Sub-attonewton force detection at millikelvin temperatures

A 290-nm-thick single-crystal silicon cantilever has been cooled in vacuum to a temperature of 110 mK in order reduce its thermal motion and thereby improve the achievable force resolution. Since the thermal conductivity of the silicon cantilever is extremely low at millikelvin temperatures, an improved optical fiber interferometer was developed to measure the subangstrom thermal motion with optical powers as low as 2 nW. At the lowest temperature, the cantilever exhibited a quality factor of 150 000 and achieved a noise temperature of 220 mK, with a corresponding force noise of 820 zN in a 1 Hz bandwidth.