Path integral derivation of Bloch-Redfield equations for a qubit weakly coupled to a heat bath: Application to nonadiabatic transitions

TL;DR

This paper derives Bloch-Redfield equations for a driven spin-boson system using path integrals, enabling analysis of nonadiabatic transitions and dissipation effects in qubits interacting with heat baths.

Contribution

It provides the first derivation of exact integro-differential equations for the driven spin-boson model and simplifies them to Bloch-Redfield equations for weak damping, applied to nonadiabatic transitions.

Findings

Dissipation effects decrease with increased passage speed.

Bloch-Redfield equations accurately describe the system's dynamics.

Fast qubit switching minimizes environmental dissipation.

Abstract

Quantum information processing has greatly increased interest in the phenomenon of environmentally-induced decoherence. The spin boson model is widely used to study the interaction between a spin-modelling a quantum particle moving in a double well potential-and its environment-modelled by a heat bath of harmonic oscillators. This paper extends a previous analysis of the static spin boson study to the driven spin boson case, with the derivation of an exact integro-differential equation for the time evolution of the propagator of the reduced spin density matrix. This is the first main result. By specializing to weak damping we then obtain the next result, a set of Bloch-Redfield equations for the equilibrium fixed spin initial condition. Finally we show that these equations can be used to solve the classic dissipative Landau-Zener problem and illustrate these solutions for the weak damping case. The effect of dissipation is seen to be minimised as the speed of passage is increased, implying that qubits need to be switched as fast as possible.