Approaching Heisenberg Scaling in a Continuous-Wave Interferometer

Continuous-wave (CW) interferometers lie at the core of many modern precision measurements including ground and space based gravitational wave detectors and measurements of local gravity. As with all measurements, quantum mechanics sets the ultimate limit on the precision of CW interferometers. In interferometers employing only classical sources of light, such as thermal sources and lasers, sensitivity is bounded by "standard quantum limit" (SQL) scaling. However, using quantum light sources, such as squeezed light, we can achieve precision beyond the SQL and achieve "Heisenberg scaling", where the available quantum resources are used with maximal efficiency. Prior to our work, a variety of theoretical and technical challenges have prevented any CW interferometer from approaching Heisenberg scaling. We show, for the first time, that it is possible to achieve Heisenberg scaling in a CW interferometer using current technology. We demonstrate this sensitivity directly and find that our measurements achieve the sensitivity predicted theoretically. Our results demonstrate that we can approach the Heisenberg limit in CW interferometers using existing technology.