Isolation of gravitational waves from displacement noise and utility of a time-delay device

TL;DR

This paper investigates how incorporating time delay in displacement-noise-free interferometers can enhance their sensitivity to gravitational waves, especially at low frequencies, making long-baseline space-based detectors more practical.

Contribution

It introduces a novel approach using time delay to improve the sensitivity of displacement-noise-free interferometers, addressing limitations of previous schemes.

Findings

Time delay improves low-frequency sensitivity by a factor of (f L/c)^(-1).

Displacement-noise-free schemes can be made more practical with this method.

Enhanced sensitivity helps in designing more effective space-based gravitational-wave detectors.

Abstract

Interferometers with kilometer-scale arms have been built for gravitational-wave detections on the ground; ones with much longer arms are being planned for space-based detection. One fundamental motivation for long baseline interferometry is from displacement noise. In general, the longer the arm length L, the larger the motion the gravitational-wave induces on the test masses, until L becomes comparable to the gravitational wavelength. Recently, schemes have been invented, in which displacement noises can be evaded by employing differences between the influence of test-mass motions and that of gravitational waves on light propagation. However, in these schemes, such differences only becomes significant when L approaches the gravitational wavelength, and shot-noise limited sensitivity becomes worse than that of conventional configurations by a factor of at least (f L/c)^(-2), for f<c/L. Such a factor, although can be overcome theoretically by employing high optical powers, makes these schemes quite impractical. In this paper, we explore the use of time delay in displacement-noise-free interferometers, which can improve their shot-noise-limited sensitivity at low frequencies, to a factor of (f L/c)^(-1) of the shot-noise-limited sensitivity of conventional configurations.