Hybrid quantum network for sensing in the acoustic frequency range
Valeriy A. Novikov, Jun Fang Jia, Túlio Brito Brasil, Andrea Grimaldi, Maïmouna Bocoum, M. V. Balabas, Jörg Helge Müller, Emil Zeuthen, Eugene Simon Polzik
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.
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.
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.
Quantum noise reduction and entanglement-enhanced sensing in the acoustic frequency range is an outstanding challenge relevant for a number of applications including magnetometry and broadband noise reduction in gravitational wave detectors. Here we experimentally demonstrate quantum behavior of a macroscopic atomic spin oscillator in the acoustic frequency range. Quantum back-action of the spin measurement, ponderomotive squeezing of light, and virtual spring softening are observed at oscillation frequencies down to the sub-kHz range. Quantum noise sources characteristic of spin oscillators operating in the near-DC frequency range are identified and means for their mitigation are presented.
Optical atomic magnetometry is a powerful tool for continuous sensing applications, yet, in the absence of quantum correlations, its sensitivity is limited by the standard quantum limit (SQL) stemming from a trade-off between optical probe imprecision and quantum measurement backaction. Beyond-SQL sensitivity requires quantum correlations that modify these measurement noise sources. Here we demonstrate such sensitivity by using entangled state of the probe light and by engineering correlations between measurement imprecision and backaction. Having first explored SQL in a broad range of frequencies, we demonstrate overcoming the limit by combining variational readout with coupling the magnetometer to one mode of a bipartite entangled light state and conditioning the results on the other entangled mode. Tuning the detected light quadratures and combining the signals from the two measurement channels, we achieve sensitivity beyond the SQL in a broad range of acoustic frequencies which has so far remained inaccessible to quantum-noise-limited optical magnetometry.
Advances in precision optomechanical measurement underpin a wide range of technologies, from inertial sensors to the detection of gravitational waves and the exploration of fundamental physics. For such applications, the ultimate sensitivity is limited by quantum noise: shot noise, which stems from the stochastic arrival of photons, dominates at high frequencies, while Quantum Back-Action (QBA) noise—arising from the random radiation-pressure force exerted by the light field—prevails at low frequencies. The interplay between these two contributions defines the Standard Quantum Limit (SQL) of interferometry, a power-dependent trade-off that constrains further performance improvements. Although several sub-SQL techniques have been demonstrated, they typically enhance narrow-band sensitivity or require long filter cavities. Coherent Quantum Noise Cancellation (CQNC), first proposed by Tsang and Caves, offers a broadband solution by introducing an Effective Negative-Mass Oscillator (ENMO) that destructively interferes with the QBA noise across the entire spectrum except at the mechanical resonance of the opto-mechanically coupled system. A mathematically equivalent scheme for broadband QBA evasion has been demonstrated—for example, using atomic ensembles. The operation of this scheme is restricted to specific atomic transition wavelengths and is therefore unsuitable for many optomechanical platforms. This thesis reports the first all-optical, table-top realisation of an ENMO capable of Coherent Quantum Noise Cancellation in quantum optomechanical sensors. The scheme employs an optical analogue of the optomechanical interaction, generated through a combination of parametric down-conversion and beam-splitting processes, yielding a tunable, wavelength-flexible, and compact system. The achieved parameters satisfy the CQNC targets established in earlier theoretical work, and the current system predicts a broadband quantum-noise reduction of 3.6 dB (77% suppression of QBA noise(in dB)) at the frequency of maximum suppression. With further reduction of intracavity losses, the projected cancellation improves to 11.9 dB (81% suppression (in dB)). The experimental realisation features two polarisation-coupled optical resonators that host a two-mode squeezing interaction; the optical resonators possess distinct detunings and linewidths. The interdependent nature of the ENMO system motivated the development of an in-situ characterisation method, which is essential for integrating the ENMO with an optomechanical system in practical applications. Adequately addressing the system’s complexity required the development of five specialised experimental techniques for parameter tuning and comprehensive characterisation. Beyond quantum-noise cancellation, this system lends itself to application in quantum-information and quantum-communication tasks, including an on-demand, high-purity single-photon source and a quantum memory. Owing to the optical platform’s immunity to thermal decoherence, it can be operated at room temperature, and the parametric down-conversion process affords flexibility in wavelength selection. The results presented here advance the state of the art in broadband quantum noise reduction in a broad class of optomechanical devices, with immediate relevance for emerging quantum-technology applications.
Broadband quantum noise reduction can be achieved in gravitational-wave detectors by injecting frequency-dependent squeezed light into the dark port of the interferometer. This frequency-dependent squeezing can be generated by combining squeezed light with external filter cavities. However, in future long baseline interferometers (LBIs), the filter cavity required to achieve the broadband squeezing has a low bandwidth---necessitating a very long cavity to mitigate the issue from optical loss. It has been shown recently that by taking advantage of Einstein-Podolsky-Rosen (EPR) entanglement in the squeezed light source, the interferometer can simultaneously act as a detector and a filter cavity. This is an attractive broadband squeezing scheme for LBIs because the length requirement for the filter cavity is naturally satisfied by the length of the interferometer arms. In this paper we present a systematic way of finding the working points for this broadband squeezing scheme in LBIs. We also show that in LBIs, the EPR scheme achieves nearly perfect ellipse rotation as compared to 4-km interferometers which have appreciable error around the intermediate frequency. Finally, we show that an approximation for the optomechanical coupling constant in the 4-km case can break down for longer baselines. These results are applicable to future detectors such as the 10-km Einstein Telescope and the 40-km Cosmic Explorer.
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.