Demonstrating the velocity response of a table-top EPR Speedmeter

The sensitivity of gravitational-wave interferometers is fundamentally limited by quantum noise, as dictated by the Heisenberg uncertainty principle, due to their continuous position measurement of the end mirrors. Speedmeter configurations, which measure mirror velocity rather than position, have been proposed as a means to suppress quantum back-action noise, but practical implementations remain at an early stage. In this work, we present a table-top realisation of the Einstein- Podolsky-Rosen (EPR) Speedmeter concept, employing an optical readout scheme based on two orthogonal polarisation modes that probe the interferometer with different effective bandwidths. Using a triangular cavity, we demonstrate that the differential optical response between the linear p- and s-polarised modes exhibits a speed-like frequency dependence: vanishing at DC and increasing linearly with signal frequency, up to the bandwidth of the slower mode. With this we show that an optical system equivalent to the EPR Speedmeter indeed performs a velocity readout of the end mirror.