Fabrication of low-loss quasi-single-mode PPLN waveguide and its application to a modularized broadband high-level squeezer

Abstract

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.

Quantum arbitrary waveform generator

“…Thanks to this, our method can be applied to recently developed efficient heralding methods [46,47] with a success probability around 10 −3 to 10 −2 . By incorporating these efficient heralding methods and Thz-bandwidth quantum light source [48,49] into our method, where the bandwidth is roughly corresponds to the trial rate of state generation, the Q-AWG in Fig. 2 (a) can operate semi-deterministically at a repetition rate over GHz.…”

“…As a broadband source, a single-pass waveguide optical parametric amplifier (OPA) is a promising candidate. In this experiment, we use a low-loss periodically poled LiNbO 3 waveguide OPA module [48] we have recently developed.…”

“…This is because the bandwidth of the generated states, that is, the bandwidth of the filter is roughly regarded as the trial rate of state generation in a heralding scheme. As fast detection of quadratures and photons are having been developed [48,60,61], generation of non-Gaussian states with GHz-bandwidth will be possible before long. If related technologies develop further and the THz-bandwidth of the waveguide OPA is fully exploited, a semi-deterministic Q-AWG with a GHz-order repetition rate can be realized.…”

“…By doing so, the temporal waveforms of the non-Gaussian states are directly decided by the impulse response of optical filters in the system. By incorporating timing control technique [24,[43][44][45], high-success-rate heralding methods [46,47], and THz-bandwidth quantum light source [48,49] into the basic idea, the Q-AWG can output states semideterministically with a repetition rate over GHz. As a demonstration, we generate non-Gaussian states with time-bin and balanced time-bin waveforms which have never been realized before.…”

Generation of Highly Pure Single-Photon State at Telecommunication Wavelength

“…The minimum value of Wiger negativity of the generated quantum state is recorded to be -0.12. Meanwhile, this experiment was performed using a waveguide optical parametric amplifier (OPA) [24] as a squeezed light source. Waveguide OPAs have the advantages of alignment-free and compactness, while they have the challenges of large optical losses, meaning that more refinements must be made before they can be applied to high-purity generation of non-Gaussian state.…”