Qubit reset beyond the Born-Markov approximation: optimal driving to overcome polaron formation

TL;DR

This paper explores advanced qubit reset techniques beyond standard approximations, using optimal control to mitigate polaron formation and improve reset fidelity in a spin-boson model of a transmon qubit.

Contribution

It introduces numerically exact methods and optimal control strategies to overcome polaron-related limitations in qubit reset beyond the Born-Markov approximation.

Findings

Optimal control improves reset fidelity beyond traditional limits.

Filtering the environment's spectral range enhances control effectiveness.

Time-dependent driving can reverse polaron formation and steer correlations.

Abstract

Qubits are typically reset into a known state by coupling them to a low-temperature environment. When treated in the Born-Markov approximation such couplings produce exponential relaxation to equilibrium, giving high reset fidelities limited only by temperature. We investigate qubit reset beyond this approximation, using numerically exact tensor network methods and the time-dependent variational principle, focussing on a spin-boson model describing a transmon qubit coupled to a resistor. Beyond the Born-Markov approximation the reset fidelity becomes limited by the buildup of system-environment correlations which corresponds to the formation of a polaron. We implement numerical optimal control to find time-dependent qubit Hamiltonians which overcome this limitation by steering the dynamics of the correlated system-environment state. The optimal controls becomes more effective when the environment is filtered to span a smaller spectral range, and remain effective when the multilevel nature of the transmon is considered. A related paper [C. Ortega-Taberner, E. O'Neill and P. R. Eastham, arXiv:XXXX.XXXX] addresses the complementary case of control via a time-dependent system-environment coupling. Our results show how limitations on reset speed and fidelity can be overcome, and how time-dependent driving can steer system-environment correlations and reverse polaron formation.