🔬 optics

Quantum squeezing in an all-resonant periodically poled lithium niobate microresonator

This paper demonstrates the first fully resonant, quasi-phase-matched $\chi^{(2)}$ cavity squeezer on a thin-film lithium niobate chip, achieving a record high on-chip squeezing level of -7.52 dB with high escape efficiency and broadband spectral coverage, thereby establishing a scalable route for power-efficient quantum-enhanced sensing and metrology.

Original authors: Xinyi Ren, Reshma Kopparapu, Tushar Sanjay Karnik, Chun-Ho Lee, Kiwon Kwon, Clayton Cheung, Yue Yu, Shi-Yuan Ma, Bo-Han Wu, Ran Yin, Lian Zhou, Quntao Zhuang, Dirk Englund, Zaijun Chen, Mengjie Yu

Published 2026-02-27

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Xinyi Ren, Reshma Kopparapu, Tushar Sanjay Karnik, Chun-Ho Lee, Kiwon Kwon, Clayton Cheung, Yue Yu, Shi-Yuan Ma, Bo-Han Wu, Ran Yin, Lian Zhou, Quntao Zhuang, Dirk Englund, Zaijun Chen, Mengjie Yu

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Taming the "Static" of Light

Imagine you are trying to listen to a very faint whisper in a crowded, noisy room. The background chatter (noise) makes it impossible to hear the whisper clearly. In the world of light, this "chatter" is called quantum noise or "shot noise." Even in a perfect vacuum, light isn't perfectly smooth; it jitters like a shaken soda can. This jitter limits how precisely we can measure things, from detecting gravitational waves to securing internet communications.

"Squeezed light" is a special state of light where scientists have managed to "squeeze" that jitter. Imagine a balloon. If you squeeze one side of the balloon (reducing the noise in one measurement), the other side puffs out (increasing the noise in a different measurement). By carefully controlling this, scientists can make the "noise" in the specific direction they care about disappear, allowing for super-sensitive measurements.

The Problem: The Old Ways Were Clunky

For a long time, making this squeezed light required massive, room-sized equipment with mirrors and lasers. It was heavy, expensive, and hard to keep stable.

Later, scientists tried to shrink this down onto computer chips (integrated photonics). However, the chips they used (mostly made of silicon) had a major flaw: they were like trying to push a boulder up a hill. To get enough "squeeze," they needed huge amounts of power, which created heat and more noise, ruining the effect. It was a trade-off: High Power = High Noise.

The Solution: The "Magic" Lithium Niobate Chip

This paper introduces a breakthrough: a tiny, high-performance chip made from Thin-Film Lithium Niobate (TFLN). Think of this material as a "super-highway" for light that interacts with itself much more strongly than silicon does.

Here is how they made it work, using a few key analogies:

1. The "Double-Resonance" Swing

Imagine a child on a swing. To get them to go high, you have to push them at exactly the right moment (the right frequency).

The Old Way: You push the swing (the pump laser), but the swing doesn't resonate well, so you have to push incredibly hard.
This New Way: The scientists built a "double-swing" system. They tuned the chip so that both the pushing force (the pump laser at 793.5 nm) and the resulting motion (the squeezed light at 1587 nm) are perfectly in sync with the swing.
The Result: Because both are resonating perfectly, they get a massive boost in efficiency. They can generate squeezed light with very little power (only 27 milliwatts—about the power of a tiny LED flashlight).

2. The "Super-Exit" Door

Usually, when you generate light inside a tiny chip, it gets stuck or bounces around inside, getting lost before it can escape to be measured.

The Innovation: The team engineered the chip to have a "wide open door" for the squeezed light to leave, while keeping the door closed for the pump laser to stay inside and keep pushing.
The Analogy: Imagine a room where the air (light) is being pumped in, but the exit is a giant sliding door that lets 91.5% of the useful air out instantly, while the pump stays trapped to keep working. This is called high escape efficiency.

3. The "Periodically Poled" Pattern

To make the light interact correctly, the material needs a specific internal pattern, like a zipper.

