Quantum frequency conversion using 4-port fiber-pigtailed PPLN module.

Quantum frequency conversion (QFC), which involves the exchange of frequency modes of photons, is a prerequisite for quantum interconnects among various quantum systems, primarily those based on telecom photonic network infrastructures. Compact and fiber-closed QFC modules are in high demand for such applications. In this paper, we report such a QFC module based on a fiber-coupled 4-port frequency converter with a periodically poled lithium niobate (PPLN) waveguide. The demonstrated QFC shifted the wavelength of a single photon from 780 to 1541 nm. The single photon was prepared via spontaneous parametric down-conversion (SPDC) with heralding photon detection, for which the cross-correlation function was 40.45 ± 0.09. The observed cross-correlation function of the photon pairs had a nonclassical value of 13.7 ± 0.4 after QFC at the maximum device efficiency of 0.73, which preserved the quantum statistical property. Such an efficient QFC module is useful for interfacing atomic systems and fiber-optic communication.

Broadband optical parametric amplification using PPLN waveguide pumped by detuned second harmonic.

Optical parametric amplification in the range of 1.3-1.8 µm was demonstrated by using a periodically poled LiNbO3 (PPLN) waveguide as a nonlinear medium by varying the detuning of the pump wavelength. A wide range of detuning was enabled by using a multiple-quasi-phase-matched (M-QPM) LiNbO3 waveguide for pump generation through second harmonic generation (SHG) and temperature control of the PPLN waveguide. Broadband optical amplification and wavelength conversion through difference frequency generation (DFG) are considered useful for widening the bandwidth of optical communication.

Parametric wavelength conversion with bidirectional utilization of a multiple QPM device.

A configuration for wavelength conversion and optical amplification by parametric interaction using a nonlinear optical device is proposed. It enables pump generation through second harmonic generation (SHG), difference frequency generation (DFG), and optical parametric amplification (OPA) using a multiple-quasi-phase-matched (M-QPM) LiNbO3 waveguide in a bidirectional manner. Wavelength conversion for the 1.4-1.6 µm band is experimentally demonstrated. In addition, it is demonstrated that the parametric gain band can be changed using various detunings between the pump and QPM wavelengths used for the DFG/OPA process. The proposed method would be useful for enabling high-capacity optical transmission outside the 1550-nm band.

Over-30-dB gain and 1-dB noise figure phase-sensitive amplification using a pump-combiner-integrated fiber I/O PPLN module.

Phase-sensitive amplifiers (PSAs) via the optical parametric amplification (OPA) process are capable of near-noiseless amplification, which can improve the performance of optical communications systems. OPA based on periodically poled lithium niobate (PPLN) waveguides is a proven means to implement a PSA with low additional nonlinear effects, such as frequency chirp, stimulated Brillouin scattering, and parametric crosstalk due to unwanted nonlinear interactions among pump and other signal waves. However, fiber compatibility is a challenge because optical coupling loss between a fiber and PPLN waveguide limits essential performance such as the gain and noise figure (NF), which makes PSAs still far from being practical. In this work, we developed a PPLN-waveguide-based pump-combiner-integrated OPA module with fiber input and output ports. With our recent development and optimization of the OPA module, we demonstrated high-performance phase-sensitive amplification with a gain of over 30 dB and an NF of 1.0 dB. In addition, we observed a 3-dB gain bandwidth of over 65 nm and flat NF characteristics in that wavelength band. The high conversion efficiency and high damage resistance of the PPLN waveguide, obtained by employing direct bonding and dry etching techniques, provide a high parametric gain. The low-loss coupling for the signal and pump between the fiber and a spot-size-converter-integrated PPLN waveguide through the dichroic beam combiner improve not only the gain but also the NF of the amplifier. Using the PSA as a preamplifier, the low-noise characteristics were confirmed by the sensitivity improvement provided by the low NF value.

100,000-spin coherent Ising machine.

