A coherent Ising machine for 2000-node optimization problems.

The analysis and optimization of complex systems can be reduced to mathematical problems collectively known as combinatorial optimization. Many such problems can be mapped onto ground-state search problems of the Ising model, and various artificial spin systems are now emerging as promising approaches. However, physical Ising machines have suffered from limited numbers of spin-spin couplings because of implementations based on localized spins, resulting in severe scalability problems.

Generation of a frequency comb spanning more than 3.6 octaves from ultraviolet to mid infrared.

We have observed an ultra-broadband frequency comb with a wavelength range of at least 0.35 to 4.4 μm in a ridge-waveguide-type periodically poled lithium niobate device.

Rapid spectrum measurement at 3 μm over 100 nm wavelength range using mid-infrared difference frequency generation source.

We demonstrate a broadband rapid scanning light source in the 3-μm region by using difference frequency generation (DFG). The DFG source consists of a module with quasi-phase-matched LiNbO ridge waveguides, a 1-μm-band wide swept range laser for the pump source, and a 1.5-μm continuous wave laser for the signal source.

Dual wavelength 3.2-μm source for isotope ratio measurements of (13)CH(4)/(12)CH(4).

Difference frequency generation using one 1.58-μm and two 1.06-μm distributed feedback Bragg-grating laser diodes and a ridge-type PPLN alternately provide two 3.

Integrated quasi-phase-matched second-harmonic generator and electro-optic phase modulator for low-noise phase-sensitive amplification.

We propose a quasi-phase-matched second-harmonic generator integrated with an electro-optic phase modulator in a directly bonded LiNbO3 (DB-LN) waveguide to obtain high signal-to-noise ratio (SNR) pump light for a phase-sensitive amplifier (PSA). This integrated device exhibits 1-MHz modulation and 1-W second-harmonic-generation properties sufficient for phase-locking between the signal and pump and for PSA gain, respectively. A novel PSA configuration based on the high-input-power tolerance of the device helps to suppress the noise from the erbium-doped fiber amplifier used for pump-light generation and leads to an improvement of the SNR of the pump light.

Properties of power dependence on low-crosstalk waveband conversion with an apodized multiperiod-QPM LiNbO₃ device.

Quasi-phase-matched (QPM) LiNbO₃ devices having a multiperiod-periodically-poled structure, which we call multiperiod-QPM LiNbO₃ devices, are capable of shifting the idler waveband during waveband conversion via cascaded difference-frequency generation (cascaded DFG). However, these multiperiod-QPM devices have a problem that they produce extra ripples between QPM peaks in the phase-matching curve. These ripples cause crosstalk between wavebands arising from sum-frequency generation (SFG) between the signal and idler wavebands and subsequent DFG between the SFG wavelength and the signal waveband.

Entanglement distribution over 300 km of fiber.

We report the distribution of time-bin entangled photon pairs over 300 km of optical fiber. We realized this by using a high-speed and high signal-to-noise ratio entanglement generation/evaluation setup that consists of periodically poled lithium niobate waveguides and superconducting single photon detectors. The observed two-photon interference fringes exhibited a visibility of 84%.

Phase sensitive amplification with noise figure below the 3 dB quantum limit using CW pumped PPLN waveguide.

The noise figure (NF) of a phase sensitive amplifier (PSA) based on a periodically poled LiNbO3 (PPLN) waveguide was evaluated in the optical and electrical domains. Phase sensitive amplification was realized using degenerate parametric amplification in the PPLN waveguide, which was pumped by the second harmonic frequency of the signal. Second harmonic pumping enables direct observation of the intrinsic amplified spontaneous emission (ASE), which determined the NF of the PSA.

Phase sensitive degenerate parametric amplification using directly-bonded PPLN ridge waveguides.

We constructed the first CW pumped degenerate parametric amplifier based on periodically poled and ZnO-doped LiNbO3 ridge waveguides. An in-phase gain of + 11 dB was achieved owing to the high conversion efficiency and high damage resistance of the waveguide obtained by employing direct bonding and dry etching techniques. Nearly identical amplification and deamplification were obtained owing to a sufficient spatial and temporal overlap between the pump and signal beams.

Phase-transparent flexible waveband conversion of 43 Gb/s RZ-DQPSK signals using multiple-QPM-LN waveguides.

We report phase-transparent waveband conversion with polarization insensitivity based on second harmonic (SH) wave pumped difference frequency generation (DFG) using multiple-quasi-phase-matched LiNbO(3) (QPM-LN) waveguides. Flexible waveband conversion is demonstrated over the entire C-band using a tunable DFB-LD array (TLA) as a pump source for a multiple-QPM-LN waveguide. The penalty free waveband conversion of 43 Gb/s return-to-zero differential quadrature phase-shift-keying (RZ-DQPSK) waveband signals is successfully achieved.

