Simultaneous Speedmeter and Position-Meter Response in a Single Tabletop Interferometer

Abstract: Quantum radiation-pressure noise (QRPN) limits the low-frequency sensitivity of gravitational wave detectors. The established method for suppressing QRPN is the injection of frequency-dependent squeezed light. It requires long-baseline filter cavities introducing substantial experimental complexity. A completely different interferometer concept is the speedmeter. It avoids QRPN at the source by measuring test mass speed instead of position. While extensively researched theoretically, speedmeters are yet to be demonstrated with a moving test mass in an optomechanical setting. In this work, we present the first experimental observation of speedmeter behavior in a system with a movable test mass. We realize a novel hybrid readout cavity configuration that enables simultaneous extraction of position and speed signals from two distinct output ports. We compare the optical transfer functions associated with each channel and observe the expected scaling behavior that distinguishes a speedmeter from a position-meter. We support our observations with a detailed theoretical model, showing how the hybrid readout cavity implements key speedmeter features. Our results underscore the relevance of the speedmeter concept as an alternative for mitigating QRPN in future detectors and lay the groundwork for further experimental exploration.