Nonadiabatic electron pumping: maximal current with minimal noise

Boosting thermoelectric efficiency using time-dependent control

Thermoelectric efficiency is defined as the ratio of power delivered to the load of a device to the rate of heat flow from the source. Till date, it has been studied in presence of thermodynamic constraints set by the Onsager reciprocal relation and the second law of thermodynamics that severely bottleneck the thermoelectric efficiency. In this study, we propose a pathway to bypass these constraints using a time-dependent control and present a theoretical framework to study dynamic thermoelectric transport in the far from equilibrium regime. The presence of a control yields the sought after substantial efficiency enhancement and importantly a significant amount of power supplied by the control is utilised to convert the wasted-heat energy into useful-electric energy. Our findings are robust against nonlinear interactions and suggest that external time-dependent forcing, which can be incorporated with existing devices, provides a beneficial scheme to boost thermoelectric efficiency.

Nonadiabatic effect on the quantum heat flux control

We provide a general formula of quantum transfer that includes the nonadiabatic effect under periodic environmental modulation by using full counting statistics in Hilbert-Schmidt space. Applying the formula to an anharmonic junction model that interacts with two bosonic environments within the Markovian approximation, we find that the quantum transfer is divided into the adiabatic (dynamical and geometrical phases) and nonadiabatic contributions. This extension shows the dependence of quantum transfer on the initial condition of the anharmonic junction just before the modulation, as well as the characteristic environmental parameters such as interaction strength and cut-off frequency of spectral density. We show that the nonadiabatic contribution represents the reminiscent effect of past modulation including the transition from the initial condition of the anharmonic junction to a steady state determined by the very beginning of the modulation. This enables us to tune the frequency range of modulation, whereby we can obtain the quantum flux corresponding to the geometrical phase by setting the initial condition of the anharmonic junction.

Characterization of qubit dephasing by Landau-Zener-Stückelberg-Majorana interferometry

Controlling coherent interaction at avoided crossings and the dynamics there is at the heart of quantum information processing. A particularly intriguing dynamics is observed in the Landau-Zener regime, where periodic passages through the avoided crossing result in an interference pattern carrying information about qubit properties. In this Letter, we demonstrate a straightforward method, based on steady-state experiments, to obtain all relevant information about a qubit, including complex environmental influences. We use a two-electron charge qubit defined in a lateral double quantum dot as test system and demonstrate a long coherence time of T2 ≃ 200 ns, which is limited by electron-phonon interaction.

Phase-dependent electron transport through a quantum wire on a surface

Electron transport through a quantum wire in the presence of external periodic energy-level modulations with different on-site phases is studied within the time evolution operator method for a tight-binding Hamiltonian. It is found that in the presence of spatial symmetry of the system and no source-drain and static gate voltages the pumping current can be generated. Moreover, for a wire which is tunnel-coupled to the underlying substrate, the current flowing through an unbiased wire does not fade away but increases with the wire-surface coupling. For randomly chosen phases at every wire site two regimes of the phase-averaged current are found which are related to small and high wire density of states.

Pumped shot noise in adiabatically modulated graphene-based double-barrier structures

Quantum pumping processes are accompanied by considerable quantum noise. Based on the scattering approach, we investigated the pumped shot noise properties in adiabatically modulated graphene-based double-barrier structures. It is found that compared with the Poisson processes, the pumped shot noise is dramatically enhanced where the dc pumped current changes flow direction, which demonstrates the effect of the Klein paradox.

Driving particle current through narrow channels using a classical pump

We study a symmetric exclusion process in which the hopping rates at two chosen adjacent sites vary periodically in time and have a relative phase difference. This mimics a colloidal suspension subjected to external time-dependent modulation of the local chemical potential. The two special sites act as a classical pump by generating an oscillatory current with a nonzero dc value whose direction depends on the applied phase difference. We analyze various features in this model through simulations and obtain an expression for the dc current via a novel perturbative treatment.

Two simple models of classical heat pumps

Motivated by recent studies of models of particle and heat quantum pumps, we study similar simple classical models and examine the possibility of heat pumping. Unlike many of the usual ratchet models of molecular engines, the models we study do not have particle transport. We consider a two-spin system and a coupled oscillator system which exchange heat with multiple heat reservoirs and which are acted upon by periodic forces. The simplicity of our models allows accurate numerical and exact solutions and unambiguous interpretation of results. We demonstrate that while both our models seem to be built on similar principles, one is able to function as a heat pump (or engine) while the other is not.