Generation of time-bin entangled photon pairs by cascaded second-order nonlinearity in a single periodically poled LiNbO(3) waveguide.

Entangled photon pairs are one of the most important resources for the development of quantum communication technologies. In order to produce pulsed photon pairs in the telecommunication band from 1.5 mum pump light, most of the previous experiments used two successive periodically poled lithium niobate (PPLN) waveguides.

Apodized multiple quasi-phase-matched LiNbO3 device for low-cross-talk waveband conversion.

We developed an apodized multiple quasi-phase-matched (QPM) LiNbO(3) device to reduce the cross talk that occurs during waveband conversion. Since we found that the cross talk arises from ripples between multiple QPM peaks in the phase-matching curve, we reduced the amplitude of the ripples by employing the apodized structure. Using this device, we succeeded in demonstrating waveband conversion with low cross talk.

In-line monitoring technique with visible light from 1.3 microm-band SHG module for optical access systems.

We propose an in-line monitoring technique that uses 650 nm visible light for performing maintenance work on Fiber-to-the-home (FTTH) network quickly without the need for measuring skills or external devices. This technique is characterized by visible light (650 nm) generated by an SHG module from the 1.3 microm-band line signal.

Polarization-independent, differential-phase-shift, quantum-key distribution system using upconversion detectors.

We propose and demonstrate a polarization-independent, differential-phase-shift, quantum-key distribution system with upconversion detectors. Even though the detectors have polarization dependency, use of alternative polarization modulation and a two-bit delay interferometer achieves polarization-insensitive operation. In an experiment, sifted key bits were polarization-independently generated over 50 km fiber.

Demonstration of carrier envelope offset locking with low pulse energy.

We demonstrate a carrier-envelope-offset (CEO)- locked frequency comb with 230-pJ fiber coupling pulse energy by using a passively mode-locked Er-fiber amplifier laser. For the generation of an octave-bandwidth spectrum in a highly nonlinear fiber and the second harmonic in a self-referenced interferometer with the lower pulse energy, we use a tellurite photonic crystal fiber and a direct-bonded quasi-phasematched LiNbO3 ridge waveguide, respectively. Our method is feasible for locking the CEO with a lower pulse energy to obtain a low-noise and highaccuracy optical frequency comb at telecommunications wavelengths.

Unequally spaced multiple mid-infrared wavelength generation using an engineered quasi-phase-matching device.

We propose a novel quasi-phase-matched (QPM) device that can generate unequally spaced multiple wavelengths. Unequally spaced multiple QPM peaks can be obtained by employing the optimized phase modulation of a periodic domain structure. We fabricated a LiNbO3 waveguide device for 3.

High-power, tunable difference frequency generation source for absorption spectroscopy based on a ridge waveguide periodically poled lithium niobate crystal.

A novel waveguide for difference frequency generation in the mid-IR spectral region at 3.52 mum is characterized. High mid-IR power of 15 mW and an external conversion efficiency of up to 19 %W( -1) have been obtained.

Simultaneous observation of CO isotopomer absorption by broadband difference-frequency generation using a direct-bonded quasi-phase-matched LiNbO3 waveguide.

Rovibration absorption lines both of 12CO and 13CO are observed simultaneously with the output of a 2 microm broadband difference frequency generated in a direct-bonded quasi-phase-matched LiNbO3 waveguide, which is a 50 mm device with a single quasi-phase-matching period that is operated at a constant temperature. The wavelength conversion efficiency and the difference-frequency generation bandwidth reach 100%/W and 100 nm, respectively. The idler output bandwidth in the 2 microm region is obtained by group-velocity matching or phase-mismatch minimization when a 0.

Generation of pulsed polarization-entangled photon pairs in a 1.55-microm band with a periodically poled lithium niobate waveguide and an orthogonal polarization delay circuit.

We report a scheme for generating pulsed polarization-entangled photon pairs based on conversion from time-bin entanglement to polarization entanglement by use of an orthogonal polarization delay circuit and post-selection. We have experimentally demonstrated the scheme, using a periodically poled lithium niobate waveguide, and successfully obtained polarization entanglement in the 1.55-microm telecom wavelength band.

Multiple quasi-phase-matched LiNbO3 wavelength converter with a continuously phase-modulated domain structure.

We have devised a novel device structure for a multiple quasi-phase-matched wavelength converter. Optimized continuous phase modulation of a periodic domain structure makes possible multichannel pumping with minimum loss of efficiency. Using the device, we demonstrate variable and simultaneous wavelength conversion of wavelength-division multiplexed signals.