The sensitivity of laser-interferometric gravitational wave detectors can be improved with squeezed light. The first squeezed-light laser aiming for a permanent operation in such a detector has now been accomplished and characterized.
A century after Einstein's formulation of general relativity, the detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO) made the first direct detection of gravitational waves. This historic achievement was the culmination of a world-wide effort and decades of instrument research. While sufficient for this monumental discovery, the current generation of gravitational-wave detectors represent the least sensitive devices necessary for the task; improved detectors will be required to fully exploit this new window on the Universe. In this paper, we review the application of squeezed vacuum states of light to gravitational-wave detectors as a way to reduce quantum noise, which currently limits their performance in much of the detection band.
Squeezed vacuum states of light show a reduction in quantum noise along one quadrature compared to the vacuum state. This characteristic is the key to improve interferometric measurements beyond their quantum noise limitation. In particular for ground-based gravitational wave detectors [1] the application of squeezed light led to a significant sensitivity improvement of up to 6 dB. Future gravitational wave detectors, like the Einstein Telescope [2] or Cosmic Explorer [3], aim for an even higher enhancement in the range of 10 dB with the injection of squeezed states of light into the dark port of their interferometers.
The worldwide network of laser interferometric gravitational-wave detectors is currently in an upgrade phase and will most likely detect gravitational waves within the next few years. Their strain sensitivity will be limited by photon shot noise over a large frequency band. A measurement beyond the shot noise level has already been accomplished via injecting squeezed vacuum states of light into the interferometers. In this proceeding, I review the current status in developements for the next-generation squeezed-light sources for future gravitational-wave detectors. A newly designed doubly-resonant squeezed light source is discussed, as well as squeezed light generated at the wavelengths of 1550 nm and 532 nm. Furthermore, high efficiency second harmonic generation of laser light for gravitational wave detectors will be presented. The injection of squeezed light as a key technology is well on schedule for improving the sensitivity of future gravitational-wave detectors.
Among the revolutionary predictions of Einstein’s theory of general relativity is the existence of gravitational waves, which are perturbations in the fabric of spacetime that travel at the speed of light. The first indirect evidence pointing to their existence came from the observation of the Hulse-Taylor pulsar in 1974[4]. More recently, gravitational wave observatories like the Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo have been built with the purpose of detecting gravitational waves (GW) directly, providing not only further evidence in support of general relativity, but also a whole new way of viewing the cosmos. The strain produced by a passing gravitational wave (namely, the relative change in the length of the matter a gravitational wave passes through) is so small that the best sensitivity achieved by the detectors has not produced a positive detection as of today. With a clear understanding that there should be gravitational waves, scientists are working on improving the sensitivity of the detectors to increase the likelihood of a detection. One of the main sources of noise for the next generation gravitational wave detectors, such as advanced LIGO, is shot noise. Shot noise dominates the noise spectrum above a few hundred Hz. This noise is due to uncertainties in quantum statistics, according to the generalized uncertainty principle. There are two ways to reduce the shot noise. The first way is to increase the intensity of the
This contribution reviews our recent progress on the generation of squeezed light [1], and also the recent squeezed-light enhancement of the gravitational wave detector GEO 600 [2]. GEO 600 is currently the only GW observatory operated by the LIGO Scientific Collaboration in its search for gravitational waves. With the help of squeezed states of light it now operates with its best ever sensitivity, which not only proves the qualification of squeezed light as a key technology for future gravitational wave astronomy but also the usefulness of quantum entanglement.
Nearly a century after Einstein first predicted the existence of gravitational waves, a global network of Earth-based gravitational wave observatories 1-4 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.