Broadband Limits on Stochastic Length Fluctuations from a Pair of Table-Top Interferometers

The Quantum-Enhanced Space-Time (QUEST) experiment consists of a pair of colocated power recycled Michelson interferometers, each designed to have a broadband, shot-noise-limited displacement sensitivity of $2×10^{−19}  m\sqrt{Hz}$ from 1 to 200 MHz. Here, we present the first results of QUEST, with a search up to 80 MHz, that set new upper limits on correlated length fluctuations from 13 to 80 MHz, constituting the first broadband constraints for a stochastic gravitational wave background at these frequencies. In a coincident observing run of $10^{4}s$, the averaging of the cross-correlation spectra between the two interferometer signals resulted in a strain sensitivity of $3×10^{-20} 1\sqrt{Hz}$, making QUEST the most sensitive table-top interferometric system to date.