Effective environments: Preparation of stationary states with inverse temperature ranging from positive to negative values

TL;DR

This paper demonstrates how periodic measurements on a spin environment can effectively prepare a two-level quantum system in a wide range of thermal states, including negative temperatures and inverted populations.

Contribution

It introduces a method to control the thermal state of a TLS using measurement-induced relaxation with adjustable parameters, valid for small and large environments.

Findings

Able to prepare states with negative effective temperature.

Analytical results hold for small environments.

Adjusting measurement timing and detuning controls the thermal state.

Abstract

In this paper, we discuss how effective environments incorporating periodic measurements can be used to prepare a two-level system (TLS) in almost arbitrary thermal states: Concretely, we study a TLS coupled to a spin environment, the magnetization of which is measured periodically. In ensemble average these measurements cause a relaxation of the TLS into a thermal (diagonal) state. By adjusting the time between the measurements and the detuning of the environmental spins, the creation of very low temperatures as well as inversion becomes possible. Our analytical results derived for large environments are numerically shown to be valid even for quite small environments, down to only a few spins.