MHz gravitational wave constraints with decameter Michelson interferometers

A new detector, the Fermilab Holometer, consists of separate yet identical 39-meter Michelson interferometers. Strain sensitivity achieved is better than 10^-21/√{Hz } between 1 to 13 MHz from a 130-h data set. This measurement exceeds the sensitivity and frequency range made from previous high frequency gravitational wave experiments by many orders of magnitude. Constraints are placed on a stochastic background at 382 Hz resolution. The 3 σ upper limit on Ω_GW, the gravitational wave energy density normalized to the closure density, ranges from 5.6 ×1 0¹² at 1 MHz to 8.4 ×1 0¹⁵ at 13 MHz. Another result from the same data set is a search for nearby primordial black hole binaries (PBHB). There are no detectable monochromatic PBHBs in the mass range 0.83 - 3.5 ×1 0²¹ g between the Earth and the Moon. Projections for a chirp search with the same data set increase the mass range to 0.59 -2.5 ×1 0²⁵ g and distances out to Jupiter. This result presents a new method for placing limits on a poorly constrained mass range of primordial black holes. Additionally, solar system searches for PBHBs place limits on their contribution to the total dark matter fraction.