A joint research team from Shanxi University and Nanjing University reported the first on-chip demonstration of two-mode squeezed light in the audio-frequency band. The study, published in Science Bulletin, combines a silica microcavity with a low-noise phase-control strategy to observe squeezing down to 60 Hz and to verify entanglement between the generated optical modes.
When light is used to measure an extremely small displacement, force, field, or phase change, the measurement is ultimately affected by quantum fluctuations of photons. These fluctuations set the shot-noise level, a standard quantum baseline for optical measurements. Squeezed light reduces the noise in the quadrature of the optical field that carries the signal while increasing it in the other. This makes squeezed light an important resource for quantum-enhanced sensing and metrology.
The audio-frequency band is especially important because many slowly varying signals occur at frequencies from tens of hertz to several kilohertz. Examples include key readout bands in gravitational-wave detection and measurements limited by radiation-pressure noise. Integrated photonics could make squeezed-light systems smaller, more stable, and easier to scale, but most previous chip-based demonstrations operated at radio-frequency sidebands. At lower frequencies, slow laser and cavity fluctuations, environmental drift, electronic pickup, and residual detector imbalance become much more significant. Long-term phase drift can also rotate the measured quadrature and wash out phase-sensitive quantum correlations even when squeezing is generated inside the cavity.
To address this problem, the researchers developed a coherent-comb control method. A weak electro-optic reference comb is derived from the same pump laser and travels through the same optical path as the pump and quantum fields. The reference is placed in an orthogonal polarization and given a fixed frequency offset, while the microcavity is designed to avoid resonating with it. After the light leaves the cavity, the reference and quantum modes are separated and detected independently. Therefore, phase detection and feedback are acted through the reference channel instead of directly applying modulation or a bright locking tone to the fragile quantum modes. In practical terms, the reference comb acts as a phase ruler that follows the optical path without directly manipulating the squeezed light.
The experiment used a silica microtoroid resonator with a diameter of 732 micrometers. Below the oscillation threshold, pairs of quantum frequency modes in a two-mode squeezed vacuum state are generated through Kerr four-wave mixing. With the homodyne detector phase locked by coherent-comb control, the researchers recorded 100 seconds of data at 10,000 samples per second, corresponding to about one million samples. The time traces showed stable access to the squeezed, anti-squeezed, and shot-noise quadratures throughout the acquisition window. The frequency spectra revealed approximately 1.0 dB of two-mode squeezing below the shot-noise level from 60 Hz to 5 kHz. Below 60 Hz, the present system was limited by residual low-frequency technical noise, and the authors did not use that region to support the squeezing claim. In the comparison of representative integrated platforms, the minimum analysis frequency is about four orders of magnitude lower than in earlier chip-scale demonstrations.
Observing joint noise below the shot-noise level confirms quantum correlation, but it is not by itself a complete entanglement test. The team therefore changed the locking angle and local-oscillator configuration to measure a set of single-mode and inter-mode quadrature combinations. From these measurements, they reconstructed the two-mode covariance matrix and applied the positive partial transposition criterion. The minimum symplectic eigenvalue was 0.395 ± 0.001, which is below the separability threshold of 0.5.
The results bridge a long-standing gap between bulk-optical audio-band squeezing and integrated quantum photonics. Because the phase reference can be separated from the quantum modes after sharing the same optical path, the architecture offers a practical route to stable quadrature measurements across multiple frequency channels. It could support future chip-scale quantum sensors for low-frequency signals, as well as arrayed and distributed continuous-variable quantum systems.
The current result is a platform demonstration rather than a mature quantum sensor. The measured squeezing is approximately 1 dB, and extending the operating range below 60 Hz will require further reduction of technical noise. The study identifies several routes for improvement, including higher cavity escape efficiency, lower optical loss and mode mismatch, stronger polarization isolation, suppression of parasitic background channels, lower technical noise in the electro-optic comb, improved servo electronics, and better electromagnetic, thermal, and acoustic isolation. These advances could enable higher squeezing levels and operation at even lower frequencies.
The work was supported by the Innovation Program for Quantum Science and Technology, the National Natural Science Foundation of China, and the Fund for Shanxi “1331 Project” Key Subjects Construction.
Science Bulletin
Experimental study