Surpassing the shot-noise limit by homodyne-mediated feedback

TL;DR

This paper demonstrates that homodyne-mediated feedback can significantly enhance the quantum Fisher information of two driven, damped qubits, surpassing the shot-noise limit and reaching the Heisenberg limit under certain conditions.

Contribution

It introduces a feedback scheme that improves steady-state quantum Fisher information, surpassing classical limits and achieving Heisenberg scaling in realistic qubit systems.

Findings

Steady QFI can be enhanced with homodyne feedback.

Shot-noise limit can be surpassed in both symmetric and nonsymmetric feedback.

Heisenberg limit achievable with appropriate parameters.

Abstract

Entangled systems with large quantum Fisher information (QFI) can be used to outperform the standard quantum limit of the separable systems in quantum metrology. However, the interaction between the system and the environments inevitably leads to decoherence and decrease of the QFI, and it is not clear whether the entanglement systems can be better resource than separable systems in the realistic physical condition. In this work, we study the steady QFI of two driven and collectively damped qubits with homodyne-mediated feedback. We show that the steady QFI can be significantly enhanced both in the cases of symmetric feedback and nonsymmetric feedback, and the shot-noise limit of separable states can be surpassed in both cases. The QFI can even achieve the Heisenberg limit for appropriate feedback parameters and initial conditions in the case of symmetric feedback. We also show that an initial-condition-independent steady QFI can be obtained by using nonsymmetric feedback.