Decoherence of a qubit due to a quantum fluctuator or to a classical telegraph noise

TL;DR

This paper compares the decoherence effects on a qubit caused by either a quantum two-level system or a classical telegraph noise, highlighting how the ratio of coupling to decoherence rate determines the appropriate model.

Contribution

It introduces a model that interpolates between quantum and classical descriptions of a TLS, analyzing how this affects qubit decoherence and mutual information.

Findings

Decoherence rates depend on the ratio of coupling to TLS decoherence rate.

Classical and quantum models converge at high temperatures.

The model identifies conditions when a classical telegraph process suffices.

Abstract

We investigate the decoherence of a qubit coupled to either a quantum two-level system (TLS) again coupled to an environment, or a classical fluctuator modeled by random telegraph noise. In order to do this we construct a model for the quantum TLS where we can adjust the temperature of its environment, and the decoherence rate independently. The model has a well-defined classical limit at any temperature and this corresponds to the appropriate random telegraph process, which is symmetric at high temperatures and becomes asymmetric at low temperatures. We find that the difference in the qubit decoherence rates predicted by the two models depends on the ratio between the qubit-TLS coupling and the decoherence rate in the pointer basis of the TLS. This is then the relevant parameter which determines whether the TLS has to be treated quantum mechanically or can be replaced by a classical telegraph process. We also compare the mutual information between the qubit and the TLS in the classical and quantum cases.