Computers based on physical systems are increasingly anticipated to overcome the impending limitations on digital computer performance. One such computer is a coherent Ising machine (CIM) for solving combinatorial optimization problems. Here, we report a CIM with 100,512 degenerate optical parametric oscillator pulses working as the Ising spins. We show that the CIM delivers fine solutions to maximum cut problems of 100,000-node graphs drastically faster than standard simulated annealing. Moreover, the CIM, when operated near the phase transition point, provides some extremely good solutions and a very broad distribution. This characteristic will be useful for applications that require fast random sampling such as machine learning.

Collective and synchronous dynamics of photonic spiking neurons.

Nonlinear dynamics of spiking neural networks have recently attracted much interest as an approach to understand possible information processing in the brain and apply it to artificial intelligence. Since information can be processed by collective spiking dynamics of neurons, the fine control of spiking dynamics is desirable for neuromorphic devices. Here we show that photonic spiking neurons implemented with paired nonlinear optical oscillators can be controlled to generate two modes of bio-realistic spiking dynamics by changing optical-pump amplitude. When the photonic neurons are coupled in a network, the interaction between them induces an effective change in the pump amplitude depending on the order parameter that characterizes synchronization. The experimental results show that the effective change causes spontaneous modification of the spiking modes and firing rates of clustered neurons, and such collective dynamics can be utilized to realize efficient heuristics for solving NP-hard combinatorial optimization problems.

Non-degenerate phase-sensitive amplification scheme using digital dispersion pre-equalization for unrepeated transmission.

We experimentally demonstrate an ultra-low-noise pre-amplification using a non-degenerate phase-sensitive amplifier (ND-PSA) with an optically dispersion-unmanaged link. Chromatic dispersion (CD) compensation is required for phase-sensitive amplification after fiber transmission. In the conventional transmitter configuration for ND-PSAs in which phase-conjugated light (idler light) is optically generated, it is necessary to optically compensate for the CD, for example, by using dispersion-compensating fibers. In this work, we propose an ND-PSA scheme using a digitally generated idler and CD pre-equalization by means of digital signal processing. We conduct an unrepeated transmission over a 200-km single-mode fiber with a 10-Gbaud 64QAM signal using the periodically poled LiNbO3-based PSA. The experimental results demonstrate that the proposed ND-PSA scheme provides a low-noise pre-amplification that outperforms the EDFA without optical CD compensation.

Continuous and long-term stabilization of degenerate optical parametric oscillators for large-scale optical hybrid computers.

The minimum requirements for an optical reservoir computer, a recent paradigm for computation using simple algorithms, are nonlinearity and internal interactions. A promising optical system satisfying these requirements is a platform based on coupled degenerate optical parametric oscillators (DOPOs) in a fiber ring cavity. We can expect advantages using DOPOs for reservoir computing with respect to scalability and reduction of excess noise; however, the continuous stabilization required for reservoir computing has not yet been demonstrated. Here, we report the continuous and long-term stabilization of an optical system by introducing periodical phase modulation patterns for DOPOs and a local oscillator. We observed that the Allan variance of the optical phase up to 100 ms was suppressed and that the homodyne measurement signal had a relative standard deviation of 1.4% over 62,500 round trips. The proposed methods represent important technical bases for realizing stable computation on large-scale optical hybrid computers.

All-optical phase-sensitive detection for ultra-fast quantum computation.

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 LiNbO3 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.

Experimental investigation of performance differences between coherent Ising machines and a quantum annealer.

Physical annealing systems provide heuristic approaches to solving combinatorial optimization problems. Here, we benchmark two types of annealing machines-a quantum annealer built by D-Wave Systems and measurement-feedback coherent Ising machines (CIMs) based on optical parametric oscillators-on two problem classes, the Sherrington-Kirkpatrick (SK) model and MAX-CUT. The D-Wave quantum annealer outperforms the CIMs on MAX-CUT on cubic graphs. On denser problems, however, we observe an exponential penalty for the quantum annealer [exp(-αDW N 2)] relative to CIMs [exp(-αCIM N)] for fixed anneal times, both on the SK model and on 50% edge density MAX-CUT. This leads to a several orders of magnitude time-to-solution difference for instances with over 50 vertices. An optimal-annealing time analysis is also consistent with a substantial projected performance difference. The difference in performance between the sparsely connected D-Wave machine and the fully-connected CIMs provides strong experimental support for efforts to increase the connectivity of quantum annealers.

