First-step experiment in developing optical-spring quantum locking for DECIGO: sensitivity optimization for simulated quantum noise by completing the square

TL;DR

This paper experimentally verifies a sensitivity optimization technique for quantum noise reduction in DECIGO, using optical-spring quantum locking with a simplified setup to enhance gravitational wave detection prospects.

Contribution

It demonstrates the feasibility of sensitivity optimization via completing the square in a tabletop experiment simulating quantum noise for DECIGO's optical-spring quantum locking.

Findings

Sensitivity improved by completing the square of detector outputs

Optimal laser power identified for maximum sensitivity

Method applicable to quantum noise-limited gravitational wave detectors

Abstract

DECi-hertz Interferometer Gravitational Wave Observatory (DECIGO) is a future mission for a space-borne laser interferometer. DECIGO has 1,000-km-long arm cavities mainly to detect the primordial gravitational waves (PGW) at lower frequencies around 0.1 Hz. Observations in the electromagnetic spectrum have lowered the bounds on the upper limit of PGW energy density ($\Omega_{\rm gw} \sim 10^{-15} \to 10^{-16}$). As a result, DECIGO's target sensitivity, which is mainly limited by quantum noise, needs further improvement. To maximize the feasibility of detection while constrained by DECIGO's large diffraction loss, a quantum locking technique with an optical spring was theoretically proposed to improve the signal-to-noise ratio of the PGW. In this paper, we experimentally verify one key element of the optical-spring quantum locking: sensitivity optimization by completing the square of multiple detector outputs. This experiment is operated on a simplified tabletop optical setup with classical noise simulating quantum noise. We succeed in getting the best of the sensitivities with two different laser powers by the square completion method.