Squeezed light offers genuine quantum advantage in enhanced sensing and quantum computation; yet the level of squeezing or quantum noise reduction generated from nanophotonic chips has been limited. In addition to strong quantum noise reduction, key desiderata for such a nanophotonic squeezer include frequency agility or tunability over a broad frequency range, and simultaneous operation in many distinct, well-defined quantum modes (qumodes). Here we present a strongly overcoupled silicon nitride squeezer based on a below-threshold optical parametric amplifier (OPA) that produces directly detected squeezing of 5.6 dB
± 0.2 dB, surpassing previous demonstrations in both continuous-wave and pulsed regimes. We introduce a seed-assisted detection technique into such nanophotonic squeezers that reveals a quantum frequency comb (QFC) of 16 qumodes, with a separation of 11~THz between the furthest qumode pair, while maintaining a strong squeezing. Additionally, we report spectral tuning of a qumode comb pair over one free-spectral range of the OPA, thus bridging the spacing between the discrete modes of the QFC. Our results significantly advance both the generation and detection of nanophotonic squeezed light in a broadband and multimode platform, establishing a scalable, chip-integrated path for compact quantum sensors and continuous-variable quantum information processing systems.
Macroscopic bright squeezed light is a crucial continuous-variable quantum resource. However, generating it in integrated Kerr microresonators typically requires strong external overcoupling, which inevitably increases the parametric oscillation threshold and forces a reliance on noisy pump amplifiers. Here, we demonstrate a foundry-compatible silicon nitride ( Si 3 N 4 ) optical parametric oscillator (OPO) that successfully breaks this fundamental trade-off. By leveraging ultralow intrinsic loss alongside a large-free-spectral-range microresonator and a broadband pulley coupler, we achieve a milliwatt-level oscillation threshold with high escape efficiencies across dual telecom bands (C and L). This architecture enables the robust, pump-amplifier-free generation of twin-beam intensity-difference squeezing. We directly observe optical squeezing approaching 3 dB in both bands, with quantum noise reduction extending to analysis frequencies of nearly 100 MHz. Furthermore, our phenomenological model of the power-dependent squeezing dynamics captures the competition among fundamental quantum correlations, detuning-locking jitter, and classical excess noise. Armed with these physical insights and our optimized low-threshold architecture, we successfully bridge the gap between macroscopic quantum squeezing and stringent on-chip power budgets, providing a practical, scalable pathway toward fully monolithic, directly diode-pumped continuous-variable quantum light sources.
Squeezed states, with their noise reduction below the standard quantum limit, are promising quantum states of light exploited for enhancing sensitivity in quantum sensing or as carriers of information in continuous variable quantum computing and communication. To make use of their quantum advantage, the main common challenge in generating these states is achieving high levels of squeezing in low loss setups.Over the years, several bulk optical nonlinear crystals and cavities have been employed for the generation of different types of high-quality squeezed states. However, recent progress in fabricating ultra-low loss integrated waveguides has shifted the interest of the community towards the integration of squeezing sources to gain advantages in terms of compactness, scalability, and reproducibility.The aim of this thesis is to investigate the main characteristics, limits, and requirements for the generation of vacuum and bright squeezed states by second- and third-order nonlinear devices, with more emphasis on integrated cavities. In particular, we demonstrate high on-chip intensity difference squeezing between bright mode pairs created in an engineered SiN microring resonator. In addition, we propose the use of the Ikeda map, a semi-classical simulation tool, for the investigation of two-mode quadrature squeezing generation in microresonators, discussing its competitiveness against standard quantum models. These results contribute to the optimization of engineered integrated devices with the aim of attaining high levels of squeezing sufficient to bring real advantages in classical and quantum applications.
Squeezed light constitutes a key resource for quantum optical technologies including quantum sensing, computing, communication and metrology. For many applications the generation of squeezed light typically requires at least two nonlinear optical stages involving careful phase and frequency matching to achieve the required mixing of squeezed and coherent light. In our work, we introduce an on-chip system that simplifies the generation of coherent-squeezed light, utilizing only a single squeezing stage. We achieve this by pumping a silicon nitride (
Si3N4) microring resonator to produce single-mode squeezed light through four-wave mixing at the same frequency as the pump mode, leveraging the inherent
χ(3)-nonlinearity of the
Si3N4 resonator. Our on-chip system demonstrates a squeezing of -4.7 dB with a clear perspective towards -10 dB squeezing. We also provide a theoretical model that describes the straightforward yet robust generation of single-mode squeezing at the injection locking point of the ring resonator. In fact, we show that a design with a normal dispersion can be used for robust generation of bright squeezed light without the need for careful suppression of unwanted nonlinear processes. Overall, our findings highlight an approach which drastically simplifies the generation of coherent-squeezed light in photonic integrated circuits.
Quantum noise limits the sensitivity of optical measurements, but squeezed states of light enable quantum-enhanced metrology, sensing, and information processing. Most on-chip squeezed-light sources rely on Kerr (
χ(3)) nonlinearities, remain limited by pump power and excess loss constraints. Quadratic (
χ(2)) platforms instead provide stronger parametric interactions, lower pump power requirements, and greater spectral engineering flexibility. Here, we demonstrate strong, broadband squeezed-light generation on a thin-film lithium niobate (TFLN) photonic chip using a dual-resonant optical parametric amplifier implemented in a single periodically poled LN (PPLN) microresonator. Near-full-depth domain inversion is achieved simultaneously with highly over-coupled resonances, exhibiting escape efficiencies exceeding 90% and intrinsic quality factors above 2.5 million in a 0.6 mm
2 X-cut TF-PPLN resonator, enabling efficient squeezing at 1587 nm when pumped at 793.5 nm. Operating in the continuous-wave regime, we directly measure -0.81 dB of squeezing below the shot-noise limit with a pump power of 27 mW, together with +4.29 dB of anti-squeezing. From these measurements, we infer an on-chip squeezing level of -7.52 dB
± 0.22 dB (95% confidence interval: [-7.96,-7.10] dB), and an on-chip anti-squeezing level of +9.62 dB
± 0.25 dB. We demonstrate single-mode squeezing at degeneracy with a squeezed-light spectrum exceeding 10.3 THz. This work reports the highest squeezing ratio among integrated
χ(2) cavity platforms and the first quasi-phase matched, fully resonant
χ(2) cavity squeezer on chip, establishing a scalable route to fully integrated power-efficient squeezed-light sources for quantum-enhanced sensing and metrology.
Integrated nonlinear microring resonators are promising sources of bright twin-beams exhibiting intensity difference squeezing. To take advantage of the noise reduction of these quantum states, many applications require high squeezing levels. However, the attainable on-chip squeezing is constrained by the ratio of intrinsic to coupling loss, which is strongly related to design and fabrication limits. Here, we demonstrate a silicon nitride microring resonator engineered to enable a set of periodic strongly overcoupled resonances from which the estimated on-chip intensity difference squeezing is 11.8 dB, exceptionally high for an integrated device. Due to significant setup loss, we directly measure 1.4±0.2 dB of squeezing, corresponding to an on-chip level of 9.2±5.1 dB. In addition, the new, to the best of our knowledge, design inherently suppresses mode competition in the modes surrounding the twin-beams, enabling a constant high squeezing level up to moderate powers. These results represent a significant step toward the miniaturization of integrated devices whose performance benefits from surpassing the quantum noise limit using squeezed states, with particular relevance to sensing applications.