  We present an interferometric sensor for investigating macroscopic quantum
mechanics on a table-top scale. The sensor consists of pair of suspended
optical cavities with a finesse in excess of 100,000 comprising 10 g
fused-silica mirrors. In the current room-temperature operation, we achieve a
peak sensitivity of \SI{0.5}{\fmasd} in the acoustic frequency band, limited by
the readout noise. With additional suppression of the readout noise, we will be
able to reach the quantum radiation pressure noise, which would represent a
novel measurement of the quantum back-action effect. Such a sensor can
eventually be utilised for demonstrating macroscopic entanglement and testing
semi-classical and quantum gravity models.
