Dual-resonator speed meter for a free test mass - art. no. 044002

A description and analysis are given of a "speed meter'' for monitoring a c lassical force that acts on a test mass. This speed meter is based on two m icrowave resonators ("dual resonators"), one of which couples evanescently to the position of the test mass. The sloshing of the resulting signal betw een the resonators, and a wise choice of where to place the resonators' out put waveguide, produce a signal in the waveguide that (for sufficiently low frequencies) is proportional to the test-mass velocity (speed) rather than its position. This permits the speed meter to achieve force-measurement se nsitivities better than the standard quantum limit (SQL), both when operati ng in a narrow-band mode and a wideband mode. A scrutiny of experimental is sues shows that it is feasible, with current technology, to construct a dem onstration speed meter that beats the wideband SQL by a factor 2. A concept is sketched for an adaptation of this speed meter to optical frequencies; this adaptation forms the basis for a possible LIGO-III interferometer that could beat the gravitational-wave standard quantum limit h(SQL), but perha ps only by a factor 1/xi=h(SQL)/h less than or similar to 3 (constrained by losses in the optics) and at the price of a Very high circulating optical power-larger by xi(-2) than that required to reach the SQL.