24 research outputs found
Coulomb effects in tunneling through a quantum dot stack
Tunneling through two vertically coupled quantum dots is studied by means of
a Pauli master equation model. The observation of double peaks in the
current-voltage characteristic in a recent experiment is analyzed in terms of
the tunnel coupling between the quantum dots and the coupling to the contacts.
Different regimes for the emitter chemical potential indicating different peak
scenarios in the tunneling current are discussed in detail. We show by
comparison with a density matrix approach that the interplay of coherent and
incoherent effects in the stationary current can be fully described by this
approach.Comment: 6 pages, 6 figure
Shot noise of coupled semiconductor quantum dots
The low-frequency shot noise properties of two electrostatically coupled
semiconductor quantum dot states which are connected to emitter/collector
contacts are studied. A master equation approach is used to analyze the bias
voltage dependence of the Fano factor as a measure of temporal correlations in
tunneling current caused by Pauli's exclusion principle and the Coulomb
interaction. In particular, the influence of the Coulomb interaction on the
shot noise behavior is discussed in detail and predictions for future
experiments will be given. Furthermore, we propose a mechanism for negative
differential conductance and investigate the related super-Poissonian shot
noise.Comment: submitted to PR
Reverse quantum state engineering using electronic feedback loops
We propose an all-electronic technique to manipulate and control interacting
quantum systems by unitary single-jump feedback conditioned on the outcome of a
capacitively coupled electrometer and in particular a single-electron
transistor. We provide a general scheme to stabilize pure states in the quantum
system and employ an effective Hamiltonian method for the quantum master
equation to elaborate on the nature of stabilizable states and the conditions
under which state purification can be achieved. The state engineering within
the quantum feedback scheme is shown to be linked with the solution of an
inverse eigenvalue problem. Two applications of the feedback scheme are
presented in detail: (i) stabilization of delocalized pure states in a single
charge qubit and (ii) entanglement stabilization in two coupled charge qubits.
In the latter example we demonstrate the stabilization of a maximally entangled
Bell state for certain detector positions and local feedback operations.Comment: 23 pages, 6 figures, to be published by New Journal of Physics (2013
Transport Statistics of Interacting Double Dot Systems: Coherent and Non-Markovian Effects
We formalize the derivation of a generalized coarse-graining -resolved
master equation by introducing a virtual detector counting the number of
transferred charges in single-electron transport. Our approach enables the
convenient inclusion of coherences and Lamb shift in counting statistics. As a
Markovian example with Lindblad-type density matrices, we consider the
Born-Markov-Secular (BMS) approximation which is a special case of the
non-Markovian dynamical coarse graining (DCG) approach. For illustration we
consider transport through two interacting levels that are either serially or
parallelly coupled to two leads held at different chemical potentials. It is
shown that the coherences can strongly influence the (frequency-dependent)
transport cumulants: In the serial case the neglect of coherences would lead to
unphysical currents through disconnected conductors. Interference effects in
the parallel setup can cause strong current suppression with giant Fano factors
and telegraph-like distribution functions of transferred electrons, which is
not found without coherences. We demonstrate that with finite coarse graining
times coherences are automatically included and, consequently, the shortcomings
of the BMS approximation are resolved.Comment: 24 pages, 18 figures, updated references, reviewer suggestions
included, to appear in PR
Influence of a Random Telegraph Process on the Transport through a Point Contact
We describe the transport properties of a point contact under the influence
of a classical two-level fluctuator. We employ a transfer matrix formalism
allowing us to calculate arbitrary correlation functions of the stochastic
process by mapping them on matrix products. The result is used to obtain the
generating function of the full counting statistics of a classical point
contact subject to a classical fluctuator, including extensions to a pair of
two-level fluctuators as well as to a quantum point contact. We show that the
noise in the quantum point contact is a sum of the (quantum) partitioning noise
and the (classical) noise due to the two-level fluctuator. As a side result, we
obtain the full counting statistics of a quantum point contact with
time-dependent transmission probabilities.Comment: 8 pages, 2 figure; a new section about experiments and a figure
showing the crossover from sub- to superpoissonian noise have been adde
Current-voltage correlations in interferometers
We investigate correlations of current at contacts and voltage fluctuations
at voltage probes coupled to interferometers. The results are compared with
correlations of current and occupation number fluctuations at dephasing probes.
We use a quantum Langevin approach for the average quantities and their
fluctuations. For higher order correlations we develop a stochastic path
integral approach and find the generating functions of voltage or occupation
number fluctuations. We also derive a generating function for the joint
distribution of voltage or occupation number at the probe and current
fluctuations at a terminal of a conductor. For energy independent scattering we
found earlier that the generating function of current cumulants in
interferometers with a one-channel dephasing or voltage probe are identical.
Nevertheless, the distribution function for voltage and the distribution
function for occupation number fluctuations differ, the latter being broader
than that of former in all examples considered here.Comment: 23 pages, 10 figures, minor changes, additional appendix, added
reference
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
Theory of Nonlinear Transport for Ensembles of Quantum Dots
This article reviews our work on the description of electronic transport through self-assembled quantum dots. Our main interest is in the effect of Coulomb interaction on quantum dot charging (capacitance-voltage characteristics), on the average current (current-voltage characteristics), and on current fluctuations (quantum shot noise) in quantum dot layers embedded in pn- or resonant tunneling devices. Our studies show the particular importance of understanding those interaction mechanisms for future device applications
Comparative studies of pelagic microbial methane oxidation within the redox zones of the Gotland Deep and Landsort Deep (central Baltic Sea)
Pelagic methane oxidation was investigated in dependence on differing
hydrographic conditions within the redox zone of the Gotland Deep (GD) and
Landsort Deep (LD), central Baltic Sea. The redox zone of both deeps, which
indicates the transition between oxic and anoxic conditions, was
characterized by a pronounced methane concentration gradient between the deep
water (GD: 1233 nM, 223 m; LD: 2935 nM, 422 m) and the surface water (GD
and LD 13C CH4
enrichment (δ13C CH4 deep water: GD −84‰, LD
−71‰; redox zone: GD −60‰, LD −20‰;
surface water: GD −47‰, LD −50‰; δ13C
CH4 vs. Vienna Pee Dee Belemnite standard), clearly indicating microbial
methane consumption within the redox zone. Expression analysis of the methane
monooxygenase identified one active type I methanotrophic bacterium in
both redox zones. In contrast, the turnover of methane within the redox zones
showed strong differences between the two basins (GD: max. 0.12 nM d−1, LD: max. 0.61 nM d−1), with a nearly four-times-lower turnover
time of methane in the LD (GD: 455 d, LD: 127 d). Vertical mixing rates for
both deeps were calculated on the base of the methane concentration profile
and the consumption of methane in the redox zone (GD:
2.5 × 10–6 m2 s−1, LD:
1.6 × 10–5 m2 s−1). Our study identified vertical
transport of methane from the deep-water body towards the redox zone as well
as differing hydrographic conditions (lateral intrusions and vertical mixing)
within the redox zone of these deeps as major factors that determine the
pelagic methane oxidation