Splitting time dependence in Lindblad master equation

Tuure Orell

Yi-Siang Wang, Parmeet Nijjar, Xin Zhou +2 more

Motivated by the need to study nonequilibrium evolutions of many-electron systems at the atomistic ab initio level, as they occur in modern devices and applications, we developed a quantum dynamics approach bridging master equations and surface hopping (SH). The Lindblad master equation (LME) allows us to propagate efficiently ensembles of particles, while SH provides nonperturbative evaluation of transition rates that evolve in time and depend explicitly on nuclear geometry. We implemented the LME-SH technique within real-time time-dependent density functional theory using global flux SH, and we demonstrated its efficiency and utility by modeling metallic films, in which charge-phonon dynamics was studied experimentally and showed an unexpectedly strong dependence on adhesion layers. The LME-SH approach provides a general framework for modeling efficiently quantum dynamics in a broad range of complex many-electron condensed-matter and nanoscale systems.

Robert S. Whitney

The perturbative master equation (Bloch–Redfield) is used extensively to study dissipative quantum mechanics—particularly for qubits—despite the 25-year-old criticism that it violates positivity (generating negative probabilities). We take an arbitrary system coupled to an environment containing many degrees-of-freedom and cast its perturbative master equation (derived from a perturbative treatment of Nakajima–Zwanzig or Schoeller–Schön equations) in the form of a Lindblad master equation. We find that the equation's parameters are time dependent . This time dependence is rarely accounted for and invalidates Lindblad's dynamical semigroup analysis. We analyse one such Bloch–Redfield master equation (for a two-level system coupled to an environment with a short but non-vanishing memory time), which apparently violates positivity. We analytically show that, once the time dependence of the parameters is accounted for, positivity is preserved.

Francisco Soto, Braulio Misael Villegas-Martínez, H. M. Moya-Cessa

Paul R. B. Hughes, Marc M. Dignam

We solve the Lindblad master equation for the quantum state of a pumped optomechanical system coupled to a thermal bath. We show that the solution to the state in the linear pumping regime is a beam-split thermal state when pumped on the red sideband of cavity resonance and a two-mode squeezed thermal state when pumped on the blue sideband. The time dependence of each state is fully described by four coupled differential equations. Using this formalism, we describe a process of first cooling the mechanical mode via the red-sideband pump, then entangling that mode using the blue-sideband pump. We find that there is an optimal strength of blue-sideband pumping to drive the correlation variance between the two modes below a desired threshold for a maximum amount of time. This optimal value depends both on the loss rates and equilibrium temperatures of the two modes, and we provide an approximate analytic expression for this relationship. We show that a long entanglement time is achievable, even at relatively high temperatures, as long as the optical loss rate is much higher than the mechanical one.

Marco Falconi, Jérémy Faupin, Jürg Martin Fröhlich +1 more