Impact of pump-phase noise in PPLN-based optical parametric amplifier and wavelength converter for digital coherent transmission.

Wideband signal amplification and optical signal processing with a high gain using an optical parametric amplifier based on a periodically poled LiNbO3 (PPLN) waveguide is attractive for constructing wideband optical fiber networks. We experimentally investigate the transfer characteristics of the phase noise of a pump laser in χ(2)-based optical parametric amplification and wavelength conversion on the basis of second-harmonic-generation and differential-frequency-generation processes. We also evaluate the effect of the transferred phase noise on signal quality in dispersion-unmanaged digital coherent fiber transmission systems. We show that the phase noise is transferred only to the wavelength-converted idler and does not affect the amplified signal even by using a pump laser with a MHz-order linewidth. We also show that the phase noise transferred to the idler light can have a similar impact on signal quality as equalization-enhanced phase noise (EEPN) in digital coherent transmission. The signal penalty including EEPN was evaluated with several pump lasers and at symbol rates of 32, 64, and 96 Gbaud. We also propose a method of using correlated pump lights between a wavelength converter pair to cancel out the transfer of phase noise.

Transmission of signal and idler using optical parametric amplifier based on PPLN waveguide.

In this study, we attempted the repeated transmission of S-band signals by compensating for the loss of the transmission fiber using an optical parametric amplifier (OPA) based on a periodically poled LiNbO3 waveguide. We examined and compared the two configurations. The first method involved wavelength conversion of the signal to an idler, while the second method amplified the signal itself. In the latter case, we demonstrated repeated transmissions using external dispersion compensation. In the former case, we demonstrated that it was possible not only to compensate for fiber loss but also to reduce the accumulation of dispersion in transmission fibers by utilizing spectral inversion.

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.

Low-loss polarization control in fiber systems for quantum computation.

Optical quantum information processing requires low loss interference of quantum light. Also, when the interferometer is composed of optical fibers, degradation of interference visibility due to the finite polarization extinction ratio becomes a problem. Here we propose a low loss method to optimize interference visibility by controlling the polarizations to a crosspoint of two circular trajectories on the Poincaré sphere. Our method maximizes visibility with low optical loss by using fiber stretchers as polarization controllers on both paths of the interferometer. We also experimentally demonstrate our method, where the visibility was maintained basically above 99.9% for three hours using fiber stretchers with an optical loss of 0.02 dB (0.5%). Our method makes fiber systems promising for practical fault-tolerant optical quantum computers.

Non-Gaussian quantum state generation by multi-photon subtraction at the telecommunication wavelength.

In the field of continuous-variable quantum information processing, non-Gaussian states with negative values of the Wigner function are crucial for the development of a fault-tolerant universal quantum computer. While several non-Gaussian states have been generated experimentally, none have been created using ultrashort optical wave packets, which are necessary for high-speed quantum computation, in the telecommunication wavelength band where mature optical communication technology is available. In this paper, we present the generation of non-Gaussian states on wave packets with a short 8-ps duration in the 1545.32 nm telecommunication wavelength band using photon subtraction up to three photons. We used a low-loss, quasi-single spatial mode waveguide optical parametric amplifier, a superconducting transition edge sensor, and a phase-locked pulsed homodyne measurement system to observe negative values of the Wigner function without loss correction up to three-photon subtraction. These results can be extended to the generation of more complicated non-Gaussian states and are a key technology in the pursuit of high-speed optical quantum computation.

Programmable time-multiplexed squeezed light source.

One of the leading approaches to large-scale quantum information processing (QIP) is the continuous-variable (CV) scheme based on time multiplexing (TM). As a fundamental building block for this approach, quantum light sources to sequentially produce time-multiplexed squeezed-light pulses are required; however, conventional CV TM experiments have used fixed light sources that can only output the squeezed pulses with the same squeezing levels and phases. We here demonstrate a programmable time-multiplexed squeezed light source that can generate sequential squeezed pulses with various squeezing levels and phases at a time interval below 100 ns. The generation pattern can be arbitrarily chosen by software without changing its hardware configuration. This is enabled by using a waveguide optical parametric amplifier and modulating its continuous pump light. Our light source will implement various large-scale CV QIP tasks.

Quantum arbitrary waveform generator.

Controlling the temporal waveform of light is the key to a versatile light source in classical and quantum electronics. Although pulse shaping of classical light is mature and has been used in various fields, more advanced applications would be realized by a light source that generates arbitrary quantum light with arbitrary temporal waveforms. We call such a device a quantum arbitrary waveform generator (Q-AWG). The Q-AWG must be able to handle various quantum states of light, which are fragile. Thus, the Q-AWG requires a radically different methodology from classical pulse shaping. Here, we invent an architecture of Q-AWGs that can operate semi-deterministically at a repetition rate over gigahertz in principle. We demonstrate its core technology via generating highly nonclassical states with temporal waveforms that have never been realized before. This result would lead to powerful quantum technologies based on Q-AWGs such as practical optical quantum computing.

Generation of Schrödinger cat states with Wigner negativity using a continuous-wave low-loss waveguide optical parametric amplifier.

Continuous-wave (CW) squeezed light is used in the generation of various optical quantum states, and thus is a fundamental resource of fault-tolerant universal quantum computation using optical continuous variables. To realize a practical quantum computer, a waveguide optical parametric amplifier (OPA) is an attractive CW squeezed light source in terms of its THz-order bandwidth and suitability for modularization. The usages of a waveguide OPA in quantum applications thus far, however, are limited due to the difficulty of the generation of the squeezed light with a high purity. In this paper, we report the first observation of Wigner negativity of the states generated by a heralding method using a waveguide OPA. We generate Schrödinger cat states at the wavelength of 1545 nm with Wigner negativity using a quasi-single-mode ZnO-doped periodically poled LiNbO3 waveguide module we developed. Wigner negativity is regarded as an important indicator of the usefulness of the quantum states as it is essential in the fault-tolerant universal quantum computation. Our result shows that our waveguide OPA can be used in wide range of quantum applications leading to a THz-clock optical quantum computer.

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.

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.

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.

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.

Multiple optical carrier generation using frequency mixing in damage resistant multiple QPM device.

We devised a method for the measurement of the phase-matching curve of multiple quasi-phase-matched (QPM) LiNbO3 waveguide under conditions of high-power second-harmonic generation. The data obtained revealed that the phase-matching condition can be preserved due to the high damage resistance of the directly bonded LiNbO3 waveguide. Based on this evaluation, we tried to generate multiple optical carriers using multi-stage frequency mixing in the multiple QPM device. The multiple optical carriers have mutual phase correlation, which is suitable for coherent wavelength division multiplexing (WDM) transmission. We also demonstrated 20 Gb/s quadrature phase shift keying (QPSK) signal generation using multiple optical carriers in order to ensure signal quality.

Optical pump phase locking to a carrier wave extracted from phase-conjugated twin waves for phase-sensitive optical amplifier repeaters.

In this paper, an optical phase-locked loop assisted by sum-frequency and second-harmonic generation (SS-OPLL) for frequency nondegenerate optical parametric phase-sensitive amplifier repeaters is experimentally demonstrated. First, theoretical derivations show that carrier extraction from phase-conjugated twin waves (PCTWs) and reference light generation are achieved by sum-frequency generation; therefore, the SS-OPLL circuit enables optical phase locking between PCTWs and a pump wave by a simple architecture based on a balanced OPLL. Then, optical phase locking between 20-Gbit/s quadrature phase-shift keying PCTWs and an individual pump source is experimentally demonstrated. Experimental results indicate that phase errors were reduced during the SS-OPLL operation.