Squeezed-Light Generation in PICs

• Squeezed-light generation in PICs is a technique that produces nonclassical states with reduced quantum noise using nonlinear processes such as χ(2) and χ(3) interactions.
• Key architectures like microring resonators, photonic crystals, and hybrid cavities enable scalable and reproducible sources with tunable spectral and temporal properties.
• Advanced noise management and on-chip integration support applications in quantum-enhanced sensing, communications, and continuous-variable quantum information processing.

Squeezed-light generation in photonic integrated circuits (PICs) encompasses on-chip realization of nonclassical optical states exhibiting reduced quantum fluctuations in one field quadrature, below the shot-noise limit. These states underpin continuous-variable (CV) quantum information protocols, quantum-enhanced sensing, and advanced optical metrology. A diverse set of physical mechanisms—employing second- and third-order optical nonlinearities, engineered waveguide and resonator architectures, and sophisticated phase-matching schemes—have enabled reproducible, scalable squeezed-light sources that are increasingly compatible with wafer-scale CMOS photonics. This article details the principal architectures, materials, nonlinear processes, device-level innovations, and experimental benchmarks for squeezed-light sources in PICs, including both single-mode and multimode (frequency-comb) regimes, as well as advanced noise management and integration strategies.

1. Mechanisms of Squeezed-Light Generation in PICs

Most PIC-based squeezed-light sources harness either $\chi^{(2)}$ (second-order) or $\chi^{(3)}$ (third-order) nonlinear processes for parametric amplification or four-wave mixing within tightly confined optical modes.

Second-order ($\chi^{(2)}$) mechanisms

• PIC devices in thin-film lithium niobate (TFLN) and hybrid ScAlN/Si$_3$N$_4$ platforms utilize processes such as degenerate optical parametric amplification and spontaneous parametric down-conversion (SPDC) (Park et al., 2023, Arge et al., 24 Jun 2024, Liu et al., 1 Aug 2025). These processes generate correlated photon pairs at half the pump frequency, inducing quadrature noise suppression in the signal field emerging from on-chip resonant cavities or waveguides.
• Achieving phase matching is critical; conventional approaches rely on periodic poling for quasi-phase matching (Chen et al., 2021), while more recent work exploits modal phase matching—engineering distinct spatial modes at fundamental and second harmonic frequencies to satisfy $$n_{\text{eff}}(2\omega_0) \approx n_{\text{eff}}(\omega_0)$$ (Arge et al., 24 Jun 2024).

Third-order ($\chi^{(3)}$) mechanisms

• In Si$_3$N$_4$ and SiN photonic circuits, degenerate four-wave mixing (FWM) in high-Q microring or photonic crystal resonators is the principal nonlinear channel (Cernansky et al., 2019, Vaidya et al., 2019, 2002.01082, Tritschler et al., 22 Feb 2025, Ulanov et al., 24 Feb 2025, Shen et al., 6 May 2025, Liu et al., 12 Sep 2025).
• Dual-pump FWM (two pump resonances straddling the target resonance) generates strongly squeezed light in the central, degenerate resonance. Spontaneous FWM via a single pump also enables squeezed vacuum generation, especially near threshold under strong pump enhancement in high-Q resonators.
• Kerr nonlinearity drives both self-phase modulation (SPM) and cross-phase modulation (XPM), which shape the intracavity field structure and can impact both squeezing efficiency and spectral purity.

Non-cavity platforms

• Integrated waveguides exploiting second-harmonic generation via quasi-phase matching, with either periodic poling (QPM) or engineered modal dispersion, can support broadband (multi-THz) squeezed-light generation in thin-film devices (Chen et al., 2021, Arge et al., 24 Jun 2024).

The device-level squeezing mechanism is universally described by an effective Hamiltonian (e.g., $$H_\text{int} = iA \hat{a}_1 \hat{a}_2 e^{i\omega_2 t} + \text{h.c.}$$ in parametric amplifiers (El-Orany et al., 2011)) whose evolution leads to quantum correlations accessible via field evolution equations featuring hyperbolic functions characteristic of parametric amplifiers.