The Challenge: Making this pattern on a tiny chip is like trying to sew a perfect zipper on a grain of rice. If the teeth aren't aligned, the light scatters and is lost.
The Success: They achieved a "near-perfect" pattern (domain inversion) that covers almost the entire length of the chip. This ensures the light stays focused and doesn't get messy.

The Results: What Did They Achieve?

By combining these techniques, the team achieved several "firsts":

Low Power: They squeezed light using only 27 mW of power. Previous chip-based methods needed hundreds of milliwatts.
High Quality: They measured a "squeezing" level of -0.81 dB directly. When they calculated for the losses in the cables and detectors outside the chip, they estimated the chip itself is squeezing light by -7.52 dB. This is a massive amount of noise reduction.
Broadband: The squeezed light isn't just one color; it's a wide rainbow of colors (a bandwidth of 10.3 THz). This is like having a whole orchestra playing in perfect harmony rather than just a single violin.

Why Does This Matter?

Think of this chip as the iPhone moment for quantum sensors.

Before: Quantum sensors were like mainframe computers—huge, expensive, and only available in a few labs.
Now: This chip is tiny (0.6 mm²), energy-efficient, and scalable.

This technology paves the way for:

Better GPS and Sensors: Detecting tiny changes in gravity or magnetic fields.
Secure Communication: Unhackable internet connections.
Medical Imaging: Seeing biological structures with much higher clarity.

In short, the researchers took a complex, room-sized physics experiment and shrunk it down to a speck on a chip, making it powerful, efficient, and ready to be mass-produced. They turned a "boulder" into a "marble" that rolls perfectly.

1. Problem Statement

Quantum noise fundamentally limits the sensitivity of optical measurements. Squeezed states of light, which suppress quantum noise in one field quadrature below the shot-noise limit, are essential for quantum-enhanced metrology, sensing, and computing. However, generating these states on photonic chips faces significant challenges:

Kerr ( $\chi^{(3)}$ ) Platforms: While widely used, they suffer from intrinsic limitations. Achieving strong squeezing requires high pump powers or extreme cavity quality factors ( $Q$ ), which introduce excess noise (e.g., thermo-refractive fluctuations, Raman scattering) and parasitic nonlinear processes. Furthermore, the spectral proximity of pump and squeezed fields in $\chi^{(3)}$ systems creates a trade-off between escape efficiency and pump power, often necessitating high-extinction-ratio filtering that degrades observable squeezing.
Quadratic ( $\chi^{(2)}$ ) Platforms: These offer stronger parametric interactions and lower pump power requirements. However, previous integrated $\chi^{(2)}$ implementations (typically in thin-film lithium niobate, TFLN) have relied on non-resonant waveguides requiring high power and large footprints, or resonant devices with suboptimal performance (low escape efficiency, low $Q$ factors, or single-resonance configurations) that fail to maximize the nonlinear interaction.

The Core Challenge: Realizing a compact, scalable, on-chip $\chi^{(2)}$ squeezer that simultaneously achieves all-resonant conditions (pump and signal/idler), high intrinsic quality factors, near-full-depth domain inversion, and high escape efficiency to enable low-power, broadband squeezing.

2. Methodology

The authors developed a dual-resonant optical parametric amplifier (OPA) on a thin-film lithium niobate (TFLN) photonic chip.

Device Architecture:
- Material: 600-nm-thick X-cut TFLN wafer.
- Structure: A single racetrack microresonator with a 3177 $\mu$ m path length (Free Spectral Range $\approx$ 42.2 GHz).
- Nonlinearity: Periodically poled lithium niobate (PPLN) section (0.9 mm long) on one straight arm of the racetrack to enable quasi-phase matching (QPM).
- Resonance Design: The cavity is engineered to be resonant at both the Second Harmonic (SH) pump wavelength ( $\approx$ 793.5 nm) and the Fundamental Harmonic (FH) squeezed light wavelength ( $\approx$ 1587 nm).
- Coupling: An optical "pulley" coupler design was used to achieve extreme over-coupling specifically for the FH (squeezed) mode while maintaining optimal coupling for the SH (pump) mode. This maximizes the escape efficiency of the generated squeezed light.
Fabrication & Poling:
- Poling: Electric-field-assisted periodic poling with a period of 4.742 $\mu$ m.
- Quality Control: The process achieved near-full-depth domain inversion with a poling duty cycle of $\approx$ 0.54 and high intensity contrast, verified via two-photon microscopy.
- Optimization: The device features optimized anti-reflection coatings on chip facets to suppress parasitic etalon effects.
Experimental Setup:
- Operation: Continuous-wave (CW) regime.
- Pump: 793.5 nm laser injected into the SH resonance.
- Detection: Balanced homodyne detection using a strong local oscillator (LO) and a balanced photodetector (96% quantum efficiency) to measure quadrature noise.

