First measurements of high frequency cross-spectra from a pair of large Michelson interferometers

TL;DR

This paper reports the first high-frequency cross-spectra measurements from a pair of large Michelson interferometers, achieving unprecedented sensitivity in the MHz range and surpassing fundamental physical limits for classical or exotic signals.

Contribution

It introduces a novel measurement of high-frequency cross-spectra using co-located Michelson interferometers with sensitivity beyond previous limits, reaching the Planck time scale.

Findings

Achieved sensitivity of 2.1×10⁻²⁰ m/√Hz to stationary signals.

Surpassed the Planck time scale for classical or exotic signals at frequencies above 11 kHz.

First measurement of high-frequency cross-spectra in the MHz range with broadband response.

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 $2\times 10^8$ independent spectral measurements with 381 Hz frequency resolution to obtain $2.1\times 10^{-20} \ \mathrm{m}/\sqrt{\mathrm{Hz}}$ 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 $t_p= 5.39\times 10^{-44}/\mathrm{Hz}$ is the Planck time.