Beating the quantum limit in gravitational wave detectors

The Advanced LIGO gravitational wave detectors operate at sensitivities where noise from vacuum fluctuation begins to dominate as quantum shot noise and quantum radiation pressure noise. In their most recent observing run, the LIGO and Virgo detectors implemented the use of special quantum states of light known as squeezed states of light in order to reduce high frequency quantum noise.The improved sensitivity from shot noise reduction was, however, accompanied by an increase in quantum radiation pressure noise at low frequencies. This is quantum backaction from the optomechanical coupling of the kilogram-sized mirrors with the light inside the detector’s optical cavities. For the current upcoming observing run, LIGO aims to counteract this backaction using 300 m filter cavities which rotate squeezed states at low frequencies in order to decrease radiation pressure noise, thus providing a broadband sensitivity improvement. The improved sensitivity of the detectors is expected to vastly increase the number of astrophysical events that the gravitational wave detector network observes, setting the stage for a lot of new and interesting astrophysics. Here, we present results from the filter cavity installation in LIGO for frequency dependent squeezing to beat the standard quantum limit, the latest milestone in decades of work on quantum back-action evasion.