Squeezing the quantum noise of a gravitational-wave detector below the standard quantum limit

The Heisenberg uncertainty principle dictates that the position and momentum of an object cannot be simultaneously measured with arbitrary precision, giving rise to an apparent limitation known as the standard quantum limit (SQL). Gravitational-wave detectors use photons to continuously measure the positions of freely falling mirrors and so are affected by the SQL. We investigated the performance of the Laser Interferometer Gravitational-Wave Observatory (LIGO) after the experimental realization of frequency-dependent squeezing designed to surpass the SQL. For the LIGO Livingston detector, we found that the upgrade reduces quantum noise below the SQL by a maximum of three decibels between 35 and 75 hertz while achieving a broadband sensitivity improvement, increasing the overall detector sensitivity during astrophysical observations. Editor’s summary Gravitational wave detectors use kilometers-long interferometers to measure tiny expansions and contractions of spacetime. The precision of those measurements is limited due to quantum mechanics. Theoretically, that quantum limit could be surpassed by using squeezed states of light, and a suitable filter cavity could extend the advantage over a wide frequency range (see the Perspective by Aso). Jia et al. measured the performance of the Laser Interferometer Gravitational-Wave Observatory (LIGO) after it was upgraded with a squeezed light and filter cavity system. The authors demonstrate that the upgrades improved the sensitivity of the detector over a wide range of frequencies. Over a smaller range, it surpassed the quantum limit. —Keith T. Smith