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Q: Fachverband Quantenoptik und Photonik

Q 61: Matter Wave Interferometry and Metrology III

Q 61.3: Talk

Thursday, March 5, 2026, 15:00–15:15, P 11

Heterodyne cavity-tracking for enhanced displacement sensing — •Nurmi Sawlanski, Shreevathsa Chalathadka Subrahmanya, and Oliver Gerberding — Institute of Experimental Physics, University of Hamburg, 22761 Hamburg, Germany

Gravitational-wave detectors allow us to observe compact binaries and other extreme astrophysical events by measuring minute changes in the distance between nominally free-falling test masses. To separate these signals from seismic motion and technical disturbances, the detectors rely on ultra-sensitive displacement sensors in their active isolation and control systems. Improving such sensors is therefore essential for future gravitational-wave observatories.

Heterodyne cavity-tracking is a laser interferometric displacement readout scheme that enables sub-femtometer precision with an operating range on the order of a wavelength. A laser is locked to the resonance of a cm-scale cavity containing the proof mass, and the resulting frequency changes are measured via a heterodyne beat with a second, stable laser using a GHz-bandwidth phasemeter. This combines high precision with a dynamic range suitable for tracking test-mass motion.

Our fiber-based setup has achieved a displacement sensitivity of about 20 fm/√Hz for frequencies above 5 Hz and a dynamic range of six orders of magnitude by tracking motions up to 0.15 µm. To further improve the achievable sensitivity, we plan to implement Pound-Drever-Hall locking for laser stabilization and to move to a free-beam vacuum setup. In this talk, we present the sensing scheme, current performance, and planned experimental steps.

Keywords: Gravitational wave detection; Displacement sensors