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 ${10}^{\ensuremath{-}21}/\sqrt{\mathrm{Hz}}$ 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 $3\ensuremath{\sigma}$ upper limit on ${\mathrm{\ensuremath{\Omega}}}_{\mathrm{GW}}$, the gravitational wave energy density normalized to the closure density, ranges from $5.6\ifmmode\times\else\texttimes\fi{}1{0}^{12}$ at 1 MHz to $8.4\ifmmode\times\else\texttimes\fi{}1{0}^{15}$ 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 $0.83--3.5\ifmmode\times\else\texttimes\fi{}1{0}^{21}\text{ }\text{ }\mathrm{g}$ between the Earth and the Moon. Projections for a chirp search with the same data set increase the mass range to $0.59\ensuremath{-}2.5\ifmmode\times\else\texttimes\fi{}1{0}^{25}\text{ }\text{ }\mathrm{g}$ 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.