A quantum-enhanced prototype gravitational-wave detector

The quantum nature of the electromagnetic field imposes a fundamental limit on the sensitivity of optical precision measurements such as spectroscopy, microscopy and interferometry(1). The so-called quantum limit is set by the zero-point fluctuations of the electromagnetic field, which constrain the precision with which optical signals can be measured(2-4). In the world of precision measurement, laser-interferometric gravitational-wave detectors(4-6) are the most sensitive position meters ever operated, capable of measuring distance changes of the order of 10(-18) mr.m.s. over kilometre separations caused by gravitational waves from astronomical sources(7). The sensitivity of currently operational and future gravitational-wave detectors is limited by quantum optical noise(6). Here, we demonstrate a 44% improvement in displacement sensitivity of a prototype gravitational-wave detector with suspended quasi-free mirrors at frequencies where the sensitivity is shot-noise-limited, by injecting a squeezed state of light(1-3). This demonstration is a critical step towards implementation of squeezing-enhancement in large-scale gravitational-wave detectors.