Nonunitary geometric phases: A qubit coupled to an environment with random noise

Abstract

We describe the decoherence process induced on a two-level quantum system in direct interaction with a nonequilibrium environment. The nonequilibrium feature is represented by a nonstationary random function corresponding to the fluctuating transition frequency between two quantum states coupled to the surroundings. In this framework, we compute the decoherence factors which have a characteristic "dip" related to the initial phases of the bath modes. We therefore study different types of environments, namely, ohmic and supraohmic. These environments present different decoherence time scales than the thermal environment we used to study. As a consequence, we compute analytically and numerically the nonunitary geometric phase for the qubit in a quasicyclic evolution under the presence of these particular nonequilibrium environments. We show in which cases decoherence effects could, in principle, be controlled in order to perform a measurement of the geometric phase using standard procedures.