Crossover of phase qubit dynamics in presence of negative-result weak measurement

TL;DR

This paper investigates how a superconducting phase qubit's dynamics are affected by negative-result measurements, revealing a transition from stable states to oscillations as driving strength increases, with implications for quantum control.

Contribution

It introduces a detailed analysis of phase qubit dynamics under negative-result measurement, highlighting a crossover from stable to oscillatory behavior with increasing microwave drive.

Findings

Weak driving leads to stable qubit states distinguished by purity and murity.

Beyond a critical drive, the qubit exhibits non-decaying oscillations.

Predictions are experimentally verifiable with existing setups.

Abstract

Coherent dynamics of a superconducting phase qubit is considered in the presence of both unitary evolution due to microwave driving and continuous non-unitary collapse due to negative-result measurement. In the case of a relatively weak driving, the qubit dynamics is dominated by the non-unitary evolution, and the qubit state tends to an asymptotically stable point on the Bloch sphere. This dynamics can be clearly distinguished from conventional decoherence by tracking the state purity and the measurement invariant (``murity''). When the microwave driving strength exceeds certain critical value, the dynamics changes to non-decaying oscillations: any initial state returns exactly to itself periodically in spite of non-unitary dynamics. The predictions can be verified using a modification of a recent experiment.