208 research outputs found
Universal oscillations in counting statistics
Noise is a result of stochastic processes that originate from quantum or
classical sources. Higher-order cumulants of the probability distribution
underlying the stochastic events are believed to contain details that
characterize the correlations within a given noise source and its interaction
with the environment, but they are often difficult to measure. Here we report
measurements of the transient cumulants > of the number n of passed
charges to very high orders (up to m=15) for electron transport through a
quantum dot. For large m, the cumulants display striking oscillations as
functions of measurement time with magnitudes that grow factorially with m.
Using mathematical properties of high-order derivatives in the complex plane we
show that the oscillations of the cumulants in fact constitute a universal
phenomenon, appearing as functions of almost any parameter, including time in
the transient regime. These ubiquitous oscillations and the factorial growth
are system-independent and our theory provides a unified interpretation of
previous theoretical studies of high-order cumulants as well as our new
experimental data.Comment: 19 pages, 4 figures, final version as published in PNA
Full counting statistics of nano-electromechanical systems
We develop a theory for the full counting statistics (FCS) for a class of
nanoelectromechanical systems (NEMS), describable by a Markovian generalized
master equation. The theory is applied to two specific examples of current
interest: vibrating C60 molecules and quantum shuttles. We report a numerical
evaluation of the first three cumulants for the C60-setup; for the quantum
shuttle we use the third cumulant to substantiate that the giant enhancement in
noise observed at the shuttling transition is due to a slow switching between
two competing conduction channels. Especially the last example illustrates the
power of the FCS.Comment: 7 pages, 3 figures; minor changes - final version as published in
Europhys. Let
Delayed feedback control in quantum transport
Feedback control in quantum transport has been predicted to give rise to
several interesting effects, amongst them quantum state stabilisation and the
realisation of a mesoscopic Maxwell's daemon. These results were derived under
the assumption that control operations on the system be affected
instantaneously after the measurement of electronic jumps through it. In this
contribution I describe how to include a delay between detection and control
operation in the master equation theory of feedback-controlled quantum
transport. I investigate the consequences of delay for the state-stabilisation
and Maxwell's-daemon schemes. Furthermore, I describe how delay can be used as
a tool to probe coherent oscillations of electrons within a transport system
and how this formalism can be used to model finite detector bandwidth.Comment: 13 pages, 5 figure
Current and current fluctuations in quantum shuttles
We review the properties of electron shuttles, i.e. nanoelectromechanical
devices that transport electrons one-by-one by utilizing a combination of
electronic and mechanical degrees of freedom. We focus on the extreme quantum
limit, where the mechanical motion is quantized. We introduce the main
theoretical tools needed for the analysis, e.g. generalized master equations
and Wigner functions, and we outline the methods how the resulting large
numerical problems can be handled. Illustrative results are given for current,
noise, and full counting statistics for a number of model systems. Throughout
the review we focus on the physics behind the various approximations, and some
simple examples are given to illustrate the theoretical concepts. We also
comment on the experimental situation.Comment: Minireview; technical level aimed at general audience, based on an
invited talk at "Transport Phenomena in Micro and Nanodevices", October 17-21
Kona, Hawai
Three-level mixing and dark states in transport through quantum dots
We consider theoretically the transport through the double quantum dot
structure of the recent experiment of C. Payette {\it et al.} [Phys. Rev. Lett.
{\bf 102}, 026808 (2009)] and calculate stationary current and shotnoise.
Three-level mixing gives rise to a pronounced current suppression effect, the
character of which charges markedly with bias direction. We discuss these
results in connexion with the dark states of coherent population trapping in
quantum dots.Comment: 6 pages, 5 fig
The influence of charge detection on counting statistics
We consider the counting statistics of electron transport through a double
quantum dot with special emphasis on the dephasing induced by a nearby charge
detector. The double dot is embedded in a dissipative enviroment, and the
presence of electrons on the double dot is detected with a nearby quantum point
contact. Charge transport through the double dot is governed by a non-Markovian
generalized master equation. We describe how the cumulants of the current can
be obtained for such problems, and investigate the difference between the
dephasing mechanisms induced by the quantum point contact and the coupling to
the external heat bath. Finally, we consider various open questions of
relevance to future research.Comment: 15 pages, 2 figures, Contribution to 5-th International Conference on
Unsolved Problems on Noise, Lyon, France, June 2-6, 200
Non-equilibrium Entanglement and Noise in Coupled Qubits
We study charge entanglement in two Coulomb-coupled double quantum dots in
thermal equilibrium and under stationary non-equilibrium transport conditions.
In the transport regime, the entanglement exhibits a clear switching threshold
and various limits due to suppression of tunneling by Quantum Zeno localisation
or by an interaction induced energy gap. We also calculate quantum noise
spectra and discuss the inter-dot current correlation as an indicator of the
entanglement in transport experiments.Comment: 4 pages, 4 figure
Measurement of finite-frequency current statistics in a single-electron transistor
Electron transport in nano-scale structures is strongly influenced by the
Coulomb interaction which gives rise to correlations in the stream of charges
and leaves clear fingerprints in the fluctuations of the electrical current. A
complete understanding of the underlying physical processes requires
measurements of the electrical fluctuations on all time and frequency scales,
but experiments have so far been restricted to fixed frequency ranges as
broadband detection of current fluctuations is an inherently difficult
experimental procedure. Here we demonstrate that the electrical fluctuations in
a single electron transistor (SET) can be accurately measured on all relevant
frequencies using a nearby quantum point contact for on-chip real-time
detection of the current pulses in the SET. We have directly measured the
frequency-dependent current statistics and hereby fully characterized the
fundamental tunneling processes in the SET. Our experiment paves the way for
future investigations of interaction and coherence induced correlation effects
in quantum transport.Comment: 7 pages, 3 figures, published in Nature Communications (open access
Frequency-dependent counting statistics in interacting nanoscale conductors
We present a formalism to calculate finite-frequency current correlations in
interacting nanoscale conductors. We work within the n-resolved density matrix
approach and obtain a multi-time cumulant generating function that provides the
fluctuation statistics, solely from the spectral decomposition of the
Liouvillian. We apply the method to the frequency-dependent third cumulant of
the current through a single resonant level and through a double quantum dot.
Our results, which show that deviations from Poissonian behaviour strongly
depend on frequency, demonstrate the importance of finite-frequency
higher-order cumulants in fully characterizing interactions.Comment: 4 pages, 2 figures, improved figures & discussion. J-ref adde
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