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

Abstract

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 $\pm$ 0.22 dB (95% confidence interval: [-7.96,-7.10] dB), and an on-chip anti-squeezing level of +9.62 dB $\pm$ 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.

Figures (5)

• Figure 1: Concept and design of the integrated dual-resonant on-chip squeezer based on TF-PPLN. a, Schematic of the microring resonator with PPLN on a nanophotonic chip. The device generates squeezed light at the FH frequency $\omega$ (red, output) from a SH frequency at $2\omega$ (blue, input) through a $\chi^{(2)}$ nonlinear interaction. b, Optical micrograph of the microring with the 0.9mm-long periodical-poled section highlighted. A SEM image of the anti-reflection–coated chip facet is shown on the top right. The bottom panel shows its microscope image of the poled domain pattern inside the resonator. c, SHG efficiency calibration of a nearby poled waveguide, yielding 2516%/W/cm^2. d, Pulley coupler design illustrating phase matching of TE$_0$ modes between bus and resonator waveguides. The FH mode is intentionally over-coupled to enable efficient external coupling. e,f, Experimental transmission spectra at FH (e) and SH (f) wavelengths, showing agreement with the coupled-mode design with the extracted external $Q$-factors of $Q_\mathrm{e} = 2.4 \times 10^{5}$, while intrinsic $Q$-factors are $Q_\mathrm{i} = 2.1 \times 10^{6}$ (SH) and $Q_\mathrm{i} = 2.6 \times 10^{6}$ (FH). The over-coupled FH resonance yields an escape efficiency of $\sim 91.5\%$. Insets show the simulated spatial mode profiles, confirming that both FH and SH modes are supported within the same waveguide, enabling dual-resonant operation.
• Figure 2: SHG characterization of the on-chip $\chi^{(2)}$ squeezer. a, SHG measurement setup. A tunable NIR laser is coupled into the squeezer chip with the generated visible light collected by a visible PD. b, SHG peaks appear at discrete positions within the QPM bandwidth, where the FH and SH resonances simultaneously align. The peak spacing of approximately 4nm arises from the resonator FSRs of the two modes. c, SHG spectra measured at different temperatures with the overall envelope matching with the simulated QPM response of the poled waveguide. d, Power-dependent SHG efficiency showing a normalized on-chip conversion efficiency of 30157e4%/W, extracted from a linear fit to experimental data.
• Figure 3: Squeezed light spectrum characterization of the on-chip $\chi^{(2)}$ squeezer. a, Measurement setup. A visible pump laser near 793.5nm drives the squeezer chip under the dual-resonant condition. An OSA records the output spectrum. b, Transmission spectra of the two modes recorded, showing alternating amplification and de-amplification features when the resonances overlap. c, When the pump excites the co-resonant SH and FH modes, both degenerate and nondegenerate mode pairs are generated, producing a broadband squeezed light spectrum with evenly spaced sidebands. d, Measured on-chip quantum frequency comb spectrum, revealing broadband parametric generation around 1587nm. The inset shows zoomed-in cavity resonances with uniform spacing matching the FSR$_{\mathrm{FH}}$.
• Figure 4: Observation of squeezing from the dual-resonant integrated squeezer. a, Layout of the experimental setup. The NIR laser is split into two paths of squeezing generation and local oscillator before merging at the balanced homodyne detector. NIR: near-infrared; LO: local oscillator; PM: phase modulator; COL: collimator; BS: beam splitter; RSA: radio-frequency spectrum analyzer. b, Normalized quantum noise under the sweeping of the squeezing angle, as shown in the lower trace of the phase-modulator drive. $-0.81dB{} \pm 0.04dB{}$ squeezing and $+4.29dB{} \pm 0.10dB{}$ anti-squeezing are observed.
• Figure 5: Literature comparison of integrated squeezed-light sources. Reported squeezing and anti-squeezing levels are plotted against on-chip pump power for integrated devices based on $\chi^{(2)}$ (green) and $\chi^{(3)}$ (magenta) nonlinearities. Marker shapes denote the device architecture of cavity (circle) and waveguide (cross). Preprint result is indicated by an asterisk. Where available, reported squeezing or squeezed light spectrum bandwidths are annotated next to the corresponding data points, including multi-terahertz bandwidths for waveguide-based devices and a 10.3THz spectral span for our device. Our work occupies the low-power, high-squeezing regime near the left side of the plot, underscoring the advantage of dual-resonant TFLN devices for efficient squeezed-light generation. Ref. [45] is not included due to no inferred value (measured -0.55dB squeezing and 1.55dB anti-squeezing).