Photon counting readout for detection and inference of gravitational waves from neutron star merger remnants

TL;DR

This paper investigates photon-counting readout schemes for gravitational wave detectors to improve detection and inference of postmerger signals from neutron star mergers, especially in low signal-to-noise conditions.

Contribution

It introduces photon counting as an effective alternative readout method for detecting and analyzing weak postmerger gravitational wave signals.

Findings

Photon counting can detect signals with SNR as low as 0.2.

Approximately 1 in 100 postmerger signals can be detected via photon counting.

Over 20,000 signals, photon counting improves neutron star radius measurements twofold.

Abstract

Gravitational waves emitted after neutron star binary coalescences and the information they carry about dense matter are a high-priority target for next-generation detectors. Even though such detectors are expected to observe millions of signals, detectable postmerger emission will remain rare. In this work, we explore postmerger detectability and inference through an alternative detector readout scheme for data dominated by quantum-noise, which is the case above $1$\,kHz: photon-counting. In such a readout, signals and noise become quantized into discrete distributions corresponding to the detection of single photons measured in a chosen basis of modes. Through simulated data, we demonstrate that photon counting can be efficient even for weak signals. We find ${\sim}1$ in 100 signals with a postmerger signal-to-noise ratio of 0.2 can result in a single photon and thus be detected. Furthermore, after $2\times10^4$ signals -- equivalent to $10^{-2}$ to $1.5$ years of observation -- photon counting results in a twofold improvement in the measurement of the radius of a $1.6\,M_\odot$ neutron star. Constraints can be further tightened if the detector classical noise is reduced. Photon counting offers a promising alternative to traditional homodyne readout techniques for extracting information from low signal-to-noise ratio postmerger signals.