Continuous measurements for control of superconducting quantum circuits

TL;DR

This paper reviews the development and application of continuous quantum measurements and feedback in circuit QED, highlighting their role in enhancing quantum control and error correction in superconducting quantum circuits.

Contribution

It provides a comprehensive overview of continuous measurement techniques and feedback mechanisms in circuit QED, emphasizing recent experimental advances and potential for fault-tolerant quantum computing.

Findings

Experimental demonstrations of continuous measurements in circuit QED

Implementation of quantum feedback for state control

Potential for fault-tolerant error correction

Abstract

Developments over the last two decades have opened the path towards quantum technologies in many quantum systems, such as cold atoms, trapped ions, cavity-quantum electrodynamics (QED), and circuit-QED. However the fragility of quantum states to the effects of measurement and decoherence still poses one of the greatest challenges in quantum technology. An imperative capability in this path is quantum feedback, as it enhances the control possibilities and allows for prolonging coherence times through quantum error correction. While changing parameters from shot to shot of an experiment or procedure can be considered feedback, quantum mechanics also allows for the intriguing possibility of performing feedback operations during the measurement process itself. This broader approach to measurements leads to the concepts of weak measurement, quantum trajectories and numerous types of feedback with no classical analogues. These types of processes are the primary focus of this review. We introduce the concept of quantum feedback in the context of circuit QED, an experimental platform with significant potential in quantum feedback and technology. We then discuss several experiments and see how they elucidate the concepts of continuous measurements and feedback. We conclude with an overview of coherent feedback, with application to fault-tolerant error correction.