Quantum entanglement, the eerie link that binds the fates of particles regardless of distance, has long been the province of elaborate laboratory setups with mirrors on tables and beams snaking through meters of free space. Now, in a milestone that could reshape how scientists build quantum-enhanced sensors, a joint team from Shanxi University and Nanjing University has demonstrated, for the first time, two-mode squeezed light generated and verified entirely on a photonic chip at audio frequencies as low as 60 hertz. The achievement, published in Science Bulletin, bridges a gap that has frustrated researchers for years: the mismatch between the compact promise of integrated photonics and the demanding, low-frequency regimes where some of the most sensitive measurements in science actually happen.
To understand why this matters, it helps to consider what limits any measurement made with light. Photons are not polite observers; they arrive randomly, and their arrival times fluctuate according to the rules of quantum mechanics. This randomness sets a floor on measurement precision known as the shot-noise limit. Whenever an interferometer tries to detect an extremely small displacement, force, magnetic field, or phase shift, the signal ultimately competes with these irreducible quantum jitters. Squeezed light offers a way around this wall. By redistributing quantum uncertainty, reducing it in the quadrature of the optical field that carries the signal while deliberately increasing it in the orthogonal quadrature, squeezed light lets experimenters measure one property with a precision that exceeds the standard quantum baseline. It is this trick that already underpins the quantum-enhanced sensitivity of gravitational-wave detectors, which inject squeezed states into their kilometer-scale interferometers to catch the faintest ripples of spacetime.
The audio-frequency band, spanning roughly tens of hertz to several kilohertz, is precisely where many of the most interesting slowly varying signals live. Gravitational-wave detectors read out their most crucial science band in this range, and radiation-pressure noise, the quantum push and pull of light on mirrors, imposes its limits there too. Any future quantum sensor designed to detect weak, slowly changing forces or fields would need squeezed light at these low frequencies. Integrated photonics would make such systems dramatically smaller, more stable, and easier to scale into arrays. Yet most chip-based demonstrations of squeezed light to date have operated at radio-frequency sidebands, far above the audio band. Pushing down to low frequencies on a chip is brutally difficult because the problems that plague precision optics grow worse as frequency drops: slow fluctuations of the laser, thermal drift of the cavity, environmental disturbances, electronic pickup, residual imbalance in the detectors, and, perhaps most insidiously, long-term phase drift that silently rotates the measured quadrature and washes out the very phase-sensitive quantum correlations the experiment is trying to observe. A chip can generate squeezed light, but holding the measurement phase stable long enough to prove it is another matter entirely.
The research team solved this problem with an elegant strategy they call coherent-comb control. The idea is to keep a phase reference close at hand without ever contaminating the delicate quantum states with a bright locking tone. A weak electro-optic reference comb is derived from the same pump laser that drives the squeezing process, and it travels through the same optical path as the pump and the quantum fields. Crucially, the reference is placed in an orthogonal polarization and given a fixed frequency offset, while the microcavity is engineered to avoid resonating with it. Once the light exits the cavity, the reference comb and the quantum modes are separated and detected independently. Phase detection and feedback then act through this reference channel rather than through the fragile quantum modes themselves. In effect, the reference comb functions as a phase ruler that rides along the entire optical path, faithfully tracking every drift and vibration, without directly manipulating the squeezed light that carries the quantum correlations. It is a bit like navigating a city by following a separate, quiet escort car that mirrors every turn of the vehicle you are escorting, rather than shining headlights directly into its face.
The physical heart of the experiment is a silica microtoroid resonator just 732 micrometers across, small enough to sit on a fingertip. Below the threshold of optical oscillation, the device exploits Kerr four-wave mixing, a nonlinear optical process in which pump photons conspire to produce photons in pairs at symmetrically placed frequencies. These paired quantum frequency modes emerge in a two-mode squeezed vacuum state, a fundamentally quantum configuration in which measurements on the two modes are correlated so strongly that their combined noise falls below the shot-noise limit, a feat impossible for any classical pair of light beams. Two-mode squeezed light is the workhorse of continuous-variable quantum information, and verifying genuine entanglement between the modes requires more than simply observing low noise.
