Torrey Cullen

The two theories that best describe the Universe, general relativity and quantum mechanics, are not complete and make contradictory predictions. New approaches for finding a theory of quantum gravity that could solve these issues predict that spacetime exhibits quantum fluctuations. These fluctuations are just out of reach of the most sensitive machines for measuring spacetime: laser interferometers such as the LIGO gravitational-wave detectors.

Modern interferometers such as LIGO have achieved sensitivities limited by quantum noise, comprising radiation pressure and shot noise. To mitigate this noise, a static system is employed that minimizes the quantum noise within the measurement band. However, since gravitational wave inspiral signals are a single frequency changing over time, only noise at the chirp frequency needs to be minimized.

Quantum mechanics places noise limits and sensitivity restrictions on physical measurements. The balance between unwanted backaction and the precision of optical measurements imposes a standard quantum limit (SQL) on interferometric systems. In order to realize a sensitivity below the SQL, it is necessary to leverage a backaction evading measurement technique, reduce thermal noise to below the level of backaction, and exploit cancellations of any excess noise contributions at the detector.