Enhanced sensitivity of the LIGO gravitational wave detector by using squeezed states of light

Nearly a century after Einstein first predicted the existence of gravitational waves, a global network of Earth-based gravitational wave observatories1–4 is seeking to directly detect this faint radiation using precision laser interferometry. Photon shot noise, due to the quantum nature of light, imposes a fundamental limit on the attometre-level sensitivity of the kilometre-scale Michelson interferometers deployed for this task. Here, we inject squeezed states to improve the performance of one of the detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) beyond the quantum noise limit, most notably in the frequency region down to 150 Hz, critically important for several astrophysical sources, with no deterioration of performance observed at any frequency. With the injection of squeezed states, this LIGO detector demonstrated the best broadband sensitivity to gravitational waves ever achieved, with important implications for observing the gravitational-wave Universe with unprecedented sensitivity. A fundamental limit to the sensitivity of a Michelson interferometer with quasi-free mirrors comes from the quantum nature of light, which reveals itself through two fundamental mechanisms: photon counting noise (shot noise), arising from statistical fluctuations in the arrival time of photons at the interferometer output, and radiation pressure noise, which is the recoil of the mirrors due to the radiation pressure arising from quantum fluctuations in the photon flux. Both sources can be attributed to the quantum fluctuations of the electromagnetic vacuum field, or vacuum fluctuations, that enter the interferometer5,6. An electromagnetic field can be described by two non-commuting conjugate operators that are associated with field amplitudes that oscillate out of phase with each other by 908, labelled as ‘in-phase’ and ‘quadrature phase’7. A coherent state of light (or vacuum, if the coherent amplitude is zero) has equal uncertainty in both quadratures, with the uncertainty product limited by the Heisenberg uncertainty principle. For a squeezed state, the uncertainty in one quadrature is decreased relative to that of the coherent state (green box in Fig. 1). Note that the uncertainty in the orthogonal quadrature is correspondingly increased, always satisfying the Heisenberg inequality. The vacuum fluctuations that limit the sensitivity of an interferometric gravitational-wave detector enter through the antisymmetric port of the interferometer, mix with the signal field produced at the beamsplitter by a passing gravitational wave, and exit the antisymmetric port to create noise on the output photodetector. Caves5,6 showed that replacing coherent vacuum fluctuations entering the antisymmetric port with correctly phased squeezed vacuum states decreases the ‘in-phase’ quadrature uncertainty, and thus the shot noise, below the quantum limit. Soon after, the first experiments showing squeezed light production through nonlinear optical media achieved modest but important reductions in noise at high frequencies8,9. However, squeezing in the audiofrequency region relevant for gravitational-wave detection and control schemes for locking the squeezed phase to that needed by the interferometer were not demonstrated until the last decade10–12. Since then, squeezed vacuum has been used to enhance the sensitivity of a prototype interferometer13. The 600-m-long GEO600 detector14 has deployed squeezing since 2010, achieving improved sensitivity at 700 Hz and above. An important motivation for the experiment we present here was to extend the frequency range down to 150 Hz while testing squeezing at a noise level close to that required for Advanced LIGO15. This lower frequency region is critically important for the most promising astrophysical sources, such as coalescences of black hole and neutron star binary systems, but also poses a significant experimental challenge. Seismic motion is huge compared to the desired sensitivity, albeit at very low frequencies of less than 1 Hz, and LIGO employs a very high-performance isolation system to attenuate the seismic motion by several orders of magnitude. This uncovers a set of nonlinear couplings that upconvert low-frequency noise into the gravitational wave band. In the past, these processes have made it difficult for gravitational-wave detectors to reach a shot-noise-limited sensitivity in their most sensitive band near 150 Hz. Any interactions between the interferometer and the outside world have to be kept at an absolute minimum. For instance, randomly scattered light reflecting back into the interferometer has to be managed at the level of 1× 10 W. Past experience has shown that measured sensitivities at higher frequencies are difficult to extrapolate to lower frequencies2. For the first time, we employ squeezing to obtain a sensitivity improvement at a gravitationalwave observatory in the critical frequency band between 150 Hz and 300 Hz. Similarly important, we observe that no additional noise above background was added by our squeezed vacuum source, firmly establishing this quantum technology as an indispensable technique in the future of gravitationalwave astronomy. The experiment was carried out towards the end of 2011 on the LIGO detector at Hanford, Washington, known as ‘H1’. The optical layout of the detector is shown in Fig. 1. The interferometer light source (‘H1 laser’) is a Nd:YAG laser (1,064 nm) stabilized in frequency and intensity. A beamsplitter splits the light into the two arms of the Michelson, and Fabry–Perot cavities increase the phase sensitivity by bouncing the light 130 times in each arm. The Michelson is operated on a dark fringe, so most of the light is reflected from the interferometer back to the laser. A partially transmitting mirror between the laser and the beamsplitter forms the power-recycling cavity, which increases the power incident on the beamsplitter by a factor of 40. To isolate them from terrestrial forces such as seismic noise, the power recycling mirror, the beamsplitter and the arm cavity mirrors are all suspended as pendula on vibrationisolated platforms.