162 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
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
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
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
Tunable few electron quantum dots in InAs nanowires
Quantum dots realized in InAs are versatile systems to study the effect of
spin-orbit interaction on the spin coherence, as well as the possibility to
manipulate single spins using an electric field. We present transport
measurements on quantum dots realized in InAs nanowires. Lithographically
defined top-gates are used to locally deplete the nanowire and to form
tunneling barriers. By using three gates, we can form either single quantum
dots, or two quantum dots in series along the nanowire. Measurements of the
stability diagrams for both cases show that this method is suitable for
producing high quality quantum dots in InAs.Comment: 8 pages, 4 figure
Bipartite Fluctuations as a Probe of Many-Body Entanglement
We investigate in detail the behavior of the bipartite fluctuations of
particle number and spin in many-body quantum systems,
focusing on systems where such U(1) charges are both conserved and fluctuate
within subsystems due to exchange of charges between subsystems. We propose
that the bipartite fluctuations are an effective tool for studying many-body
physics, particularly its entanglement properties, in the same way that noise
and Full Counting Statistics have been used in mesoscopic transport and cold
atomic gases. For systems that can be mapped to a problem of non-interacting
fermions we show that the fluctuations and higher-order cumulants fully encode
the information needed to determine the entanglement entropy as well as the
full entanglement spectrum through the R\'{e}nyi entropies. In this connection
we derive a simple formula that explicitly relates the eigenvalues of the
reduced density matrix to the R\'{e}nyi entropies of integer order for any
finite density matrix. In other systems, particularly in one dimension, the
fluctuations are in many ways similar but not equivalent to the entanglement
entropy. Fluctuations are tractable analytically, computable numerically in
both density matrix renormalization group and quantum Monte Carlo calculations,
and in principle accessible in condensed matter and cold atom experiments. In
the context of quantum point contacts, measurement of the second charge
cumulant showing a logarithmic dependence on time would constitute a strong
indication of many-body entanglement.Comment: 30 pages + 25 pages supplementary materia
Counting statistics of collective photon transmissions
We theoretically study cooperative effects in the steady-state transmission
of photons through a medium of radiators. Using methods from quantum
transport, we find a cross-over in scaling from to in the current and
even higher powers of in the higher cumulants of the photon counting
statistics as a function of the tunable source occupation. The effect should be
observable for atoms confined within a nano-cell with a pumped optical cavity
as photon source.Comment: extended results, 9 pages, 2 figures, to appear in Annals of Physic
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