Proof-of-Principle Experiment on a Displacement-Noise-Free Neutron Interferometer for Gravitational Wave Detection

TL;DR

This work addresses the challenge of displacement noise in gravitational-wave interferometry by employing a neutron displacement-noise-free interferometer (DFI) with four unidirectional speeds to preserve GW signals while canceling mirror and BS displacement noise. The authors implement a proof-of-principle experiment at J-PARC using pulsed neutrons, Al phase plates, and beam-splitting etalons to realize a constrained neutron DFI, and they develop a phase-compensation and frequency-multiplexing analysis to emulate independent actuator effects. The key contribution is the first experimental demonstration that BS displacement noise can be canceled in a neutron DFI and that GW-like signals can be preserved under realistic hardware constraints, supported by close agreement between measured and simulated modulation signals and a validated DFI coefficient ratio of approximately $c_{14}/c_{23}\approx-4.31$. This work establishes a practical pathway for neutron-based GW detection concepts and informs future refinements in neutron optics and source capabilities. In the longer term, these ideas could enable sub-Hz GW sensitivity using neutron interferometry, complementing laser-based detectors.

Abstract

The displacement-noise-free interferometer (DFI) is designed to eliminate all displacement-induced noise while retaining sensitivity to gravitational wave (GW) signals. Ground-based DFIs suffer from physical arm-length limitations, resulting in poor sensitivity at frequencies below 1 kHz. To address this, previous research introduced a neutron-based DFI, which replaces laser light with neutrons and achieves exceptional sensitivity down to a few hertz. In this study, we conducted a proof-of-principle experiment using a pulsed neutron source at the Japan Proton Accelerator Research Complex (J- PARC). Despite practical constraints that led to deviations from the ideal experimental design, we optimized the setup and developed a novel analysis method that successfully cancels displacement noise while preserving simulated GW signals. This work presents the first successful demonstration of a neutron DFI and a neutron interferometer for GW detection.

Figures (41)

• Figure 1: Mach-Zehnder interferometer illuminated with four-speed unidirectional neutrons. This interferometer comprises two beam splitters, $\mathrm{BS_U}$ and $\mathrm{BS_L}$, and two mirrors, $\mathrm{M_1}$ and $\mathrm{M_2}$. The red, blue, green, and magenta arrows indicate the trajectories of neutrons with the speeds of $v_\mathrm{1}$, $v_\mathrm{2}$, $v_\mathrm{3}$, $v_\mathrm{4}$, respectively.
• Figure 2: Phasor diagram of BS displacement noises. Solid arrows originating from the origin represent the displacement noises of $\mathrm{BS_U}$ (upper panel) and $\mathrm{BS_L}$ (lower panel). Arrow lengths correspond to the normalized amplitude of the displacement noise. Dashed circle shows the reference of normalized amplitude. Tangerine and purple arrows indicate the residual BS noises after all mirror noises are canceled in Eqs. (\ref{['eq:1']})-(\ref{['eq:2']}). In the upper panel, the length of the rotating arrows corresponds to the phase shift of BS displacement noises.
• Figure 3: Phasor diagram of GW signals. Solid arrows originating from the origin represent the $\mathrm{GW_U}$ (upper panel) and $\mathrm{GW_L}$ (lower panel) signals. Arrow lengths correspond to the normalized amplitude of the GW signal. Dashed circle shows the reference of normalized amplitude. Tangerine and purple arrows indicate the residual GW signals after all mirror noises are canceled in Eqs. (\ref{['eq:1']})-(\ref{['eq:2']}). In the upper panel, the length of rotating arrows corresponds to the phase shift of GW signals.
• Figure 4: Straightforward experimental setup for the ideal proof-of-principle experiment on the four-speed unidirectional neutron DFI. The red, blue, green, and magenta arrows indicate the trajectories of neutrons with the speeds of $v_\mathrm{1}$, $v_\mathrm{2}$, $v_\mathrm{3}$, and $v_\mathrm{4}$, respectively. The purple and grey rectangles represent actuation systems for BS noises and mirror noises. The mesh circles colored yellow show actuation systems for GW signals.
• Figure 5: Actual setup for the neutron DFI used at J-PARC (left panel), and a photograph of the setup with the inserted sample displayed (right panel). In the left panel, neutron mirrors and BSs correspond to magenta lines and the paths of interfering neutrons are illustrated by the solid (Path 1) and dashed (Path 2) black arrows. In the photograph of the right panel, two beam-splitting etalons (BSEs) are highlighted by yellow squares, and the neutron beam enters there from the top side of the picture.