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International Journal of Environmental Sciences

Broadband Quantum Noise Reduction In A Small-Scale Suspended Interferometer With Quantum Entangled Squeezed Beams

W. Ali, B. Garaventa, S. Di Pace, F. De Marco, S. Lee, V. Sequino

Published Aug 4, 2025• 0 citations
Publisher

Abstract

General relativity predicts the existence of gravitational waves (GWs), ripples in spacetime that are now observable to highly sensitive interferometers. However, quantum noise poses a significant challenge for gravitational wave (GW) interferometers, influencing the entire bandwidth range (10-10,000 Hz) of the current GW detectors. At higher frequencies, shot noise (SN) predominates, while at lower frequencies, radiation-pressure noise (RPN) becomes the limiting factor, reducing both sensitivity and measurement precision. Squeezing techniques are implemented to address this issue by confining uncertainty within one quadrature of the light field. Starting from the observing run, O4 Virgo and LIGO have enhanced this method through frequency-dependent squeezing (FDS). By employing a 300-meter-long detuned filter cavity, the squeezing ellipse is rotated, allowing phase squeezing at high frequencies to reduce and amplitude squeezing at low frequencies to mitigate RPN, ultimately refining detection capabilities across the frequency spectrum. A promising alternative technique to reduce quantum noise across the entire sensitivity band uses Einstein-Podolsky-Rosen (EPR) quantum entanglement. This approach involves injecting two entangled squeezed beams at different frequencies into the dark port of the interferometer, enabling it to act as both a filter cavity and a gravitational wave detector. The Suspended Interferometer for Ponderomotive Squeezing (SIPS) adopts a large-scale GW detector layout, with a Michelson configuration and Fabry-Perot arm cavities. This system was initially thought to generate FDS in the GW frequency band using the ponderomotive technique. To enhance stability, a precise local control system was developed for the main optics of SIPS, which are suspended in a double pendulum configuration with monolithic fibers to reduce thermal noise. Advanced PXI-based data acquisition and position-sensitive detectors (PSDs) monitor angular and linear displacements, while real-time signal processing in LabVIEW enables corrective actuation, marking a significant step toward improved alignment and noise reduction in GW interferometry. The paper highlights the current status of SIPS, focusing on the local control strategy, underscoring its potential impact on the field.

