The Holometer: an instrument to probe Planckian quantum geometry

Abstract

This paper describes the Fermilab Holometer, an instrument for measuring correlations of position variations over a fourdimensional volume of space-time. The apparatus consists of two co-located, but independent and isolated, 40-m power-recycled Michelson interferometers, whose outputs are cross-correlated to 25 MHz. The data are sensitive to correlations of differential position across the apparatus over a broad band of frequencies up to and exceeding the inverse light crossing time, 7.6 MHz. A noise model constrained by diagnostic and environmental data distinguishes among physical origins of measured correlations, and is used to verify shot-noise-limited performance. These features allow searches for exotic quantum correlations that depart from classical trajectories at spacelike separations, with a strain noise power spectral density sensitivity smaller than the Planck time. The Holometer in current and future configurations is projected to provide precision tests of a wide class of models of quantum geometry at the Planck scale, beyond those already constrained by currently operating gravitational wave observatories.

Novel signatures of dark matter in laser-interferometric gravitational-wave detectors

“…Using Eqs. (4), (16), (18) and (19), we estimate the current sensitivities of GEO 600 [13,14] and the Fermilab holometer using both of its co-located interferometers [15,16] to the linear interactions of the DM field φ with the photon and electron in (1). We present these estimates as solid lines in Fig.…”

“…Using Eqs. (4), (16), (23) and (24), we estimate the sensitivities of a single small-scale Michelson interferometer and a pair of co-located smallscale Michelson interferometers using the above narrowband approach and operating near room temperature to the linear interactions of the DM field φ with the photon and electron in (1), assuming that the measurements are limited by Brownian thermal noise and that all of the dimensions of the beam-splitter are altered in a proportional manner. We present these estimates as the dashed and thin dashed purple lines, respectively, in Fig.…”

“…where we have again used Eqs. (4) and (16). The freelyfloating test masses will, therefore, undergo the following time-varying centre-of-mass displacements:…”

“…Laser interferometry has been optimised over decades to develop ultra-sensitive gravitational-wave detectors, which have recently been employed spectacularly to directly observe gravitational waves on Earth for the first time [20,21]. Additionally, smaller-scale interferometers have more recently been utilised to search for non-gravitational-wave phenomena, such as quantumgeometry effects that may arise at the Planck scale [15,16].…”

Axion interferometry

“…As we do not know the mass of ALP dark matter, we wish to design a broadband detector rather than a resonant detector. We thus choose our equivalent gravitational wave interferometer to be a typical Michelson interferometer such as those used in experiments such as LIGO [15] and the Holometer [16].…”