Broadband Quantum Enhancement of the LIGO Detectors with Frequency-Dependent Squeezing

Quantum noise imposes a fundamental limitation on the sensitivity of interferometric gravitational-wave detectors like LIGO, manifesting as shot noise and quantum radiation pressure noise. Here, we present the first realization of frequency-dependent squeezing in full-scale gravitational-wave detectors, resulting in the reduction of both shot noise and quantum radiation pressure noise, with broadband detector enhancement from tens of hertz to several kilohertz. In the LIGO Hanford detector, squeezing reduced the detector noise amplitude by a factor of 1.6 (4.0 dB) near 1 kHz; in the Livingston detector, the noise reduction was a factor of 1.9 (5.8 dB). These improvements directly impact LIGO’s scientific output for high-frequency sources (e.g., binary neutron star postmerger physics). The improved low-frequency sensitivity, which boosted the detector range by 15%–18% with respect to no squeezing, corresponds to an increase in the astrophysical detection rate of up to 65%. Frequency-dependent squeezing was enabled by the addition of a 300-meter-long filter cavity to each detector as part of the LIGO $\mathrm{A}+$ upgrade.

Popular Summary

The sensitivity of interferometric gravitational-wave detectors like LIGO is ultimately limited by quantum fluctuations of electromagnetic fields that create noise on the readout photodetectors. One way to mitigate this quantum noise is to employ “squeezed vacuum states.” This is where the electromagnetic ground state is manipulated so that the quantum uncertainty in either the phase or amplitude is reduced at the expense of the other. Here, as part of a detector upgrade, we present the first implementation of “frequency-dependent squeezing” at LIGO. The subsequent noise reduction boosts the detector range by up to 18%, corresponding to an increase in detection rate of up to 65%.

In their third observing run, LIGO and other gravitational-wave detectors employed frequency-independent squeezing, which reduced the shot noise from quantum fluctuations, but at the expense of increased noise from radiation pressure. To circumvent this, we built a 300-meter-long optical cavity (called a “filter cavity”) at both LIGO sites to enable frequency-dependent squeezing. The squeezed vacuum states are reflected off this long cavity before injection into the interferometer, enabling a frequency-dependent rotation of the squeezed state.

Two decades after frequency-dependent squeezing was initially proposed, this latest result fully establishes it as a new fundamental technology for current and future gravitational-wave detectors.

Figure 1

Experimental setup of frequency-dependent squeezing in the LIGO detectors. The blue panel shows a simplified overview of the main experimental components. The LIGO detector is a modified Michelson interferometer with Fabry-Perot arm cavities and both power recycling and signal extraction cavities [1]. The LIGO squeezer [8] generates squeezed vacuum at 1064 nm using a subthreshold optical parametric oscillator, pumped at 532 nm. The squeezed beam reflects from the 300-m filter cavity, and a movable beam diverter opens to inject squeezing at the output port of the interferometer. Three Faraday isolators prevent stray interferometer light from reaching the squeezer and filter cavity. Active steering optics [34] enable alignment and mode matching of the squeezed beam to the filter cavity and interferometer. The squeezed beam copropagates with the outgoing interferometer beam through the output mode cleaner cavity, for measurement at the readout photodetectors [1]. The yellow panels illustrate uncertainty ellipses of squeezed states in phase space [20], where vacuum fluctuations are squeezed along the vertical readout quadrature. The left depicts the frequency-dependent rotation impressed by filter cavity resonance (upper) upon the generated squeezed vacuum state (lower). The right shows how the interferometer backaction affects the injected squeezed state, which is either frequency independent (light) or frequency dependent (dark). In either case, the ellipse is rotated and stretched, corresponding to rotation and gain of the squeezed state. For frequency-independent squeezing, uncertainty in the readout quadrature is increased at low frequencies, as indicated by the vertical red arrows. For frequency-dependent squeezing prepared with the appropriate rotation to counteract backaction, a reduced uncertainty in the readout quadrature is recovered at low frequencies.

Figure 2

Observation of frequency-dependent squeezing in the LIGO detectors. The top and bottom show strain noise spectra of the LIGO Hanford (H1) and Livingston (L1) detectors in amplitude spectral density units, measured in the commissioning period leading up the fourth observing run, O4. Reference measurements of detector noise spectra without squeezing are shown in black and measured with the squeezed beam diverted away from the detector. Without squeezing, the classical noise estimate (gray), i.e., the sum of nonquantum noises, is obtained by subtracting the calculated quantum noise (red) from the measured detector noise (black). Frequency-independent squeezing spectra (green) are measured with the squeezed beam injected and the filter cavity end mirror misaligned, to have the input mirror act as a high reflector. With frequency-independent squeezing, shot noise reductions of 4.0 (H1) and 5.8 dB (L1) are observed around 1 kHz, alongside the corresponding increase in quantum radiation pressure noise below a few hundred hertz. Frequency-dependent squeezing spectra (purple) are obtained by locking the filter cavity near resonance, demonstrating the broadband reduction of detector quantum noise. In addition to the squeezed shot noise reduction, the filter cavity reduces total detector noise by 1–2 dB from 60–100 Hz in both detectors, with quantum enhancement visible from kilohertz down to tens of hertz.

Figure 3

Detailed optical and controls layout of frequency-dependent squeezing instrumentation in the LIGO detectors, described in Appendix pp1. To generate squeezed vacuum, the squeezer pump laser is frequency stabilized to the main interferometer laser, frequency doubled using SHG, and sent to pump the in-vacuum OPO. A series of AOMs generate both 1064 nm squeezer control sidebands: the CLF at 3.125 MHz for squeeze angle control [8, 52] and the resonant locking field (RLF) at 3.020 MHz for filter cavity length and detuning control [23]. Both sidebands are injected through the OPO and copropagate with squeezed vacuum to the filter cavity, interferometer, and readout photodetectors. A movable beam diverter can redirect the squeezed beam to an in-air balanced homodyne detector for diagnostics. After first acquiring lock at 532 nm, filter cavity length control is transferred to lock on RLF resonance such that the CLF is off resonant; the optical beat note of the RLF and CLF at 105 kHz is, thus, phase sensitive to filter cavity resonance. In reflection of the filter cavity, a 1% pickoff sends this 105 kHz beat note to a pair of quadrant photodiodes for filter cavity length and alignment control. For long-term operation, alignment is controlled between the OPO, filter cavity, and interferometer, and intensity stabilization holds the control sideband powers and generated squeezing levels constant.

Figure 4

Frequency-dependent squeezing on a diagnostic balanced homodyne detector in H1. The blue and yellow traces show frequency-independent squeezing spectra measured with the filter cavity end mirror misaligned. The red and green traces show frequency-dependent squeezing spectra, measured with the filter cavity locked at a detuning of $\delta \sim 1\mathrm{kHz}$ from the local oscillator. Dotted lines show a model of frequency-dependent squeezing for each configuration, given measured optical properties of the filter cavity [27]. From the model, the plot legend gives the corresponding squeeze angle $\varphi$ and filter cavity detuning $\delta$ for each trace.