Resonant tunneling in the presence of a two-level fluctuator: Average transparency

Energy conductance across a mesoscopic junction in a nonequilibrium spin-boson model under the influence of telegraph noise

A spin-boson model in the presence of a telegraph noise (TN) source is employed to calculate the energy conductance between a tunnel junction and two bosonic baths. A polaron-transformed coupling term with the bosonic baths allows for treating quantum damping to arbitrary orders of strength. However, the polaron transformation yields a dressed tunneling frequency which is assumed small and treated perturbatively as is familiar in the noninteracting blip approximation in the context of the nonequilibrium spin-boson model. While the coupling with the bosonic baths leads to decoherence in an otherwise coherent tunneling process, the TN induces a different kind of fluctuation, that is, in the asymmetry of the underlying two-level system. It is the interplay of these two different relaxation effects, one triggered by the two quantum (bosonic) baths and the other through a classical bath (creating a TN), that is investigated here in detail. The TN that mimics the classical, fluctuating environment makes a nontrivial contribution to the self-energy that helps compute the imaginary part of the spin susceptibility which, in turn, determines the energy transfer across the junction. The range of validity of the TN is clarified at the outset and its efficacy in tuning the environmental influence is pointed out. The present paper complements an earlier similar study-albeit for fermionic baths-and provides additional input in terms of the TN to a previous investigation of energy transfer between a nanojunction and bosonic reservoirs without, however, the noisy environment.

Non-Gaussian low-frequency noise as a source of qubit decoherence

We study decoherence in a qubit with the distance between the two levels affected by random flips of bistable fluctuators. For the case of a single fluctuator we evaluate explicitly an exact expression for the phase-memory decay in the echo experiment with a resonant ac excitation. The echo signal as a function of time shows a sequence of plateaus. The position and the height of the plateaus can be used to extract the fluctuator switching rate gamma and its coupling strength v. At small times the logarithm of the echo signal is proportional to t3. The plateaus disappear when the decoherence is induced by many fluctuators. In this case the echo signal depends on the distribution of the fluctuators parameters. According to our analysis, the results significantly deviate from those obtained in the Gaussian model as soon as v greater than or approximately equal gamma.