1,339 research outputs found
Coulomb Blockade due to Quantum Phase-Slips Illustrated with Devices
In order to illustrate the emergence of Coulomb blockade from coherent
quantum phase-slip processes in thin superconducting wires, we propose and
theoretically investigate two elementary setups, or "devices". The setups are
derived from Cooper-pair box and Cooper-pair transistor, so we refer to them as
QPS-box and QPS-transistor, respectively. We demonstrate that the devices
exhibit sensitivity to a charge induced by a gate electrode, this being the
main signature of Coulomb blockade. Experimental realization of these devices
will unambiguously prove the Coulomb blockade as an effect of coherence of
phase-slip processes. We analyze the emergence of discrete charging in the
limit strong phase-slips. We have found and investigated six distinct regimes
that are realized depending on the relation between three characteristic energy
scales: inductive and charging energy, and phase-slip amplitude. For
completeness, we include a brief discussion of dual Josephson-junction devices
Full Current Statistics in the Regime of Weak Coulomb Interaction
We evaluate the full statistics of the current via a Coulomb island that is
strongly coupled to the leads. This strong coupling weakens Coulomb
interaction. We show that in this case the effects of the interaction can be
incorporated into the renormalization of transmission eigenvalues of the
scatterers that connect the island and the leads. We evaluate the Coulomb
blockade gap in the current-voltage characteristics, the value of the gap being
exponentially suppressed as compared to the classical charging energy of the
island.Comment: 4 pages, 3 figure
Inelastic Interaction Corrections and Universal Relations for Full Counting Statistics
We analyze in detail the interaction correction to Full Counting Statistics
(FCS) of electron transfer in a quantum contact originating from the
electromagnetic environment surrounding the contact. The correction can be
presented as a sum of two terms, corresponding to elastic/inelastic electron
transfer. Here we primarily focus on the inelastic correction.
For our analysis, it is important to understand more general -- universal --
relations imposed on FCS only by quantum mechanics and statistics with no
regard for a concrete realization of a contact. So we derive and analyze these
relations. We reveal that for FCS the universal relations can be presented in a
form of detailed balance. We also present several useful formulas for the
cumulants.
To facilitate the experimental observation of the effect, we evaluate
cumulants of FCS at finite voltage and temperature. Several analytical results
obtained are supplemented by numerical calculations for the first three
cumulants at various transmission eigenvalues.Comment: 10 pages, 3 figure
Current fluctuations in composite conductors: Beyond the second cumulant
Employing the non-linear -model we analyze current fluctuations in
coherent composite conductors which contain a diffusive element in-between two
tunnel barriers. For such systems we explicitly evaluate the
frequency-dependent third current cumulant which also determines the leading
Coulomb interaction correction to shot noise. Our predictions can be directly
tested in future experiments.Comment: 6 pages, 1 figur
Coulomb interacting Dirac fermions in disordered graphene
We study interacting Dirac quasiparticles in disordered graphene and find
that an interplay between the unscreened Coulomb interactions and
pseudo-relativistic quasiparticle kinematics can be best revealed in the
ballistic regime, whereas in the diffusive limit the behavior is qualitatively
(albeit, not quantitatively) similar to that of the ordinary 2DEG with
parabolic dispersion. We calculate the quasiparticle width and density of
states that can be probed by photoemission, tunneling, and magnetization
measurements.Comment: Latex, 4 page
Quadratic Quantum Measurements
We develop a theory of quadratic quantum measurements by a mesoscopic
detector. It is shown that quadratic measurements should have non-trivial
quantum information properties, providing, for instance, a simple way of
entangling two non-interacting qubits. We also calculate output spectrum of a
quantum detector with both linear and quadratic response continuously
monitoring coherent oscillations in two qubits.Comment: 5 pages, 2 figure
The Effect of Mechanical Resonance on Josephson Dynamics
We study theoretically dynamics in a Josephson junction coupled to a
mechanical resonator looking at the signatures of the resonance in d.c.
electrical response of the junction. Such a system can be realized
experimentally as a suspended ultra-clean carbon nanotube brought in contact
with two superconducting leads. A nearby gate electrode can be used to tune the
junction parameters and to excite mechanical motion. We augment theoretical
estimations with the values of setup parameters measured in the samples
fabricated.
We show that charging effects in the junction give rise to a mechanical force
that depends on the superconducting phase difference. The force can excite the
resonant mode provided the superconducting current in the junction has
oscillating components with a frequency matching the resonant frequency of the
mechanical resonator. We develop a model that encompasses the coupling of
electrical and mechanical dynamics. We compute the mechanical response (the
effect of mechanical motion) in the regime of phase bias and d.c. voltage bias.
We thoroughly investigate the regime of combined a.c. and d.c. bias where
Shapiro steps are developed and reveal several distinct regimes characteristic
for this effect. Our results can be immediately applied in the context of
experimental detection of the mechanical motion in realistic superconducting
nano-mechanical devices.Comment: 18 pages, 11 figure
Statistics of Measurement of Non-commuting Quantum Variables: Monitoring and Purification of a qubit
We address continuous weak linear quantum measurement and argue that it is
best understood in terms of statistics of the outcomes of the linear detectors
measuring a quantum system, for example, a qubit. We mostly concentrate on a
setup consisting of a qubit and three independent detectors that simultaneously
monitor three noncommuting operator variables, those corresponding to three
pseudospin components of the qubit. We address the joint probability
distribution of the detector outcomes and the qubit variables. When analyzing
the distribution in the limit of big values of the outcomes, we reveal a high
degree of correspondence between the three outcomes and three components of the
qubit pseudospin after the measurement. This enables a high-fidelity monitoring
of all three components. We discuss the relation between the monitoring
described and the algorithms of quantum information theory that use the results
of the partial measurement. We develop a proper formalism to evaluate the
statistics of continuous weak linear measurement. The formalism is based on
Feynman-Vernon approach, roots in the theory of full counting statistics, and
boils down to a Bloch-Redfield equation augmented with counting fields.Comment: 30 pages, 5 figure
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