Decoherence due to an excited state quantum phase transition in a two-level boson model

TL;DR

This paper investigates how a qubit's decoherence is affected by its environment undergoing excited state quantum phase transitions, highlighting that continuous transitions significantly impact decoherence behavior.

Contribution

It introduces a mean field approach to analyze decoherence caused by environment phase transitions, emphasizing the effect of continuous transitions on qubit fidelity.

Findings

Decoherence is notably affected only by continuous excited state phase transitions.

Maximum decoherence occurs when the environment reaches the critical point of the transition.

Decoherence factor scales with system size at the critical point.

Abstract

The decoherence induced on a single qubit by its interaction with the environment is studied. The environment is modelled as a scalar two-level boson system that can go through either first order or continuous excited state quantum phase transitions, depending on the values of the control parameters. A mean field method based on the Tamm-Damkoff approximation is worked out in order to understand the observed behaviour of the decoherence. Only the continuous excited state phase transition produces a noticeable effect in the decoherence of the qubit. This is maximal when the system-environment coupling brings the environment to the critical point for the continuous phase transition. In this situation, the decoherence factor (or the fidelity) goes to zero with a finite size scaling power law.