3. Key Contributions

First All-Resonant $\chi^{(2)}$ Cavity Squeezer: This is the first demonstration of a fully resonant, quasi-phase-matched $\chi^{(2)}$ cavity squeezer on a chip, where both pump and squeezed modes are resonantly enhanced.
Record-Breaking Performance Metrics:
- Intrinsic Quality Factor ( $Q_i$ ): Achieved $Q_i > 2.5$ million for the FH mode (highest reported for PPLN cavities).
- Escape Efficiency: Engineered to exceed 90% (specifically 91.5%) for the squeezed mode, a critical factor for extracting nonclassical light.
- Poling Fidelity: Near-full-depth domain inversion with a duty cycle approaching 50%, enabling high nonlinear efficiency.
Scalable Architecture: The device operates at low pump power (27 mW) with a compact footprint (0.6 mm $^2$ ), overcoming the trade-offs between bandwidth, squeezing ratio, and power consumption typical in previous integrated squeezers.

4. Key Results

Squeezing Levels:
- Measured (Off-chip): $-0.81 \pm 0.04$ dB of squeezing and $+4.29 \pm 0.10$ dB of anti-squeezing at 27 mW pump power.
- Inferred (On-chip): After accounting for coupling losses, facet transmission, and detection efficiency, the on-chip squeezing is estimated at $-7.52 \pm 0.22$ dB (95% confidence interval: $[-7.96, -7.10]$ dB), with on-chip anti-squeezing of $+9.62 \pm 0.25$ dB.
Bandwidth:
- The generated squeezed light exhibits a broadband spectrum with a 3-dB bandwidth exceeding 10.3 THz.
- This corresponds to 244 correlated signal-idler mode pairs and a squeezed-light power of 320 pW at degeneracy.
Nonlinear Efficiency:
- Second-harmonic generation (SHG) efficiency was calibrated at 2516 %/W/cm $^2$ , matching theoretical predictions based on the inferred poling duty cycle.
- Normalized on-chip conversion efficiency reached $30,157 \times 10^{-4}$ %/W.
Comparison: The device achieves the highest squeezing ratio among integrated $\chi^{(2)}$ cavity platforms and operates at significantly lower pump power than state-of-the-art $\chi^{(3)}$ integrated squeezers.

5. Significance

This work establishes a scalable route to fully integrated, power-efficient squeezed-light sources.

Quantum Sensing & Metrology: By enabling high-fidelity squeezing at low power and small footprints, this platform is ideal for deploying quantum-enhanced sensors (e.g., gravitational wave detection, biosensing) in practical, chip-scale networks.
Overcoming Limitations: The dual-resonant, over-coupled design breaks the traditional trade-off between squeezing ratio and bandwidth found in single-pass waveguides and $\chi^{(3)}$ resonators.
Future Applications: The platform supports not only squeezing but also low-noise parametric amplification, quantum frequency combs, and high-brightness spontaneous parametric down-conversion (SPDC). The ability to engineer dispersion and extend bandwidth to multi-terahertz scales opens new avenues for continuous-variable quantum computation and simulation.

In summary, this paper demonstrates a breakthrough in integrated quantum photonics by combining advanced nanofabrication, precise domain engineering, and resonant cavity design to produce high-performance squeezed light on a chip, paving the way for the next generation of quantum technologies.