Biexponential decay and ultralong coherence of a qubit

Abstract

A quantum two-state system, weakly coupled to a heat bath, is traditionally studied in the Born-Markov regime under the secular approximation with completely positive linear master equations. Despite its success, this microscopic approach exclusively predicts exponential decays and Lorentzian susceptibility profiles, in disagreement with a number of experimental findings. On the contrary, in the absence of the secular approximation they can be explained but with the risk of jeopardizing the positivity of the density matrix. To avoid these drawbacks, we use a physically motivated nonlinear master equation being both thermodynamically and statistically consistent. We find that, beyond a temperature-dependent threshold, a bifurcation in the decoherence time T 2 takes place; it gives rise to a biexponential decay and a susceptibility profile being neither Gaussian nor Lorentzian. This implies that, for suitable initial states, a major prolongation of the coherence can be obtained in agreement with recent experiments. Moreover, T 2 is no longer limited by the energy relaxation time T 1 offering novel perspectives to elaborate devices for quantum information processing.