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According to quantum measurement theory, "speed meters" -- devices thatmeasure the momentum, or speed, of free test masses -- are immune to thestandard quantum limit (SQL). It is shown that a Sagnac-interferometergravitational-wave detector is a speed meter and therefore in principle it canbeat the SQL by large amounts over a wide band of frequencies. It is shown,further, that, when one ignores optical losses, a signal-recycled Sagnacinterferometer with Fabry-Perot arm cavities has precisely the sameperformance, for the same circulating light power, as the Michelson speed-meterinterferometer recently invented and studied by P. Purdue and the author. Theinfluence of optical losses is not studied, but it is plausible that they befairly unimportant for the Sagnac, as for other speed meters. With squeezedvacuum (squeeze factor $e^{-2R} = 0.1$) injected into its dark port, therecycled Sagnac can beat the SQL by a factor $ \sqrt{10} \simeq 3$ over thefrequency band $10 {\rm Hz} \alt f \alt 150 {\rm Hz}$ using the samecirculating power $I_c\sim 820$ kW as is used by the (quantum limited)second-generation Advanced LIGO interferometers -- if other noise sources aremade sufficiently small. It is concluded that the Sagnac optical configuration,with signal recycling and squeezed-vacuum injection, is an attractive candidatefor third-generation interferometric gravitational-wave detectors (LIGO-III andEURO).Comment: 12 pages, 6 figure

Sagnac interferometer as a speed-meter-type, quantum-nondemolition gravitational-wave detector