EPR Speedmeter Demonstrates Frequency-Dependent Velocity Measurement in Table-Top Interferometer

Gravitational-wave detectors constantly struggle with noise that limits their sensitivity, a fundamental constraint dictated by the laws of quantum physics, and researchers are exploring innovative ways to overcome this challenge. A team led by S. L. Kranzhoff and S. L. Danilishin from Maastricht University, along with S. Steinlechner from the same institution and T. Zhang from the University of Birmingham, now demonstrates a working prototype of a device called an EPR Speedmeter, which measures the velocity of a mirror rather than its position. This approach, based on probing an interferometer with different polarisation modes of light, effectively suppresses the quantum noise that hinders detection, and the team’s table-top experiment confirms the device responds to velocity, exhibiting a frequency dependence characteristic of a speed measurement. The successful realisation of this system represents a significant step towards building more sensitive gravitational-wave detectors and potentially unlocking new insights into the universe.

Squeezed Light Improves Gravitational Wave Detection

This document details the development and theoretical basis of an EPR Speedmeter, a novel optical system designed to enhance the sensitivity of gravitational wave detectors like LIGO and Virgo. The core challenge in detecting these faint ripples in spacetime lies in overcoming quantum noise, a fundamental limit to measurement precision. The EPR Speedmeter addresses this by manipulating the quantum state of light, a technique known as squeezing, which reduces noise in one aspect of the light wave. This system innovatively functions like a speedmeter, responding to the velocity of a mirror rather than its displacement, a crucial step towards reducing low-frequency noise and improving detection capabilities.

The system employs lasers and mirrors to generate and manipulate squeezed states of light, creating a response proportional to the velocity of a test mass within the interferometer. Detailed mathematical models underpin the system’s behavior, describing the relationship between mirror velocity and the resulting signal. This research is directly relevant to gravitational wave astronomy, and is being considered as a potential upgrade for existing detectors like LIGO and Virgo. The technology could also be incorporated into future, more sensitive detectors like the Einstein Telescope and Cosmic Explorer, contributing to the broader field of quantum metrology.

The research builds upon foundational work by pioneers in squeezed light and quantum noise reduction. It acknowledges existing squeezing technology already implemented in LIGO and Virgo, and connects to research on advanced detector concepts like the Einstein Telescope and Cosmic Explorer. The EPR Speedmeter represents a potentially significant innovation in gravitational wave detection, grounded in a strong theoretical framework, and offering the potential for substantial improvements in detector sensitivity. Future investigations will focus on experimental verification, practical implementation, comparison to existing noise reduction techniques, and optimizing the efficiency of generating squeezed states of light.

Velocity Measurement Circumvents Quantum Noise Limits

Researchers have developed a novel approach to gravitational wave detection by directly measuring the velocity of a mirror, rather than its position, effectively bypassing a fundamental limitation imposed by quantum mechanics. Recognizing that standard interferometers are limited by quantum noise arising from the need to precisely determine mirror position, the team sought a method to circumvent this constraint by focusing on speed. This innovative strategy hinges on the principle that measuring velocity directly can reduce the impact of back-action noise, a key impediment to detecting faint gravitational waves. The core of their methodology lies in exploiting the differing responses of two polarization states of light within a specially designed triangular cavity.

By carefully controlling the reflectivity of the cavity mirrors for different polarizations, they created two distinct optical pathways with differing bandwidths, essentially tuning how quickly each polarization state responds to changes in the mirror’s position. This difference in response is crucial, as it allows the system to effectively combine information from different points in time, transforming a position measurement into a velocity readout. The team’s setup utilizes the frequency-dependent reflectivity of the triangular cavity, where the response to mirror motion varies depending on the polarization of light and the frequency of the signal. This differential response, arising from the bandwidth disparity between the two polarization modes, effectively encodes the mirror’s velocity, allowing for a more sensitive and accurate detection of subtle movements. This approach represents a significant departure from conventional interferometry, as it directly measures velocity rather than inferring it from position changes. The use of a triangular cavity, combined with precise control over polarization, provides a compact and efficient means of realizing this velocity readout.

Velocity Measurement Circumvents Quantum Noise Limit

Researchers have demonstrated a novel approach to gravitational wave detection by building a tabletop experiment that measures the velocity of a mirror, rather than its position, effectively circumventing a fundamental limitation imposed by the Heisenberg uncertainty principle. Traditional gravitational wave detectors are limited by quantum noise arising from the need to precisely measure the position of mirrors; this new technique aims to reduce that noise by focusing on velocity measurements. The experiment utilizes a triangular cavity, carefully designed to manipulate the bandwidth of light polarized in two different directions. The core principle involves exploiting the differing responses of these two polarization modes to mirror motion; one mode responds quickly, while the other responds more slowly.

This difference in response creates a system that effectively measures velocity, as the signal vanishes at very low frequencies and increases linearly with higher frequencies. The team successfully demonstrated this “speed-like” frequency dependence, confirming that the system behaves as a velocity readout. This is achieved through precise control of light polarization and the reflectivity of the cavity mirrors, creating a system where the two polarization modes experience different optical paths and, consequently, different bandwidths. Importantly, this tabletop experiment represents a proof-of-principle demonstration, showing that velocity measurements are indeed possible and can be realized with existing optical components. While current gravitational wave detectors rely on measuring incredibly small changes in distance, this new technique offers a potential pathway to overcome the limitations imposed by quantum noise, potentially enhancing the sensitivity of future detectors.

Velocity Readout Demonstrates EPR Speedmeter Concept

This research presents a table-top demonstration of the Einstein-Podolsky-Rosen (EPR) Speedmeter concept, a technique proposed to reduce noise in gravitational-wave detectors. The team successfully built an optical system employing two polarisation modes within a triangular cavity, demonstrating a ‘speed-like’ response to mirror motion, meaning the system effectively measures the velocity of the mirror, rather than its position. The measured response aligned with theoretical predictions, showing vanishing sensitivity at very low frequencies and increasing linearly with frequency up to a specific limit.