a part of the packet is reflected and a Gaussian barrier increases the
average momentum of the transmitted packet and its variance in momentum space.Comment: 23 pages, 5 figure
Wave attenuation model for dephasing and measurement of conditional times
Inelastic scattering induces dephasing in mesoscopic systems. An analysis of
previous models to simulate inelastic scattering in such systems is presented
and also a relatively new model based on wave attenuation is introduced. The
problem of Aharonov-Bohm(AB) oscillations in conductance of a mesoscopic ring
is studied. We have shown that conductance is symmetric under flux reversal and
visibility of AB oscillations decay to zero as function of the incoherence
parameter, signalling dephasing. Further wave attenuation is applied to a
fundamental problem in quantum mechanics, i.e., the
conditional(reflection/transmission) times spent in a given region of space by
a quantum particle before scattering off from that region.Comment: 8 pages, 6 figures. Based on presentations by A. M. J and C. B at the
2nd Winter Institute on Foundations of Quantum theory, Quantum Optics and QIP
held at S N Bose National Centre for Basic Sciences, Kolkata, India, from
January 2-11, 200
Is there a renormalization of the 1D conductance in Luttinger Liquid model?
Properties of 1D transport strongly depend on the proper choice of boundary
conditions. It has been frequently stated that the Luttinger Liquid (LL)
conductance is renormalized by the interaction as . To
contest this result I develop a model of 1D LL wire with the interaction
switching off at the infinities. Its solution shows that there is no
renormalization of the universal conductance while the electrons have a free
behavior in the source and drain reservoirs.Comment: 5 pages, RevTex 2.0, attempted repair of tex error
Shot noise suppression in multimode ballistic Fermi conductors
We have derived a general formula describing current noise in multimode
ballistic channels connecting source and drain electrodes with Fermi electron
gas. In particular (at ), the expression describes the
nonequilibrium ''shot'' noise, which may be suppressed by both Fermi
correlations and space charge screening. The general formula has been applied
to an approximate model of a 2D nanoscale, ballistic MOSFET. At large negative
gate voltages, when the density of electrons in the channel is small, shot
noise spectral density approaches the Schottky value , where
is the average current. However, at positive gate voltages, when the
maximum potential energy in the channel is below the Fermi level of the
electron source, the noise can be at least an order of magnitude smaller than
the Schottky value, mostly due to Fermi effects.Comment: 4 page
Edge state transmission, duality relation and its implication to measurements
The duality in the Chalker-Coddington network model is examined. We are able
to write down a duality relation for the edge state transmission coefficient,
but only for a specific symmetric Hall geometry. Looking for broader
implication of the duality, we calculate the transmission coefficient in
terms of the conductivity and in the diffusive
limit. The edge state scattering problem is reduced to solving the diffusion
equation with two boundary conditions
and
.
We find that the resistances in the geometry considered are not necessarily
measures of the resistivity and () holds only
when is quantized. We conclude that duality alone is not sufficient
to explain the experimental findings of Shahar et al and that Landauer-Buttiker
argument does not render the additional condition, contrary to previous
expectation.Comment: 16 pages, 3 figures, to appear in Phys. Rev.
Quantum Force in Superconductor
Transitions between states with continuous (called as classical state) and
discrete (called as quantum state) spectrum of permitted momentum values is
considered. The persistent current can exist along the ring circumference in
the quantum state in contrast to the classical state. Therefore the average
momentum can changes at the considered transitions. In order to describe the
reiterated switching into and out the quantum state an additional term is
introduced in the classical Boltzmann transport equation. The force inducing
the momentum change at the appearance of the persistent current is called as
quantum force. It is shown that dc potential difference is induced on ring
segments by the reiterated switching if the dissipation force is not
homogeneous along the ring circumference. The closing of the superconducting
state in the ring is considered as real example of the transition from
classical to quantum stateComment: 4 pages, RevTex, 0 figure
Zero Frequency Current Noise for the Double Tunnel Junction Coulomb Blockade
We compute the zero frequency current noise numerically and in several limits
analytically for the coulomb blockade problem consisting of two tunnel
junctions connected in series. At low temperatures over a wide range of
voltages, capacitances, and resistances it is shown that the noise measures the
variance in the number of electrons in the region between the two tunnel
junctions. The average current, on the other hand, only measures the mean
number of electrons. Thus, the noise provides additional information about
transport in these devices which is not available from measuring the current
alone.Comment: 33 pages, 10 figure
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