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 co-located 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 × 10⁻²⁰ m/√H̅z̅ 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.

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