Open Quantum Devices Research Articles

Detection of the Quantum Capacitance of a Point Contact via Dispersive Gate Sensing

The technique of dispersive gate sensing (DGS) uses a single electrode to readout a qubit by detecting the change in quantum capacitance due to single electron tunnelling. Here, we extend DGS from the detection of discrete tunnel events to the open regime, where many electrons are transported via partially- or fully-transmitting quantum modes. Comparing DGS with conventional transport shows that the technique can resolve the Van Hove singularities of a one-dimensional ballistic system, and also probe aspects of the potential landscape that are not easily accessed with dc transport. Beyond readout, these results suggest that gate-sensing can also be of use in tuning-up qubits or probing the charge configuration of open quantum devices in the regime where electrons are delocalized.

Synthesizing and controlling helical indirect exchange interactions out of equilibrium

We study the nonequilibrium effects of spin and/or electric currents on the helical indirect exchange interactions of local spins that embedded in general open electronic systems. Especially, besides the synthesized anisotropic Heisenberg interactions, we find that the synthetic helical indirect exchange interactions possess two parts: antisymmetric (Dzyaloshinskii-Moriya interaction) and symmetric (Kaplan-Shekhtman-Entin-Wohlman-Aharony interaction), which are all formulated in terms of Keldysh nonequilibrium Green's functions. The presence of either spin-orbit coupling or spin polarized currents alone is able to synthesize and control the antisymmetric Dzyaloshinskii-Moriya exchange interactions, as the same direction as spin splitting. However, the appearance of symmetric Kaplan-Shekhtman-Entin-Wohlman-Aharony interactions requires both, i.e., the spin-orbit coupling and spin polarized currents with different splitting directions. Our results show the detailed scheme of controlling the sign, magnitude, and direction of indirect Dzyaloshinskii-Moriya vectors and Kaplan-Shekhtman-Entin-Wohlman-Aharony interactions at nonequilibrium in open quantum devices.

Self-consistent quantum transport theory: Applications and assessment of approximate models

We have implemented a fully self-consistent non-equilibrium Green’s function approach for vertical quantum transport in open quantum devices with contacts and study theoretically quantum well heterostructures, resonant tunneling diodes and quantum cascade laser structures in this formalism. We systematically investigate the role and consequences of several widely used approximations such as decoupling the equations for the scattering states and their occupation, neglect of inelastic scattering, and neglect of nonlocal scattering self-energies.

Modeling of open quantum devices within the closed-system paradigm

We present an alternative simulation strategy for the study of nonequilibrium carrier dynamics in quantum devices with open boundaries. We propose to replace the usual modeling of open quantum systems based on phenomenological injection/loss rates with a kinetic description of the system-reservoir thermalization process. In this simulation scheme the partial carrier thermalization induced by the device spatial boundaries is treated within the standard Boltzmann-transport approach via an effective scattering mechanism between the highly nonthermal device electrons and the thermal carrier distribution of the reservoir. Applications to state-of-the-art semiconductor nanostructures are discussed. Finally, the proposed approach is extended to the quantum-transport regime; to this end, we introduce an effective Liouville superoperator, able to describe the effect of the device spatial boundaries on the time evolution of the single-particle density matrix.

Generalized Weyl–Wigner formalism for the simulation of open quantum devices: a density-matrix approach

A density-matrix approach for the description of quantum-transport phenomena in systems with open boundaries is proposed. We shall show that the application of the conventional Wigner-function formalism within a source-term boundary-condition scheme leads to unphysical results, such as injection of coherent electronic states from the contacts. To overcome such basic limitations, we propose a generalization of the standard Weyl–Wigner formulation. The latter can be regarded as a quantum mechanical derivation of the phenomenological injection model commonly employed in the simulation of open quantum devices.

Monte Carlo simulation of hot-carrier phenomena in open quantum devices: A kinetic approach

An alternative simulation strategy for the study of nonequilibrium carrier dynamics in quantum devices with open boundaries is presented. In particular, we propose replacing the usual modeling of open quantum systems based on phenomenological injection/loss rates with a kinetic description of the system-reservoir thermalization process. More specifically, in this simulation scheme the partial carrier thermalization induced by the device spatial boundaries is treated within the standard Monte Carlo approach via an effective scattering mechanism between the highly nonthermal device electrons and the thermal carrier distribution of the reservoir. The proposed simulation strategy is applyed to state-of-the-art semiconductor nanostructures.

Microscopic Modelling of Quantum Open Systems: A Generalized Wigner-Function Approach

A microscopic approach for the simulation of quantum-transport phenomena in systems with open boundaries is proposed. We show that the application of the conventional Wigner-function formalism to this problem leads to unphysical results, like injection of coherent electronic states from the contacts. To overcome such basic limitation, we propose a generalization of the standard Wigner-function formulation, able to properly describe the incoherent nature of carrier injection at the device spatial boundaries. The latter constitutes a quantum mechanical derivation of the phenomenological injection model commonly employed in the simulation of open quantum devices.

On the problem of generalizing the semiconductor Bloch equation from a closed to an open system

A microscopic theory for the description of quantum-transport phenomena in systems with open boundaries is proposed. We shall show that the application of the conventional Wigner-function formalism to this problem leads to unphysical results, such as injection of coherent electronic states from the contacts. To overcome such basic limitation, we propose a generalization of the standard Wigner-function formulation, able to properly describe the incoherent nature of carrier injection at the device spatial boundaries as well as the interplay between phase coherence and energy relaxation/dephasing within the device active region. The proposed theoretical scheme constitutes a quantum-mechanical derivation of the phenomenological injection model commonly employed in the simulation of open quantum devices.

Open boundary conditions for a time-dependent analysis of the resonant tunneling structure

Exact boundary conditions for open quantum devices with constant potential asymptotics are presented and implemented into a numerical treatment of the one-dimensional time-dependent Schrödinger equation. A time-dependent analysis of the resonant tunneling double-barrier (RTDB) structure demonstrates their superiority over previous approaches. Our self-consistent study reveals microscopic details of the transient current in the system under an external time-dependent bias. Two time regimes characterize the charge transport in a RTDB in response to an applied bias. The ultrafast response of the system occurs in the contact regions within approximately 100 fs. This response time is of the order of the inverse bulk plasma frequency and is essentially independent of the applied bias. The second time regime is characterized by current oscillations which sensitively depend on the applied bias and structural parameters.