Optimal control in large open quantum systems: the case of transmon readout and reset

TL;DR

This paper introduces a scalable, efficient framework combining the adjoint-state method and reverse-time backpropagation to optimize control in large open quantum systems, demonstrated on transmon qubit readout and reset.

Contribution

It develops a novel, scalable optimization framework for large open quantum systems and applies it to improve transmon qubit readout and reset protocols.

Findings

Adding a transmon drive improves readout fidelity and duration by 2x.

Pulse shaping enhances reset fidelity and duration by 2x.

The method is applicable to various quantum control problems.

Abstract

We present a framework that combines the adjoint-state method together with reverse-time backpropagation to solve prohibitively large open-system quantum control problems. Our approach enables the optimization of arbitrary cost functions with fully general controls applied on large open quantum systems described by a Lindblad master equation. It is scalable, computationally efficient, and has a low-memory footprint. We apply this framework to optimize two inherently dissipative operations in superconducting qubits which lag behind in terms of fidelity and duration compared to other unitary operations: the dispersive readout and all-microwave reset of a transmon qubit. Our results show that while standard pulses for dispersive readout are nearly optimal, adding a transmon drive during the protocol can yield 2x improvements in fidelity and duration. We further demonstrate a 2x improvement in reset fidelity and duration through pulse shaping, indicating significant potential for enhancement in reset protocols. Our approach can readily be applied to optimize quantum controls in a vast range of applications such as reservoir engineering, autonomous quantum error correction, and leakage-reduction units.