Quantum noise in laser-interferometer gravitational-wave detectors with a heterodyne readout scheme

TL;DR

This paper analyzes quantum noise in laser-interferometer gravitational-wave detectors using heterodyne readout, comparing it to homodyne detection, and discusses implications for optimizing detector sensitivity and quantum non-demolition capabilities.

Contribution

It provides a theoretical framework for evaluating heterodyne versus homodyne readout schemes in advanced gravitational-wave detectors.

Findings

Heterodyne readout can measure multiple quadratures simultaneously.

Heterodyne scheme introduces additional quantum noise.

Cannot achieve broadband quantum non-demolition measurement due to Heisenberg uncertainty.

Abstract

We analyze and discuss the quantum noise in signal-recycled laser interferometer gravitational-wave detectors, such as Advanced LIGO, using a heterodyne readout scheme and taking into account the optomechanical dynamics. Contrary to homodyne detection, a heterodyne readout scheme can simultaneously measure more than one quadrature of the output field, providing an additional way of optimizing the interferometer sensitivity, but at the price of additional noise. Our analysis provides the framework needed to evaluate whether a homodyne or heterodyne readout scheme is more optimal for second generation interferometers from an astrophysical point of view. As a more theoretical outcome of our analysis, we show that as a consequence of the Heisenberg uncertainty principle the heterodyne scheme cannot convert conventional interferometers into (broadband) quantum non-demolition interferometers.