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

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 2×10⁸ independent spectral measurements with 381 Hz frequency resolution to obtain 2.1×10^-20m/√Hz sensitivity to stationary signals. For signal bandwidths Δf>11 kHz, the sensitivity to strain h or shear power spectral density of classical or exotic origin surpasses a milestone PSD_δh<tₚ where tₚ=5.39×10^-44/Hz is the Planck time.