Understanding dynamics of coherent Ising machines through simulation of large-scale 2D Ising models.

Many problems in mathematics, statistical mechanics, and computer science are computationally hard but can often be mapped onto a ground-state-search problem of the Ising model and approximately solved by artificial spin-networks of coupled degenerate optical parametric oscillators (DOPOs) in coherent Ising machines. To better understand their working principle and optimize their performance, we analyze the dynamics during the ground state search of 2D Ising models with up to 1936 mutually coupled DOPOs. For regular as well as frustrated and disordered 2D lattices, the machine finds the correct solution within just a few milliseconds. We determine that calculation performance is limited by freeze-out effects and can be improved by controlling the DOPO dynamics, which allows to optimize performance of coherent Ising machines in various tasks. Comparisons with Monte Carlo simulations reveal that coherent Ising machines behave like low temperature spin systems, thus making them suitable for optimization tasks.

Robust respiration rate estimation using adaptive Kalman filtering with textile ECG sensor and accelerometer.

An adaptive Kalman filter-based fusion algorithm capable of estimating respiration rate for unobtrusive respiratory monitoring is proposed. Using both signal characteristics and a priori information, the Kalman filter is adaptively optimized to improve accuracy. Furthermore, the system is able to combine the respiration-related signals extracted from a textile ECG sensor and an accelerometer to create a single robust measurement. We measured derived respiratory rates and, when compared to a reference, found root-mean-square error of 2.11 breaths-per-minute (BrPM) while lying down, 2.30 BrPM while sitting, 5.97 BrPM while walking, and 5.98 BrPM while running. These results demonstrate that the proposed system is applicable to unobtrusive monitoring for various applications.

Silica-based PLC with heterogeneously-integrated PDs for one-chip DP-QPSK receiver.

To realize a DP-QPSK receiver PLC, we heterogeneously integrated eight high-speed PDs on a silica-based PLC platform with a PBS, 90-degree optical hybrids and a VOA. The use of a 2.5%-Δ waveguide reduced the receiver PLC size to 11 mm x 11 mm. We successfully demonstrated 32 Gbaud DP-QPSK signal demodulation with the receiver PLC.

Silica-based, compact and variable-optical-attenuator integrated coherent receiver with stable optoelectronic coupling system.

We demonstrate a compact and variable-optical-attenuator (VOA) integrated coherent receiver with a silica-based planar lightwave circuit (PLC). To realize the compact receiver, we integrate a VOA in a single PLC chip with polarization beam splitters and optical 90-degree hybrids, and employ a stable optoelectronic coupling system consisting of micro lens arrays and photodiode (PD) subcarriers with high-speed right-angled signal lines. We integrate a VOA and a coherent receiver in a 27x40x6 mm package, and successfully demodulate a 128-Gbit/s polarization division multiplexed (PDM) quadrature phase shift keying (QPSK) signal with a VOA-assisted wide dynamic range of more than 30 dB.

Temperature insensitive and ultra wideband silica-based dual polarization optical hybrid for coherent receiver with highly symmetrical interferometer design.

To extend the operation wavelength range of dual-polarization optical hybrids (DPOH), we propose a highly symmetrical interferometer design for a polarization beam splitter and an optical hybrid to reduce temperature and wavelength dependence. The design successfully decreases this dependence, and a fabricated DPOH with silica-based planar lightwave circuits provides temperature-insensitive performance with a polarization extinction ratio of over 25 dB and phase errors of less than 3 degrees over the entire C- and L-bands.

100-Gbit/s PDM-QPSK coherent receiver with wide dynamic range and excellent common-mode rejection ratio.

We report the design and fabrication of a hybrid-integration-type coherent receiver. We optimize the receiver building blocks, and achieve a -25-dB common-mode rejection ratio and a 20-dB signal input power dynamic range.