2. Device Architectures and Engineering Strategies

The sophistication of integrated squeezed-light sources resides in their material selection, waveguide and resonator engineering, and integration of advanced coupling or filtering techniques:

• Microring and photonic crystal resonators: Si$_3$N$_4$ microring resonators with loaded Q factors above $10^6$ and engineered anomalous/normal dispersion are foundational for both single- and multimode squeezed-light generation (Cernansky et al., 2019, Vaidya et al., 2019, Tritschler et al., 22 Feb 2025, Ulanov et al., 24 Feb 2025, Shen et al., 6 May 2025, Liu et al., 12 Sep 2025). Photonic crystal rings with nano-corrugations allow suppression of parasitic nonlinear processes (e.g., SP-SFWM, BS-FWM) via selective mode splitting and resonance hybridization (Ulanov et al., 24 Feb 2025, Zhang et al., 2020).
• Hybrid and monolithic cavities: TFLN OPOs integrating on-chip SHG stages, tunable directional couplers, and balanced homodyne detectors (BHD) exemplify the monolithic approach with degenerate squeezed vacuum generation (Park et al., 2023).
• Asymmetric directional couplers: Pairing a nonlinear and a linear waveguide (nondegenerate amplifier coupling) enables direct transfer of nonclassical states between guides (El-Orany et al., 2011).
• Periodic or aperiodic corrugations: Surface corrugations introduce controlled back-scattering, enhancing effective nonlinearity, phase-matching, and spectral engineering (Jr, 2013, Ulanov et al., 24 Feb 2025).
• Wafer-scale integration: Recent demonstrations on 4-inch Si$_3$N$_4$ wafers achieve quadrature squeezing uniformity better than 0.2 dB across multiple dies with co-integrated pump-rejection filters, edge couplers, and microheaters for resonance tuning and stabilization (Liu et al., 12 Sep 2025).

A summary of these approaches is shown below:

Architecture	Nonlinearity	Main Function
Microring/PhC resonator	$\chi^{(3)}$	FWM-based squeezed/combed multimode sources
Nanophotonic molecule	$\chi^{(3)}$	Photonic molecule with parasitic mode suppression for degenerate squeezing
TFLN ring/OPO	$\chi^{(2)}$	Monolithic degenerate OPO, on-chip BHD
Asymmetric coupler	$\chi^{(2)}$	Nondegenerate amplifier, single-mode squeezed transfer
Corrugated waveguide	$\chi^{(2)}$	Enhanced nonlinearity/band shaping, pulsed squeezing

3. Spectral, Temporal, and Spatial Properties

Integrated sources offer access to widely tunable spectral and temporal squeezing regimes.

• Broadband and comb-like squeezing: Si$_3$N$_4$ microcombs provide multi-THz bandwidth with two-mode squeezed vacua across 16 or more distinct qumodes, with up to 11 THz separation between furthest modes (Shen et al., 6 May 2025). Advanced seed-assisted detection strategies enable direct measurement of these multimode states, bypassing the need for complex local oscillator phase locking.
• Pulse and mode purity: Cavity-based parametric down-conversion in photonic microresonators can concentrate squeezing nearly into a single temporal mode, with the effective mode number approaching unity under optimized pumping, filter, and linewidth conditions (Cui et al., 2020). This is crucial for CV quantum logic and error correction as mode impurity (large K) leads to decoherence and protocol overhead.
• Topological and spatial protection: Topological photonic lattices (e.g., SSH-type dimerized chains) provide spatial localization of the pump field, protecting nonlinear interactions and generated squeezed states against fabrication imperfections and cross-talk (Ren et al., 2021). Cross-correlation and squeezing metrics remain robust even at long evolution distances, confirming their utility in multi-photon circuits.

4. Noise Management and Performance Benchmarks

Key advances in loss and noise mitigation have driven observed squeezing levels in PICs to within reach of those found in bulk optical platforms.

