This study investigates the optimization of hybrid quantum networks, integrating two-color Einstein-Podolsky Rosen (EPR) light sources and tunable spin oscillators, to enhance Gravitational-Wave Detection (GWD) and distributed quantum sensing. Initial challenges with high quadrature amplitudes were addressed by adjusting frequency and coupling scales, significantly reducing noise below the standard quantum limit. Time evolution analyses demonstrated damped oscillations stabilizing over time, while Power Spectral Density (PSD) plots confirmed effective broadband noise suppression. Design enhancements, such as cryogenic cooling and low loss cavities, further lowered thermal noise and decay rates, improving system scalability. However, residual amplitude clipping and PSD peaks indicate potential nonlinearities and decoherence, suggesting areas for refinement. The study advocates for experimental validation using unclipped models and cost-effective cooling strategies, highlighting the framework’s potential for quantum-enhanced metrology in astrophysics and geophysics. Future research should address scalability and cost challenges to enable practical deployment.
This study investigates the optimization of hybrid quantum networks, integrating two-color Einstein-Podolsky Rosen (EPR) light sources and tunable spin oscillators, to enhance Gravitational-Wave Detection (GWD) and distributed quantum sensing. Initial challenges with high quadrature amplitudes were addressed by adjusting frequency and coupling scales, significantly reducing noise below the standard quantum limit. Time evolution analyses demonstrated damped oscillations stabilizing over time, while Power Spectral Density (PSD) plots confirmed effective broadband noise suppression. Design enhancements, such as cryogenic cooling and low loss cavities, further lowered thermal noise and decay rates, improving system scalability. However, residual amplitude clipping and PSD peaks indicate potential nonlinearities and decoherence, suggesting areas for refinement. The study advocates for experimental validation using unclipped models and cost-effective cooling strategies, highlighting the framework’s potential for quantum-enhanced metrology in astrophysics and geophysics. Future research should address scalability and cost challenges to enable practical deployment.
Ultimate limits for the sensing of fields and forces are set by the quantum noise of a sensor1–3. Entanglement allows for suppression of such noise and for achieving sensitivity beyond standard quantum limits4–7. Applicability of quantum optical sensing is often restricted by fixed wavelengths of available photonic quantum sources. Another ubiquitous limitation is associated with challenges of achieving quantum-noise-limited sensitivity in the acoustic noise frequency range relevant for several applications. Here we demonstrate a tool for broadband quantum sensing by performing quantum state processing that can be applied to a wide range of the optical spectrum and by suppressing quantum noise over an octave in the acoustic frequency range. An atomic spin ensemble is strongly coupled to one of the frequency-tunable beams of an Einstein–Podolsky–Rosen (EPR) source of light. The other EPR beam of light, entangled with the first one, is tuned to a disparate wavelength. Engineering the spin ensemble to act as a negative-mass or positive-mass oscillator, we demonstrate frequency-dependent quantum noise reduction for measurements at the disparate wavelength. The tunability of the spin ensemble enables targeting quantum noise in a variety of systems with dynamics ranging from kHz to MHz. As an example of broadband quantum noise reduction in the acoustic frequency range, we analyse the applicability of our approach to gravitational-wave detectors (GWDs). Other possible applications include continuous-variable quantum repeaters and distributed quantum sensing. A hybrid quantum network combining entangled light with an atomic spin ensemble engineered to act as a negative-mass oscillator enables frequency-dependent quantum noise reduction for measurements in the acoustic noise frequency range relevant for gravitational-wave detection.
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
Quantum noise is one of the limiting factors in laser-interferometric gravitational-wave (GW) detectors. The application of squeezed states in these interferometers allows the reduction of quantum noise in one quadrature. Due to opto-mechanical coupling in a GW detector the squeezed quadrature needs to be rotated within the spectrum to achieve a broadband noise reduction. So far, the implementation of additional filter cavities is considered that allow for the optimal, frequency-dependent rotation of the squeezed quadrature. However, these cavities need to have low loss, a length in the order of \unit[100]{m} and must be situated in the vacuum system, making them cost-intensive. In 2017, Ma and coworkers proposed a scheme for the broadband quantum-noise reduction without the need of additional filter cavities. It was shown by Brown et al. that a similar scheme can be used to broadband-enhance interferometers with a detuned signal-recycling cavity. Here, we performed a proof-of-principle experiment of the proposal on a table-top-scale. Squeezed states were produced detuned to the carrier field of a \unit[2.5]{m}-linear cavity and read out in a bichromatic homodyne detection. The frequencies of the lower and upper local oscillator were at entangled sidebands of the squeezed field. Depending on the relations between the involved frequencies, we can address both variants of the proposal. We show, that the frequency-dependences of the resulting noise spectra fit to a theoretical model we derived from the theory used by Ma et al. With this work we set the path towards an implementation of these schemes in a GW-detector prototype, where the compatibility of the approach with a low-frequency suspended interferometer can be tested. Moreover, we used the same setup to show nonclassical interferometer enhancement at low frequencies by high-frequency squeezed states. Here, a heterodyne readout scheme was implemented to avoid limiting noises at low frequencies. The application of squeezed states centered around the local oscillator frequency yielded an improvement in signal-to-noise ratio of $\unit[3.4]{dB}\pm\unit[0.3]{dB}$. Additionally, I designed, built and characterized a compact source of squeezed vacuum-states at \unit[1064]{nm} with a footprint of just $\unit[0.8]{m^2}$. I show measurements of squeezed states from this source with a reduction of quantum noise of $\unit[10.7]{dB}\pm\unit[0.2]{dB}$ below the vacuum noise and present a noise reduction in the frequency range from \unit[70]{kHz} to \unit[65]{MHz}.
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.
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.
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.