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.
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.
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.
Quantum cryptography is a method to exchange secret messages with unconditional security over a potentially hostile environment using single photons. Previous implementations of quantum cryptography have relied on highly attenuated laser light to approximate single photo states. Such sources are vulnerable to eavesdropping attacks based on photon splitting. Here we present an experimental demonstration of quantum cryptography using a single photon source based on Indium Arsenide quantum dots. We achieve a communication rate of 25kbits/s. This source allows secure communication over a quantum channel with up to 28dB of channel loss, as opposed to only 23dB for an attenuated laser.
A high efficiency, triggered single photon source with applications to quantum communications is discussed. The sources is formed from an InAs-based quantum dot located in the center of a micropost cavity formed from GaAs, with top and bottom GaAs/AlAs distributive Bragg reflector pairs, and lateral processing. When pumped above band into the semiconductor host, correlation measurements show a reduction in the two-photon probability to 0.14, compared to unity for a Poisson source. The external efficiency of this structure is 0.24.
This paper presents novel quantum key distribution (QKD) schemes that use differential phases of sequential pulses as an information carrier. Alice sends a photon in a linear superposition state of three temporal slots, in which each amplitude is phase-modulated by {0,π}. Bob measures the phase difference of an asymmetric interferometer, and tells Alice time-instance at which the photon was counted. From this time information and her modulation data, Alice knows which detector counted the photon in Bob s site. Then, the
two parties create a secret key. The scheme is suitable for
fiber transmission systems and offers key creation efficiency higher than conventional phase-encoding QKD. The above scheme utilizes fully non-orthogonal four states. Differential phase shift QKD utilizing two non-orthogonal states is also presented. Alice sends a coherent pulse train with less than one photon per pulse, which is phase
modulated by {0,π} for each pulse, and Bob measures the pulse train by a one-bit delay circuit. The system has a simple configuration such that Alice has no interferometer and sends no intense reference pulse, unlike to conventional QKD scheme using two non-orthogonal states, and also has an advantage of high key creation efficiency.
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