Control of a qubit under Markovian and non-Markovian noise

An essential action in quantum information processing is the manipulation (control) of a single qubit, ideally a closed two-level system. However, in realistic applications, quantum processes are often under the influence of the external environment, e.g., presenting some degree of dissipation and decoherence. In this paper we address the emerging difficulties in the (tracking) quantum control of a two-level system under the influence of both Markovian and non-Markovian noise. We employ a same framework to treat both situations, a Lindblad-type equation, but considering that for the former (latter) case, the decay rate ⠂ is time independent (dependent). We discuss the conditions leading to a breakdown of the quantum control and eventual ways to overcome the problem, like employing a fast control scheme or controlling the off-diagonal terms of the system density matrix. Surprisingly, for Markovian noise such breakdown time decreases with ⠂ not as an exponential but as a power law. This indicates that the quantum control should be possible for a coupling between the system and the environment stronger than previously expected. Moreover, we find that for non-Markovian noise, the breakdown time is longer when there is backflow, i.e., ⠂(t) can be negative. The present theoretical results point to certain favorable scenarios to operate qubits even in a noisy medium.

Automated summary

This paper discusses the quantum control problem of a two-level system under the influence of both Markovian and non-Markovian noise, and proposes solutions such as employing a fast control scheme or controlling the off-diagonal terms of the system density matrix. The experimental results reveal that the breakdown time of quantum control is related to the coupling strength under Markovian noise, and is longer when there is backflow under non-Markovian noise.