The maximum independent set (MIS) problem is an optimization problem in graph theory and is related to various applications such as network design and resource allocation. Here we report that a coherent Ising machine based on a network of optical parametric oscillators can find large independent sets in graphs that consist of thousands of nodes faster than simulated annealing implemented many-core CPU.
Random numbers which are unpredictable and definitely unknown by anyone before they are generated are now used in a large number of real-world applications ranging from authentication, gaming, many online activities to simulations and optimizations. The development of a trusted randomness source is thus a necessity. In this work we present a simple design of a certifiable quantum random number generation and its. In particular we show how real-time low-latency randomness can be generated from measurements on time-bin photonic states every 0.12s. We generate a block of 2^13 random bits certifiable against the most powerful quantum adversary with its error bounded by 2^-64. Further our device is suitable for continuous operation giving it a potential application as a quantum randomness beacon.
We introduce our recent photonic quantum information processing experiments using the high-speed electro-optic modulator (EOM) based on a lithium niobate waveguides. First, we demonstrated the frequency shifter for single photons by using an optical single sideband (OSSB) modulator, which is based on the nested configuration of four phase modulators (PMs). By properly setting the RF modulation signal and the bias voltage to each PM, we can suppress unwanted sidebands to realize a frequency shifter that can tune the frequency of single photons with accuracy of an RF synthesizer. Using the OSSB modulator, we eliminated the frequency distinguishability between two single photons whose frequencies were different by 25 GHz, and as a result observed a Hong-Ou- Mandel interference with a visibility exceeding 90%. We also proposed and demonstrated a two-qubit controlled logic gate for time-bin qubits using a two-input, two-output optical switch. The switch enables us to individually perform different functions for two temporal bases as a time-dependent beam splitter, so that we can realize controlled-phase (C-Phase) gate for a time-bin qubit. We showed that the states of two separable single photons were entangled as a result of the C-Phase gate operation. We performed the quantum state tomography to obtain density matrices of the output states from the C-Phase gate for some specific input states, and confirmed that the averaged state fidelity to the pure states was 85%. These results indicate that the EOMs will be a useful tool for realizing advanced photonic quantum communication systems.
We describe a large-scale degenerate optical parametric oscillator (DOPO) network for a coherent Ising machine that solves combinatorial optimization problems. By pumping a fiber-based phase-sensitive amplifier placed in a 1-km fiber cavity at a 2-GHz repetition frequency, we generated more than 10,000 DOPOs multiplexed in the time domain. We confirmed that the DOPO phases were discretized to {0, π} indicating that they could be used as stable artificial spins. We also implemented a one-dimensional Ising model by optically coupling adjacent DOPOs, and confirmed that the DOPOs well simulated the behavior of low-temperature spins.
We report the first entanglement swapping experiment using entangled photon-pair sources based on spontaneous four-wave mixing (SFWM). The 1.5-μm band entangled photon pairs generated by SFWM in two independent 500-m dispersion shifted fibers exhibited quantum
interference, thanks to the negligible walk-off between the pump and photon pairs. The use of 500-MHz gated-mode InGaAs/InP avalanche photodiodes based on the sine-wave gating technique increased the fourfold coincidence rate significantly. As a result, the formation of an entanglement between photons from independent sources was successfully observed.
A unique quantum key distribution (QKD) protocol, called DPS (differential-phase-shift) QKD, has been proposed and
developed at NTT and Osaka University, which utilizes a coherent pulse train instead of individual photons as in
traditional QKD protocols such as BB84. Its security is based on the fact that every phase difference of a highlyattenuated
coherent pulse train cannot be fully measured. This protocol has features of simple setup, potential for a high
key creation rate, and robustness against photon-number-splitting attack. This paper presents recent research activities on
DPS-QKD.
We demonstrate efficient nonlinear functions using silicon nanophotonic structures. In the ultrasmall core of the
waveguides and cavities, nonlinear phenomena are significantly enhanced. Applying the two-photon absorption effect,
we have confirmed all optical modulation, in which the modulation speed is improved to around 50 ps by eliminating
free carriers. Applying the four-wave-mixing effect, we have achieved high-efficiency wavelength conversion. The
conversion efficiency is -11 dB, and the efficiency will be further improved by eliminating free carriers. Using the four-wave-
mixing effect, we have also realized a low-noise entangled photon pair source. The source does not need a
refrigeration system for noise reduction, which is a great advantage for practical application.
A novel type of quantum key distribution (QKD) protocol, called DPS (differential-phase-shift) QKD, was proposed
several years ago. A sender transmits a highly-attenuated coherent pulse train with {0, π} phase, and a receiver receives
it with a one-bit delay Mach-Zehnder interferometer followed by photon detectors. A secret key is created from photon
detection events, whose security is based on the fact that an eavesdropper cannot perfectly measure the phase
information of a highly-attenuated coherent pulse train. This protocol has some features of simple setup, potential for a
high key creation rate, and robustness against photon-number-splitting attack. This paper overviews DPS-QKD. The
operation mechanism is described, and then some experimental efforts are introduced, featuring use of a glass waveguide
Mach-Zehnder interferometer and advanced single-photon detectors. The highest key rate and the longest distance have
been achieved with the DPS-QKD protocol. Some modified schemes are also presented, including that utilizing quantum
entanglement, that using decoy pulses, and that using macroscopic coherent light.
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