With the homodyne detector’s phase locked by the coherent-comb control scheme, the researchers recorded 100 seconds of continuous data at 10,000 samples per second, roughly one million samples in total, giving an unusually thorough statistical picture of the quantum state. The time traces showed stable, repeatable access to the squeezed, anti-squeezed, and shot-noise quadratures throughout the entire acquisition window, a testament to the robustness of the phase-locking approach. In the frequency domain, the spectra revealed approximately 1.0 decibel of two-mode squeezing below the shot-noise level across the band from 60 hertz to 5 kilohertz. The on-chip inferred squeezing reached about 1.73 decibels once optical losses were accounted for. Below 60 hertz, residual low-frequency technical noise still dominated, and the authors were careful not to lean on that region in support of their claim, an honesty that strengthens the credibility of the demonstration. The lowest verified sideband frequency of 60 hertz sits roughly four orders of magnitude below the minimum analysis frequencies of previous chip-scale demonstrations, a comparison that highlights just how far this single result moves the frontier.
But the team did not stop at showing noise reduction. Observing joint noise below the shot-noise level confirms quantum correlation, yet a complete proof of entanglement demands a stricter test. The researchers changed the locking angle and the local oscillator configuration to measure a full set of single-mode and inter-mode quadrature combinations. From these measurements they reconstructed the two-mode covariance matrix, the complete second-order statistical fingerprint of the quantum state. Applying the positive partial transposition criterion, the gold standard for certifying entanglement in Gaussian states, they obtained a minimum symplectic eigenvalue of 0.395 plus or minus 0.001, comfortably below the separability threshold of 0.5. In plain terms, the two optical modes leaving the chip were provably entangled, their quantum natures woven together in a way no classical explanation can reproduce.
The implications extend well beyond the single device demonstrated here. Because the phase reference can be separated from the quantum modes after sharing the same optical path, the architecture naturally supports stable quadrature measurements across multiple frequency channels simultaneously, opening a practical route to multi-mode quantum optics on a chip. Future applications could include chip-scale quantum sensors for low-frequency signals, arrayed continuous-variable quantum processors, and distributed quantum networks in which many entangled channels are generated and monitored in parallel on a single photonic platform. The coherent-comb control technique itself may prove as influential as the squeezing record, since the phase-stability problem it solves is generic to low-frequency quantum optics everywhere.
The authors are candid that the current result is a platform demonstration rather than a mature quantum sensor. One decibel of observed squeezing, while historically significant for an on-chip audio-band system, is modest compared with the ten or more decibels achieved in mature bulk-optical experiments. Extending the operating range below 60 hertz will require further suppression of technical noise. The study maps out a clear improvement path: higher cavity escape efficiency, lower optical loss and better mode matching, stronger polarization isolation, suppression of parasitic background channels, quieter electro-optic comb generation, improved servo electronics, and enhanced electromagnetic, thermal, and acoustic isolation. Each of these advances translates directly into deeper squeezing levels and access to even lower frequencies.
Still, the moment deserves to be savored. For decades, audio-frequency squeezed light lived only in the cathedral-scale instruments of gravitational-wave astronomy, maintained by armies of engineers and heroic isolation systems. This work shows that the same quantum resource can now be born from a whisper of glass on a chip, its entanglement certified with rigor, and its phase tracked by a clever reference that never touches the quantum light itself. The door to compact, scalable, quantum-enhanced sensing in the audio band has swung open, and what walks through it next may well redefine what small devices can measure about the large world.
Subject of Research: On-chip generation and entanglement verification of audio-frequency two-mode squeezed light using Kerr four-wave mixing in a silica microtoroid resonator with coherent-comb phase control.
Subject of Research: Chemistry
Article Title: Quantum entanglement on a chip reaches audio frequency
Article References: Zhu, X., Cao, Y., Liu, R., He, Y., Zhang, F., Zhang, Y., Du, S., Wang, M., Jiang, X., & Su, X. (2026). On-chip squeezed light in the audio frequency band. Science Bulletin. https://doi.org/10.1016/j.scib.2026.08.054
Image Credits: AI Generated
DOI: 10.1016/j.scib.2026.08.054
Keywords: squeezed light, quantum entanglement, integrated photonics, audio frequency, two-mode squeezing, silica microresonator, Kerr four-wave mixing, shot noise, covariance matrix, positive partial transposition, quantum sensing, homodyne detection
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Katie Riggs. (September 9, 2026). Quantum entanglement on a chip achieves audio-frequency operation. Scienmag. https://scienmag.com/quantum-entanglement-on-a-chip-achieves-audio-frequency-operation/
Katie Riggs. “Quantum entanglement on a chip achieves audio-frequency operation.” Scienmag, 9 September 2026, https://scienmag.com/quantum-entanglement-on-a-chip-achieves-audio-frequency-operation/. Accessed 9 September 2026.
Katie Riggs. “Quantum entanglement on a chip achieves audio-frequency operation.” Scienmag. September 9, 2026. https://scienmag.com/quantum-entanglement-on-a-chip-achieves-audio-frequency-operation/
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