Appearance and disappearance of quantum correlations in measurement-based feedback control of a mechanical oscillator

TL;DR

This paper investigates how measurement-based feedback control can enhance or diminish quantum correlations in a nano-optomechanical system, revealing fundamental limits and potential for manipulating quantum states.

Contribution

It demonstrates experimentally how feedback can control quantum correlations, showing both enhancement and suppression effects in a nano-optomechanical system.

Findings

Feedback increases the visibility of sideband asymmetry.

Noise squashing reduces the sideband asymmetry visibility.

Quantum correlations can be manipulated via measurement-based feedback.

Abstract

Quantum correlations between imprecision and back-action are a hallmark of continuous linear measurements. Here we study how measurement-based feedback can be used to improve the visibility of quantum correlations due to the interaction of a laser field with a nano-optomechanical system. Back-action imparted by the meter laser, due to radiation pressure quantum fluctuations, gives rise to correlations between its phase and amplitude quadratures. These quantum correlations are observed in the experiment both as squeezing of the meter field fluctuations below the vacuum level in a homodyne measurement, and as sideband asymmetry in a heterodyne measurement, demonstrating the common origin of both phenomena. We show that quantum feedback, i.e. feedback that suppresses measurement back-action, can be used to increase the visibility of the sideband asymmetry ratio. In contrast, by operating the feedback loop in the regime of noise squashing, where the in-loop photocurrent variance is reduced below the vacuum level, the visibility of the sideband asymmetry is reduced. This is due to feedback back-action arising from vacuum noise in the homodyne detector. These experiments demonstrate the possibility, as well as the fundamental limits of measurement-based feedback as a tool to manipulate quantum correlations.