The quantum squeezing techniques implemented in the current generation of gravitational wave detectors improves the sensitivity of the detectors only in the high frequency regime (>100 Hz). In order to achieve a broadband reduction in quantum noise, the squeezed states need to be rotated at the cross-over between quantum radiation pressure (quantum back-action noise) and shot noise (also known as photon counting noise). To do so, we require a high finesse detuned filter cavity which can be technically challenging to control. This need for a filter cavity can be circumvented by using entangled non-degenerate squeezed states which are subject to different resonance conditions in the interferometer. While one set of entangled sidebands are resonant in the interferometer, the other set sees the whole interferometer as a filter cavity!

Simplified schematic of the experiment. The entangled signal (red) and idler (blue) fields generated by the OPO are directed towards the test cavity. The reflected fields (which are picked off with a Faraday isolator) are then sent towards the measurement stage where a triangular output mode cleaner (OMC) cavity is used to spatially separate the entangled fields. The two fields are then measured with two homodyne detectors, and are electronically recombined for the final readout measurement. B )Frequency spacing diagram of the signal, idler and control fields. The OPO pumped at frequency 2ω₀ + Δ entangles sidebands between the signal and idler fields. The sidebands of the idler field experience a differential phase shift when reflected off a detuned cavity. The × indicates a sideband pointing into the page [1].

Noise spectrum for different test cavity detuning. a, Frequency spacing diagram of the signal (red) and idler (blue) fields with respect to test cavity resonances for same detunings on the signal (δ_sig), idler (δ_idl) and test cavity (γ_tc) on the left, δ_sig = 0, δ_idl = γtc (middle), and δ_sig = −δ_idl = 0.5γ_tc (right). b, Noise spectrum at the combined output measured at readout angle 0 and π (Maroon and green respectively). Solid dark colour lines are fits to the theoretical model. c, Measured noise spectrum as a function of readout angle. d, Associated modelled noise spectrum from the theoretical model. Dotted lines in c and d correspond to the spectra in b. [1]

[1] Yap, M.J., Altin, P., McRae, T.G. et al. Generation and control of frequency-dependent squeezing via Einstein–Podolsky–Rosen entanglement. Nat. Photonics 14, 223–226 (2020). https://doi.org/10.1038/s41566-019-0582-4