A system is proposed for detecting the effect of a weak classical force on a macroscopic mechanical oscillator. The novel feature is a type of ac bridge coupled to a SQUID amplifier. A detailed sensitivity calculation for one type of bridge excitation is reported which shows what is required to reach the standard quantum "limit." Another type of excitation should make it a "back-action evasion" device capable of exceeding the standard quantum "limit".

• Received 7 August 1981

DOI:https://doi.org/10.1103/PhysRevLett.47.1184