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arXiv:2004.02407 · APL Photonics

Continuous-wave 6-dB-squeezed light with 2.5-THz-bandwidth from single-mode PPLN waveguide

Takahiro Kashiwazaki, Naoto Takanashi, Taichi Yamashima, Takushi Kazama, Koji Enbutsu, Ryoichi Kasahara, Takeshi Umeki, Akira Furusawa

Published Mar 1, 2020• 149 citations
arXivHTMLPDF

Abstract

Terahertz (THz)-bandwidth continuous-wave (CW) squeezed light is essential for integrating quantum processors with time-domain multiplexing (TDM) by using optical delay line interferometers. Here, we utilize a single-pass optical parametric amplifier (OPA) based on a single-spatial-mode periodically poled ZnO:LiNbO3 waveguide, which is directly bonded onto a LiTaO3 substrate. The single-pass OPA allows THz bandwidth, and the absence of higher-order spatial modes in the single-spatial-mode structure helps avoid degradation of squeezing. In addition, the directly bonded ZnO-doped waveguide has durability for high-power pump and shows small photorefractive damage. Using this waveguide, we observe CW 6.3-dB squeezing at 20-MHz sideband by balanced homodyne detection. This is the first realization of CW squeezing with a single-pass OPA at a level exceeding 4.5 dB, which is required for the generation of a two-dimensional cluster state. Furthermore, the squeezed light shows 2.5-THz spectral bandwidth. The squeezed light will lead to the development of a high-speed on-chip quantum processor using TDM with a centimeter-order optical delay line.

