A force-measuring system composed of a macroscopic harmonic oscillator, mechanical-electrical transducer, and linear amplifier is described and the sensitivity is calculated for a "back-action evasion" mode of operation. It is shown that it is possible to surpass the standard amplifier limit with this device, and the specific requirements to do so are given. These results may also apply to a quantum-limited amplifier and thus give the requirements to perform quantum-nondemolition measurements.

• Received 10 February 1982

DOI:https://doi.org/10.1103/PhysRevLett.48.1371