First Measurements of High Frequency Cross-Spectra from a Pair of Large Michelson Interferometers

Abstract

Measurements are reported of the cross-correlation of spectra of differential position signals from the Fermilab Holometer, a pair of colocated 39 m long, high power Michelson interferometers with flat broadband frequency response in the MHz range. The instrument obtains sensitivity to high frequency correlated signals far exceeding any previous measurement in a broad frequency band extending beyond the 3.8 MHz inverse light-crossing time of the apparatus. The dominant but uncorrelated shot noise is averaged down over 2 x 10(8) independent spectral measurements with 381 Hz frequency resolution to obtain 2.1 x 10(-20)m/root Hz sensitivity to stationary signals. For signal bandwidths Delta f > 11 kHz, the sensitivity to strain h or shear power spectral density of classical or exotic origin surpasses a milestone PSD delta h < t(p) where t(p) = 5.39 x 10(-44)/Hz is the Planck time