# Gravitational wave detection beyond the standard quantum limit using a negative-mass spin system and virtual rigidity

## TL;DR

This paper proposes a refined method for gravitational wave detection that surpasses the standard quantum limit by linking a standard interferometer to a negative-mass spin system using virtual rigidity, enabling quantum noise evasion without modifying core optics.

## Contribution

It introduces a feasible scheme matching susceptibilities of the interferometer and spin system via virtual rigidity, improving quantum noise evasion in gravitational wave detectors.

## Key findings

- Achieves quantum noise reduction beyond the standard quantum limit.
- Requires only phase adjustments, no core optics modifications.
- Feasible with current state-of-the-art spin systems.

## Abstract

Gravitational wave detectors (GWDs), which have brought about a new era in astronomy, have reached such a level of maturity that further improvement necessitates quantum-noise-evading techniques. Numerous proposals to this end have been discussed in the literature, e.g., invoking frequency-dependent squeezing or replacing the current Michelson interferometer topology by that of the quantum speedmeter. Recently, a proposal based on the linking of a standard interferometer to a negative-mass spin system via entangled light has offered an unintrusive and small-scale new approach to quantum noise evasion in GWDs [Phys. Rev. Lett. $\mathbf{121}$, 031101 (2018)]. The solution proposed therein does not require modifications to the highly refined core optics of the present GWD design and, when compared to previous proposals, is less prone to losses and imperfections of the interferometer. In the present article, we refine this scheme to an extent that the requirements on the auxiliary spin system are feasible with state-of-the-art implementations. This is accomplished by matching the effective (rather than intrinsic) susceptibilities of the interferometer and spin system using the virtual rigidity concept, which, in terms of implementation, requires only suitable choices of the various homodyne, probe, and squeezing phases.