DOI
10.64252/22ctj311

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General relativity predicts the existence of gravitational waves (GWs), ripples in spacetime that are now observable to highly sensitive interferometers. However, quantum noise poses a significant challenge for gravitational wave (GW) interferometers, influencing the entire bandwidth range (10-10,000 Hz) of the current GW detectors. At higher frequencies, shot noise (SN) predominates, while at lower frequencies, radiation-pressure noise (RPN) becomes the limiting factor, reducing both sensitivity and measurement precision. Squeezing techniques are implemented to address this issue by confining uncertainty within one quadrature of the light field. Starting from the observing run, O4 Virgo and LIGO have enhanced this method through frequency-dependent squeezing (FDS). By employing a 300-meter-long detuned filter cavity, the squeezing ellipse is rotated, allowing phase squeezing at high frequencies to reduce and amplitude squeezing at low frequencies to mitigate RPN, ultimately refining detection capabilities across the frequency spectrum. A promising alternative technique to reduce quantum noise across the entire sensitivity band uses Einstein-Podolsky-Rosen (EPR) quantum entanglement. This approach involves injecting two entangled squeezed beams at different frequencies into the dark port of the interferometer, enabling it to act as both a filter cavity and a gravitational wave detector. The Suspended Interferometer for Ponderomotive Squeezing (SIPS) adopts a large-scale GW detector layout, with a Michelson configuration and Fabry-Perot arm cavities. This system was initially thought to generate FDS in the GW frequency band using the ponderomotive technique. To enhance stability, a precise local control system was developed for the main optics of SIPS, which are suspended in a double pendulum configuration with monolithic fibers to reduce thermal noise. Advanced PXI-based data acquisition and position-sensitive detectors (PSDs) monitor angular and linear displacements, while real-time signal processing in LabVIEW enables corrective actuation, marking a significant step toward improved alignment and noise reduction in GW interferometry. The paper highlights the current status of SIPS, focusing on the local control strategy, underscoring its potential impact on the field.
General relativity predicts the existence of Gravitational Waves (GWs), ripples in spacetime that are now observable to highly sensitive interferometers. However, Quantum Noise (QN) poses a significant challenge for Gravitational Wave (GW) interferometers, influencing the entire bandwidth range (10-10,000 Hz) of the current GW detectors. At higher frequencies, Shot Noise (SN) predominates, while at lower frequencies, Radiation Pressure Noise (RPN) becomes the limiting factor, reducing both sensitivity and measurement precision. Squeezing techniques are implemented to address this issue by confining uncertainty within one quadrature of the light field. During O4, LIGO successfully implemented Frequency-Dependent Squeezing (FDS), while Virgo integrated and commissioned the FDS system without actively operating it during the science run. By employing a 300-meterlong detuned Filter Cavity (FC), the squeezing ellipse is rotated, allowing phase squeezing at high frequencies to reduce and amplitude squeezing at low frequencies to mitigate RPN, ultimately refining detection capabilities across the frequency spectrum. This doctoral thesis focuses on a promising alternative technique for future GW detectors, that achieves broadband QN reduction by exploiting Einstein–Podolsky–Rosen (EPR) quantum entanglement. This approach involves injecting two entangled, squeezed beams at different frequencies into the dark port of the interferometer, enabling it to act as both a FC and a GW detector. To experimentally investigate these techniques, the suspended Interferometer for Ponderomotive Squeezing (SIPS) platform adopts a large-scale GW detector layout featuring a Michelson configuration with Fabry-Perot (FP) arm cavities. To guarantee the extreme optical alignment necessary for quantum noise mitigation, a high-precision, real-time local control system was developed for the SIPS main optics. Using a dummy mirror suspended in a double-pendulum configuration with monolithic fibers, a custom real-time signal processing and feedback control architecture was developed. This subsystem utilizes a PXI hardware framework and position-sensitive detectors (Power Spectral Densitys (PSDs)) implemented in LabVIEW to actively damp the mechanical resonances of the suspensions. Extending these core activities toward future infrastructures, this work directly contributed to the realization and commissioning of the new PNRR ETIC-GALILEO quantum optics cleanroom facility in Genoa for the Einstein Telescope (ET). Within this infrastructure, preliminary optical studies were initiated for Einstein Telescope Low Frequency (ET-LF) low-loss Faraday isolators operating at 1550 nm. Concurrently, numerical simulations were performed using Finesse3 to model and investigate candidate excess noise sources currently limiting the sensitivity of Advanced Virgo Plus
We are developing a small-scale interferometer with monolithic suspension of test masses (SIPS), that will be sensitive to the quantum radiation pressure noise in the audio frequency band of gravitational wave detectors. In the same time, a table-top experiment for the frequency dependent squeezing generation, through the Einstein Podolsky Rosen (EPR) principle, is under construction. SIPS interferometer, being designed to achieve the quantum radiation pressure noise limit exploiting high finesse optical cavities, turns out to be a suitable test bench for the EPR technique, before a possible integration in Virgo for broadband quantum noise reduction. In this work, we describe the concept of the SIPS experiment, the most important noises affecting this setup, and we briefly present the current status and the plan for the integration with the EPR squeezing experiment.
Frequency-dependent squeezing (FDS) represents a well established way to address quantum noise (QN) in Gravitational-Wave (GW) Earth-based detectors, such as LIGO, Virgo, and KAGRA. This technique is realized with the use of an external detuned optical resonator, the filter cavity. The experiment we present here is a table-top prototype that will probe a cheaper, more compact and more flexible strategy for broadband QN reduction (Ma et al., 2017). This scheme is based on two-mode Einstein–Podolsky–Rosen (EPR) entangled squeezed light, and it works without any filter cavity. The EPR-entangled beams will propagate in a small-scale suspended interferometer with high-finesse arm-cavities. This experiment aims at validating the EPR conditional squeezing at audio frequencies, suited for GW detection, implementing also innovative optical techniques.
The astrophysical reach of current and future ground-based gravitational-wave detectors is mostly limited by quantum noise, induced by vacuum fluctuations entering the detector output port. The replacement of this ordinary vacuum field with a squeezed vacuum field has proven to be an effective strategy to mitigate such quantum noise and it is currently used in advanced detectors. However, current squeezing cannot improve the noise across the whole spectrum because of the Heisenberg uncertainty principle: when shot noise at high frequencies is reduced, radiation pressure at low frequencies is increased. A broadband quantum noise reduction is possible by using a more complex squeezing source, obtained by reflecting the squeezed vacuum off a Fabry-Perot cavity, known as filter cavity. Here we report the first demonstration of a frequency-dependent squeezed vacuum source able to reduce quantum noise of advanced gravitational-wave detectors in their whole observation bandwidth. The experiment uses a suspended 300-m-long filter cavity, similar to the one planned for KAGRA, Advanced Virgo, and Advanced LIGO, and capable of inducing a rotation of the squeezing ellipse below 100 Hz.
Next-generation Earth-based gravitational-wave (GW) detectors, such as the Einstein Telescope and Cosmic Explorer, will be at least an order of magnitude larger and more sensitive than current ones. Their quantum noise reduction systems, using linear filter cavities, will also scale up. To reduce system size and complexity, the EPR conditional squeezing scheme has been proposed as a cost-effective alternative to km-long filter cavities, while also aiming at lower optical losses. This experiment is designed to validate EPR conditional squeezing in a suspended small-scale interferometer limited by quantum radiation pressure noise, in the frequency band relevant to large-scale GW observatories, ranging from a few tens of Hz to a few kHz. It incorporates advanced methods such as reflective mode-matching telescopes and an automated data acquisition system.
Abstract Quantum Noise limits the sensitivity of gravitational wave interferometers across the full detection band (10 Hz – 10 kHz). Current detectors mitigate quantum noise through frequency-dependent squeezing using large, detuned filter cavities. However, such infrastructure poses challenges for future detectors like the Einstein Telescope, which require more compact and cost-effective solutions. A promising alternative uses Einstein-Podolsky-Rosen (EPR) entanglement to achieve frequency-dependent squeezing without external filter cavities. We are developing an experiment in Virgo’s R&D squeezing laboratory at the European Gravitational Observatory to investigate EPR-based frequency-dependent squeezing in the audio band. The project involves researchers from INFN groups and universities in Italy and institutions in South Korea. After a brief introduction to the scientific motivation, this work describes the main components and summarizes the current status of our experiment and its potential impact on third-generation gravitational wave detectors.