185 research outputs found
Mach-Zehnder interferometry with periodic voltage pulses
We investigate a Mach-Zehnder interferometer driven by a time-dependent
voltage. Motivated by recent experiments, we focus on a train of Lorentzian
voltage pulses which we compare to a sinusoidal and a constant voltage. We
discuss the visibilities of Aharonov-Bohm oscillations in the current and in
the noise. For the current, we find a strikingly different behavior in the
driven as compared to the static case for voltage pulses containing multiple
charges. For pulses containing fractional charges, we find a universality at
path-length differences equal to multiples of the spacing between the voltage
pulses. These observations can be explained by the electronic energy
distribution of the driven contact. In the noise oscillations, we find
additional features which are characteristic to time-dependent transport.
Finite electronic temperatures are found to have a qualitatively different
influence on the current and the noise.Comment: Published version; 11 pages, 5 figure
Electron waiting times in coherent conductors are correlated
We evaluate the joint distributions of electron waiting times in coherent
conductors described by scattering theory. Successive electron waiting times in
a single-channel conductor are found to be correlated due to the fermionic
statistics encoded in the many-body state. Our formalism allows us also to
investigate the waiting times between charge transfer events in different
outgoing channels. As an application we consider a quantum point contact in a
chiral setup with one or both input channels biased by either a static or a
time-dependent periodic voltage described by Floquet theory. The theoretical
framework developed here can be applied to a variety of scattering problems and
can in a straightforward manner be extended to joint distributions of several
electron waiting times.Comment: 14 pages, 7 figure
Electron waiting times for the mesoscopic capacitor
We evaluate the distribution of waiting times between electrons emitted by a
driven mesoscopic capacitor. Based on a wave packet approach we obtain analytic
expressions for the electronic waiting time distribution and the joint
distribution of subsequent waiting times. These semi-classical results are
compared to a full quantum treatment based on Floquet scattering theory and
good agreement is found in the appropriate parameter ranges. Our results
provide an intuitive picture of the electronic emissions from the driven
mesoscopic capacitor and may be tested in future experiments.Comment: 11 pages, 7 figures, invited contribution to special issue in Physica
E on "Frontiers in quantum electronic transport - in memory of Markus
B\"uttiker
On-demand entanglement generation using dynamic single-electron sources
We review our recent proposals for the on-demand generation of entangled
few-electron states using dynamic single-electron sources. The generation of
entanglement can be traced back to the single-electron entanglement produced by
quantum point contacts acting as electronic beam splitters. The coherent
partitioning of a single electron leads to entanglement between the two
outgoing arms of the quantum point contact. We describe our various approaches
for generating and certifying entanglement in dynamic electronic conductors and
we quantify the influence of detrimental effects such as finite electronic
temperatures and other dephasing mechanisms. The prospects for future
experiments are discussed and possible avenues for further developments are
identified.Comment: Published version, 11 pages, 7 figures, short review for focus issue
on 'Single-electron control in solid-state devices'. in Phys. Status Solidi B
(2016
Quasi-probability distributions for observables in dynamic systems
We develop a general framework to investigate fluctuations of non-commuting observables. To this end, we consider the Keldysh quasi-probability distribution (KQPD). This distribution provides a measurement-independent description of the observables of interest and their time-evolution. Nevertheless, positive probability distributions for measurement outcomes can be obtained from the KQPD by taking into account the effect of measurement back-action and imprecision. Negativity in the KQPD can be linked to an interference effect and acts as an indicator for non-classical behavior. Notable examples of the KQPD are the Wigner function and the full counting statistics, both of which have been used extensively to describe systems in the absence as well as in the presence of a measurement apparatus. Here we discuss the KQPD and its moments in detail and connect it to various time-dependent problems including weak values, fluctuating work, and Leggett-Garg inequalities. Our results are illustrated using the simple example of two subsequent, non-commuting spin measurements
Quantum Thermal Machine as a Thermometer
We propose the use of a quantum thermal machine for low-temperature
thermometry. A hot thermal reservoir coupled to the machine allows for
simultaneously cooling the sample while determining its temperature without
knowing the model-dependent coupling constants. In its most simple form, the
proposed scheme works for all thermal machines which perform at Otto efficiency
and can reach Carnot efficiency. We consider a circuit QED implementation which
allows for precise thermometry down to 15 mK with realistic parameters.
Based on the quantum Fisher information, this is close to the optimal
achievable performance. This implementation demonstrates that our proposal is
particularly promising in systems where thermalization between different
components of an experimental setup cannot be guaranteed.Comment: Main text: 5 pages, 4 figures; Supplement: 5 page
Optimal work extraction from quantum states by photo-assisted Cooper pair tunneling
The theory of quantum thermodynamics predicts fundamental bounds on work
extraction from quantum states. As these bounds are derived in a very general
and abstract setting, it is unclear how relevant they are in an experimental
context, where control is typically limited. Here we address this question by
showing that optimal work extraction is possible for a realistic engine. The
latter consists of a superconducting circuit, where a LC-resonator is coupled
to a Josephson junction. The oscillator state fuels the engine, providing
energy absorbed by Cooper pairs, thus producing work in the form of an
electrical current against an external voltage bias. We show that this machine
can extract the maximal amount of work from all Gaussian and Fock states.
Furthermore, we consider work extraction from a continuously stabilized
oscillator state. In both scenarios, coherence between energy eigenstates is
beneficial, increasing the power output of the machine. This is possible
because the phase difference across the Josephson junction provides a phase
reference.Comment: Published versio
Superfluid drag of two-species Bose-Einstein condensates in optical lattices
We study two-species Bose-Einstein condensates in quasi two-dimensional
optical lattices of varying geometry and potential depth. Based on the
numerically exact Bloch and Wannier functions obtained using the plane-wave
expansion method, we quantify the drag (entrainment coupling) between the
condensate components. This drag originates from the (short range)
inter-species interaction and increases with the kinetic energy. As a result of
the interplay between interaction and kinetic energy effects, the
superfluid-drag coefficient shows a non-monotonic dependence on the lattice
depth. To make contact with future experiments, we quantitatively investigate
the drag for mass ratios corresponding to relevant atomic species.Comment: 6 pages, 4 figures. Accepted in its original form but minor changes
have been don
Markovian master equations for quantum thermal machines: local vs global approach
The study of quantum thermal machines, and more generally of open quantum
systems, often relies on master equations. Two approaches are mainly followed.
On the one hand, there is the widely used, but often criticized, local
approach, where machine sub-systems locally couple to thermal baths. On the
other hand, in the more established global approach, thermal baths couple to
global degrees of freedom of the machine. There has been debate as to which of
these two conceptually different approaches should be used in situations out of
thermal equilibrium. Here we compare the local and global approaches against an
exact solution for a particular class of thermal machines. We consider
thermodynamically relevant observables, such as heat currents, as well as the
quantum state of the machine. Our results show that the use of a local master
equation is generally well justified. In particular, for weak inter-system
coupling, the local approach agrees with the exact solution, whereas the global
approach fails for non-equilibrium situations. For intermediate coupling, the
local and the global approach both agree with the exact solution and for strong
coupling, the global approach is preferable. These results are backed by
detailed derivations of the regimes of validity for the respective approaches.Comment: Published version. See also the related work by J. Onam Gonzalez et
al. arXiv:1707.0922
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