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. To leave this limited paradigm, we use a phenomenological positive 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.

Zero-temperature limit of thermodynamic quantum master equations

“…Even for T = 1, the deviation from the zero-temperature result is very small. In view of the initial condition (14), the real and imaginary parts of ρ 10 t in Fig. 5 start at α = 0.1 and 0, respectively, and approach zero by damped oscillations.…”

“…The final step of the development of thermodynamic quantum master equations has been made in [5], where the setting has been generalized so that the linear Davies Lindblad master equations [13] are contained as a special class of thermodynamic master equations. It has been shown in [14] that the nonlinear quantum master equations proposed in [5] have the potential to explain ultralong coherence of a qubit.…”

Spin State Dynamics in a Bichromatic Microwave Field: Role of Bright and Dark States in coupling with Reservoir

“…These resonances can be regarded as Fourier transforms of multiexponential decays of populations and coherences of spin-states which represent decoherence and relaxation. In a more fundamental context, such behavior reflects nonlinear and nonmarkovian dynamics of open quantum systems coupled to a heat bath (reservoir) [32,33]. This Letter presents alternative approach to the issue of multiexponent decays and reduction of relaxation by lineshape analysis of composite hole-burning resonances, rather than by measurements of time evolution.…”