Fighting Decoherence by Feedback-controlled Dissipation

TL;DR

This paper presents a feedback-controlled dissipation method that uses measurement-based feedback to purify quantum states and combat decoherence, applicable even at high reservoir temperatures without detailed knowledge of decoherence channels.

Contribution

It introduces a feedback control approach that effectively purifies quantum states through repeated measurements and feedback, without needing precise decoherence models.

Findings

Single-qubit state purification via measurement tuning.

Bell state purification with continuous two-local measurements.

Method effective at high reservoir temperatures.

Abstract

Repeated closed-loop control operations acting as piecewise-constant Liouville superoperators conditioned on the outcomes of regularly performed measurements may effectively be described by a fixed-point iteration for the density matrix. Even when all Liouville superoperators point to the completely mixed state, feedback of the measurement result may lead to a pure state, which can be interpreted as selective dampening of undesired states. Using a microscopic model, we exemplify this for a single qubit, which can be purified in an arbitrary single-qubit state by tuning the measurement direction and two qubits that may be purified towards a Bell state by applying a special continuous two-local measurement. The method does not require precise knowledge of decoherence channels and works for large reservoir temperatures provided measurement, processing, and control can be implemented in a continuous fashion.