A high-finesse suspended interferometric sensor for macroscopic quantum mechanics with femtometer sensitivity

We present our development of a table-top quantum sensor with femtometer sensitivity within the acoustic frequency band. Our sensor utilises the expertise developed within the gravitational-wave community, particularly in the control of high-finesse, suspended interferometers, and the mitigation of seismic and thermal noise. We construct a system consisting of a cryostat-mounted four-stage suspension supporting two cavities with optical finesse in excess of 350,000. We use active isolation of the entire cryostat system to achieve an order-of-magnitude of reduction of the seismic noise prior to the passive isolation of the internal suspension. We achieve the femtometer sensitivity using a custom-made phase lock loop. With further enhancements to the readout electronics and suspension characteristics, we aim to reach the quantum radiation pressure noise. This sensor holds the potential for future applications in demonstrating macroscopic entanglement and testing semi-classical and quantum gravity theories.