DOI
10.1063/1.5142437

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quant-pharXiv:2603.02744•Mar 4, 2026•1 citation
Terahertz (THz)-bandwidth continuous-wave (CW) squeezed light is essential for integrating quantum processors with time-domain multiplexing (TDM) by using optical delay line interferometers. Here, we utilize a single-pass optical parametric amplifier (OPA) based on a single-spatial-mode periodically poled ZnO:LiNbO3 waveguide, which is directly bonded onto a LiTaO3 substrate. The single-pass OPA allows THz bandwidth, and the absence of higher-order spatial modes in the single-spatial-mode structure helps avoid degradation of squeezing. In addition, the directly bonded ZnO-doped waveguide has durability for high-power pump and shows small photorefractive damage. Using this waveguide, we observe CW 6.3-dB squeezing at 20-MHz sideband by balanced homodyne detection. This is the first realization of CW squeezing with a single-pass OPA at a level exceeding 4.5 dB, which is required for the generation of a two-dimensional cluster state. Furthermore, the squeezed light shows 2.5-THz spectral bandwidth. The squeezed light will lead to the development of a high-speed on-chip quantum processor using TDM with a centimeter-order optical delay line.
A continuous-wave (CW) broadband high-level optical quadrature squeezer is essential for high-speed large-scale fault-tolerant quantum computing on a time-domain-multiplexed continuous-variable optical cluster state. CW THz-bandwidth squeezed light can be obtained with a waveguide optical parametric amplifier (OPA); however, the squeezing level has been insufficient for applications of fault-tolerant quantum computation because of degradation of the squeezing level due to their optical losses caused by the structural perturbation and pump-induced phenomena. Here, by using mechanical polishing processes, we fabricated a low-loss quasi-single-mode periodically poled LiNbO3 (PPLN) waveguide, which shows 7% optical propagation loss with a waveguide length of 45 mm. Using the waveguide, we assembled a low-loss fiber-pigtailed OPA module with a total insertion loss of 21%. Thanks to its directly bonded core on a LiTaO3 substrate, the waveguide does not show pump-induced optical loss even under a condition of hundreds of milliwatts pumping. Furthermore, the quasi-single-mode structure prohibits excitation of higher-order spatial modes and enables us to obtain larger squeezing level. Even with including optical coupling loss of the modularization, we observe 6.3-dB squeezed light from the DC component up to a 6.0-THz sideband in a fully fiber-closed optical system. By excluding the losses due to imperfections of the modularization and detection, the squeezing level at the output of the PPLN waveguide is estimated to be over 10 dB. Our waveguide squeezer is a promising quantum light source for high-speed large-scale fault-tolerant quantum computing.
We achieved continuous-wave 8.3-dB squeezed light generation using a terahertz-order-broadband waveguide optical parametric amplifier by improving a measurement setup from our previous work [T. Kashiwazaki et al., Appl. Phys. Lett. 119, 251104 (2021)], where a low-loss periodically poled lithium niobate (PPLN) waveguide had shown 6.3-dB squeezing at a 6 THz frequency. First, to improve efficiency of the squeezed light detection, we reduced effective optical loss to about 12% by removing extra optics and changing the detection method into a low-loss balanced homodyne measurement. Second, to minimize phase-locking fluctuation, we constructed a frequency-optimized phase-locking system by comprehending its frequency responses. Finally, we found optimal experimental parameters of a measurement frequency and a pump power from their dependences for the squeezing levels. The measurement frequency was decided as 11 MHz to maximize a clearance between shot and circuit noises. Furthermore, pump power was optimized as 660 mW to get higher squeezing level while suppressing anti-squeezed-noise contamination due to an imperfection of phase locking. Note that this over-8-dB squeezing is achieved without any loss-correction and circuit-noise correction. Moreover, it is shown that the squeezing level soon after our PPLN waveguide is estimated at over 10 dB, which is thought to be mainly restricted by the waveguide loss. This broadband highly squeezed light opens the possibility to realize fault-tolerant ultra-fast optical quantum computers.
Optical parametric amplification is expected as a key technique for high-speed optical fiber communication and optical quantum computing.We developed a high-gain and broadband optical parametric amplifier (OPA) module by using a periodically poled lithium niobate (PPLN) waveguide, the single-mode core of which is directly bonded to a lithium tantalate substrate.The OPA is assembled as a pump-combiner-integrated module with four optical-fiber input/output ports.By improving the fabrication processes of the PPLN waveguide and the assembly techniques of OPA modules, we achieved over-30-dB phase sensitive amplification with a continuous-wave pump.By using the PPLN waveguide, we successfully obtained over-6-dB squeezed light, the spectral bandwidth of which was 2.5 THz.This high-gain broadband OPA module will help build an advanced information society.
Phase-sensitive detection is the essential projective measurement for measurement-based continuous-variable quantum information processing. The bandwidth of conventional electrical phase-sensitive detectors is up to several gigahertz, which would limit the speed of quantum computation. It is theoretically proposed to realize terahertz-order detection bandwidth by using all-optical phase-sensitive detection with an optical parametric amplifier (OPA). However, there have been experimental obstacles to achieve large parametric gain for continuous waves, which is required for use in quantum computation. Here, we adopt a fiber-coupled χ (2) OPA made of a periodically poled LiNbO 3 waveguide with high durability for intense continuous-wave pump light. Thanks to that, we manage to detect quadrature amplitudes of broadband continuous-wave squeezed light. 3 dB of squeezing is measured up to 3 THz of sideband frequency with an optical spectrum analyzer. Furthermore, we demonstrate the phase-locking and dispersion compensation of the broadband continuous-wave squeezed light, so that the phase of the squeezed light is maintained over 1 THz. The ultra-broadband continuous-wave detection method and dispersion compensation would help to realize all-optical quantum computation with over-THz clock frequency.
We demonstrate the generation of 12.1±0.212.1 \pm 0.212.1±0.2 dB squeezed light from a periodically poled lithium niobate (PPLN) waveguide optical parametric amplifier (OPA). While single-pass OPAs offer squeezed light with THz-order bandwidths, loss from spatial mode mismatch between the squeezed light and the local oscillator (LO) previously capped the squeezing level at $\sim$10 dB [K. Hirota et al., Opt. Express 34, 7958 (2026)]. In this work, we minimize this loss by introducing a machine-learning-optimized spatial light modulator (SLM) in the path of the LO. Specifically, we employed a double-reflection configuration to increase the spatial degrees of freedom, and directly used the measured squeezing level as the optimization's objective function.