Squeezed Light Generation in Periodically Poled Thin-Film Lithium Niobate Waveguides

TL;DR

This work addresses the challenge of integrating squeezed-light sources for continuous-variable quantum technologies on photonic chips. It demonstrates on-chip broadband vacuum squeezing in periodically poled thin-film lithium niobate waveguides by coupling a PPLN nanophotonic waveguide to fiber with a low-loss edge coupler, achieving a measured vacuum squeezing of 1.4 ± 0.1 dB at 38 mW on-chip pump and inferring ~4.7 dB on-chip squeezing after accounting for losses; a longer device without the coupler indicates potential on-chip squeezing up to ~10 dB at 62 mW. The study also reports a broadband SPDC spectrum with a HWHM of ~6.3 THz, and provides a lossy-model underpinning that matches observed squeezing trends: $S_{} = 10\log_{10}(1 - T + T e^{\pm 2 \sqrt{\eta P}})$. These results suggest that with reduced coupling and propagation losses, TFLN can achieve significantly higher on-chip squeezing, enabling scalable CV quantum sensing and information processing on integrated platforms.

Abstract

Squeezed states of light play a key role in quantum-enhanced sensing and continuous-variable quantum information processing. Realizing integrated squeezed light sources is crucial for developing compact and scalable photonic quantum systems. In this work, we demonstrate on-chip broadband vacuum squeezing at telecommunication wavelengths on the thin-film lithium niobate (TFLN) platform. Our device integrates periodically poled lithium niobate (PPLN) nanophotonic waveguides with low-loss edge couplers, comprising bilayer inverse tapers and an SU-8 polymer waveguide. This configuration achieves a fiber-to-chip coupling loss of 1.4 dB and a total homodyne detection loss of 4 dB, enabling a measured squeezing level of 1.4 dB. Additional measurements in a more efficient PPLN waveguide (without low-loss couplers) infer an on-chip squeezing level of over 10 dB at a pump power of 62 mW. These results underscore the potential of TFLN platform for efficient and scalable squeezed light generation.

Figures (4)

• Figure 1: Fabrication and characterization of periodically poled lithium niobate (PPLN) nanophotonic waveguides.a, Top-view optical micrograph, b, cross-section SEM, and c, top-view laser-scanning SHG microscopy image of a fabricated PPLN nanophotonic waveguide. d, Measured SHG spectrum from the PPLN waveguide, where the main peak is at 1560.7 nm. e, On-chip SHG power as a function of on-chip pump power in the 1 cm long PPLN waveguide. A linear fitting reveals an on-chip SHG efficiency of 918$\pm$24% W$^{-1}$.
• Figure 2: Efficient and broadband chip-to-fiber coupling on thin-film lithium niobate (TFLN) integrated platform.a, A schematic of the coupling scheme in 600 nm thick TFLN. Light couples from a 600 nm thick LN rib waveguide to 300 nm thick LN ridge waveguide through an inverse taper in the top 300 nm thick LN layer, and evanescently couples to the SU-8 waveguide through a second inverse taper in the bottom 300 nm thick LN layer. b, Micrograph of the fabricated chip-to-fiber coupler. Zoomed-in SEMs of the inverse tapers in the inset (i) top and inset (ii) bottom LN layers after etching. Inset (iii) Cross-section SEM of the SU-8 edge coupler. c, Measured transmission spectrum of a chip-to-fiber coupler, showing a coupling loss of 1.4 dB around 1560 nm with a 1 dB transmission bandwidth of 80 nm.
• Figure 3: Squeezed light generation in a PPLN nanophotonic waveguide.a, Measurement setup. CW laser: continuous-wave laser, EDFA: erbium-doped fiber amplifier, SHG: second-harmonic generation module, WDM: wavelength division multiplexer, PC: polarization controller, LPF: long-pass filter, BS: beam splitter, balanced detector, ESA: electrical spectrum analyzer. b, Measured and simulated squeezing and anti-squeezing levels as a function of pump power. The errors in noise fluctuations are on the order of 0.1 dB, and therefore, not marked on the plot. c, Normalized noise power (red line) at a pump power of 38 mW, while the LO phase is being tuned continuously. Black line shows the shot-noise level. d, Squeezed light spectrum showing a half-width at half-maximum (HWHM) of $\sim$6.3 THz.
• Figure 4: 10 dB on-chip squeezing in an efficient PPLN nanophotonic waveguide.a, Measured SHG spectrum from the 1.1 cm long PPLN waveguide, with a sharp peak at a pump wavelength of 1560.6 nm. b, SHG power as a function of pump power, showing a high SHG efficiency of 3282 $\pm$ 60 % W$^{-1}$. c, Simulated and measured squeezing and anti-squeezing levels as a function of pump power, for different assumed losses, indicating an on-chip squeezing of $\sim$10 dB. d, Normalized noise power as a function of time with phase tuning at a pump power of 68 mW.