Unraveling of a generalized quantum Markovian master equation and its application in feedback control of a charge qubit

TL;DR

This paper introduces a novel unraveling of a generalized quantum Markovian master equation into quantum trajectories and demonstrates a feedback control method that can sustain coherent oscillations in a charge qubit, surpassing traditional measurement bounds.

Contribution

It presents a new unraveling technique for the generalized quantum Markovian master equation and a feedback control protocol to maintain qubit coherence longer.

Findings

Coherent oscillations can be maintained for arbitrarily long with sufficient feedback.

Feedback improves the detector's signal-to-noise ratio beyond the Korotkov-Averin bound.

The protocol enhances the potential for quantum computation with charge qubits.

Abstract

In the context of a charge qubit under continuous monitoring by a single electron transistor, we propose an unraveling of the generalized quantum Markovian master equation into an ensemble of individual quantum trajectories for stochastic point process. A suboptimal feedback algorism is implemented into individual quantum trajectories to protect a desired pure state. Coherent oscillations of the charge qubit could be maintained in principle for an arbitrarily long time in case of sufficient feedback strength. The effectiveness of the feedback control is also reflected in the detector's noise spectrum. The signal-to-noise ratio rises significantly with increasing feedback strength such that it could even exceed the Korotkov-Averin bound in quantum measurement, manifesting almost ideal quantum coherent oscillations of the qubit. The proposed unraveling and feedback protocol may open up the prospect to sustain ideal coherent oscillations of a charge qubit in quantum computation algorithms.