• Coupling and propagation loss: Wafer-scale Si$_3$N$_4$ processes reach propagation losses $\sim$0.1–1 dB/cm and edge coupling efficiencies of 75–90% (Liu et al., 12 Sep 2025). Escape efficiencies and coupling rates are tuned (e.g., via pulley couplers and overcoupled designs) for optimal extraction of nonclassical light.
• Thermal/environmental noise: Thermorefractive noise, scaling as $\Omega^{-2}$, is a dominant excess noise contribution in SiN chips at low frequencies. Cryogenic operation and enhanced interferometer contrast (up to 60 dB) are proposed for further noise suppression (Cernansky et al., 2019).
• Uniformity and fabrication reproducibility: Across-wafer variation below 0.2 dB has been demonstrated for squeezed vacuum sources, facilitated by robust process control and independent parameter extraction for theoretical-experimental consistency (Liu et al., 12 Sep 2025).
• Experimental squeezing levels: Directly measured quadrature squeezing ranges from $\sim$0.45–0.55 dB (TFLN, SiN rings, single-pass platforms) up to $5.6$ dB (Silicon nitride microresonator OPA) and $7.8$ dB (SiN photonic-crystal ring with corrugations), with on-chip corrected values approaching or surpassing 10 dB (Ulanov et al., 24 Feb 2025, Shen et al., 6 May 2025, Tritschler et al., 22 Feb 2025, Zhang et al., 2020).

5. Integration with Advanced PIC Functionality

Monolithic integration strategies facilitate the co-location of squeezed-light generation with filtering, modulation, and detection:

• Integrated balanced homodyne detectors: Demonstrated on-chip in TFLN circuits, enabling direct quadrature tomography with minimized loss and complexity (Park et al., 2023).
• Cascaded filters (add-drop microrings): Suppress strong pump tones for clean extraction of squeezed twin beams, essential for preventing spectral leakage and device heating (Liu et al., 12 Sep 2025).
• Microheaters and thermo-optic tuning: Employed to realize frequency agility, resonator stabilization, and spectral tuning of generated quantum frequency combs over full FSRs (Shen et al., 6 May 2025, Liu et al., 12 Sep 2025).
• Hybrid material systems: Si$_3$N$_4$–ScAlN architectures combine ultra-low-loss waveguides with high-$\chi^{(2)}$ materials (ScAlN) for CMOS-compatible, on-chip parametric devices (Liu et al., 1 Aug 2025).

6. Practical Applications and Implications

Integrated squeezed-light sources are central to a wide range of quantum technologies:

• Quantum information processing: CV cluster states and multimode squeezing are the backbone of measurement-based CV quantum computation and bosonic error correction (2002.01082, Shen et al., 6 May 2025).
• Quantum-enhanced sensing: Squeezed states enable sub-shot-noise precision in metrological settings (e.g., interferometric sensors). Strong on-chip squeezing and frequency-division multiplexing enhance the sensitivity and channel density for sensor arrays (Shen et al., 6 May 2025).
• Quantum communications and networking: Telecom-band on-chip squeezing is compatible with established fiber-optic infrastructure, facilitating high-rate, entanglement-based quantum communication protocols (Mondain et al., 2018, Lenzini et al., 2018).
• Scalable quantum devices: Wafer-scale reproducibility and uniformity are prerequisites for realizing massively parallel entanglement sources, reconfigurable CV quantum processors, and multiplexed quantum memories (Liu et al., 12 Sep 2025).

7. Ongoing Developments and Future Directions

Active research tracks focus on further increasing achievable on-chip squeezing (toward or beyond –10 dB), extending frequency agility and comb size, and integrating more sophisticated quantum functionalities in robust, manufacturable platforms.

• Reductions in parasitic loss and enhancement of escape efficiency (targeting $\eta_\text{esc}>0.95$), improved photonic materials (hybrid $\chi^{(2)}/\chi^{(3)}$ platforms, ferroelectric films), and process uniformity are poised to raise both device and system-level performance (Ulanov et al., 24 Feb 2025, Shen et al., 6 May 2025, Liu et al., 1 Aug 2025).
• Implementation of synthetic reflection self-injection locking, on-chip optical parametric amplifiers, and full BHD integration are anticipated to enable compact, power-efficient squeezed-light sources for both laboratory and field applications (Ulanov et al., 24 Feb 2025, Park et al., 2023).
• Squeezed-lasing schemes stabilize macroscopic squeezed states with laser-like coherence, promising direct application in quantum-enhanced interferometry and atomic metrology (Muñoz et al., 2020).
• Topological protection, mode-division multiplexing, and mode-sorting are emerging as strategies for constructing robust and high-dimensional resource states (Ren et al., 2021).

Squeezed-light generation in PICs is at the core of scalable, CMOS-compatible quantum photonics, providing an enabling resource for fault-tolerant CV quantum information processing, high-performance quantum sensors, and future reconfigurable quantum networks.