Dynamics of periodic anticrossings: Decoherence, pointer states and hysteresis curves

TL;DR

This paper studies a driven two-level quantum system interacting with an environment, analyzing decoherence, pointer states, and hysteresis, with implications for quantum information and nanomagnet applications.

Contribution

It introduces a Markovian dynamical model without the rotating wave approximation to describe the system's evolution towards Floquet states under decoherence.

Findings

System evolves towards Floquet states regardless of initial conditions

Hysteresis curves vary qualitatively with temperature

Landau-Zener transitions remain useful for state preparation at moderate temperatures

Abstract

We consider a strongly driven two-level (spin) system, with a periodic external field that induces a sequence of avoided level crossings. The spin system interacts with a bosonic reservoir which leads to decoherence. A Markovian dynamical equation is introduced without relying on the rotating wave approximation in the system-external field interaction. We show that the time evolution of the two-level system is directed towards an incoherent sum of periodic Floquet states regardless of the initial state and even the type of the coupling to the environment. Analyzing the time scale of approaching these time-dependent pointer states, information can be deduced concerning the nature and strength of the system-environment coupling. The inversion as a function of the external field is usually multi-valued, and the form of these hysteresis curves is qualitatively different for low and high temperatures. For moderate temperatures we found that the series of Landau-Zener-St\"{u}ckelberg-type transitions still can be used for state preparation, regardless of the decoherence rate. Possible applications include quantum information processing and molecular nanomagnets.