Fig. 1: White light signal recycling (WLSR) interferometer configuration using optomechanical negative dispersion.

a Gravitational waves (GW) of frequency Ω modulate the interferometer carrier ω₀, producing sidebands at ω₀ ± Ω. The input test mass (ITM) and signal extraction mirror (SEM) are impedance matched for maximum sideband transmission. The signal recycling mirror (SRM) couples the interferometer dark port, filter cavity and output photodetector. The filter cavity is pumped with light blue detuned by the mechanical resonance ω_m. The cavity itself is 5 cm long but may be contained inside a larger housing. b Illustration of the phononic crystal (PNC) resonator consisting of a silicon nitride membrane in a phononic lattice. The colour scale represents out-of-plane mechanical mode displacement. The inset shows a magnified view of the resonator. (Image acknowledgement: E.C. Langman, Neils Bohr Institute) c Quantum noise-limited sensitivity curves of various GW detectors. “Advanced LIGO” denotes the nominal design sensitivity shown in the ref. ⁴⁹ at 800 kW arm cavity power. “Sloshing SR” refers to a detector where the signal recycling cavity is tuned to achieve an optical resonance at a specific frequency²⁵. WLSR setups using the PNC and bulk acoustic wave (BAW) resonator are denoted with their respective abbreviations. The curve “BAW Ultra-low loss” shows the speculative improvement when the BAW resonator is cooled to 1 K. Apart from “Advanced LIGO”, all curves use 4.0 MW arm cavity power and 10 dB frequency dependent squeezing.