Abstract:A secure communication network with quantum key distribution in a metropolitan area is reported. Six different QKD systems are integrated into a mesh-type network. GHz-clocked QKD links enable us to demonstrate the world-first secure TV conferencing over a distance of 45km. The network includes a commercial QKD product for long-term stable operation, and application interface to secure mobile phones. Detection of an eavesdropper, rerouting into a secure path, and key relay via trusted nodes are demonstrated in this network. ©2011 Optical Society of AmericaOCIS codes: (270.5568) Quantum cryptography; (060.5565) Quantum communications. References and links1. N. Gisin, G. Ribordy, W. Tittel, and H. Zbinden, "Quantum cryptography," Rev. Mod. Phys. 74(1), 145-195 (2002). 2. V. Scarani, H. Bechmann-Pasquinucci, N. J. Cerf, M. Dušek, N. N. Lütkenhaus, and M. Peev, "The security of practical quantum key distribution," Rev. Mod. Phys. 81(3), 1301-1350 (2009

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. We report a 2000-spin network with all-to-all spin-spin couplings. Using a measurement and feedback scheme, we coupled time-multiplexed degenerate optical parametric oscillators to implement maximum cut problems on arbitrary graph topologies with up to 2000 nodes. Our coherent Ising machine outperformed simulated annealing in terms of accuracy and computation time for a 2000-node complete graph.

Unconventional, special-purpose machines may aid in accelerating the solution of some of the hardest problems in computing, such as large-scale combinatorial optimizations, by exploiting different operating mechanisms than those of standard digital computers. We present a scalable optical processor with electronic feedback that can be realized at large scale with room-temperature technology. Our prototype machine is able to find exact solutions of, or sample good approximate solutions to, a variety of hard instances of Ising problems with up to 100 spins and 10,000 spin-spin connections.

Quantum key distribution (QKD) offers an unconditionally secure means of communication based on the laws of quantum mechanics [1]. Currently, a major challenge is to achieve a QKD system with a 40 dB channel loss, which is required if we are to realize global scale QKD networks using communication satellites [2]. Here we report the first QKD experiment in which secure keys were distributed over 42 dB channel loss and 200 km of optical fibre. We employed the differential phase shift quantum key distribution (DPS-QKD) protocol [3] implemented with a 10-GHz clock frequency, and superconducting single photon detectors (SSPD) based on NbN nanowire [4,5]. The SSPD offers a very low dark count rate (a few Hz) and small timing jitter (60 ps full width at half maximum). These characteristics allowed us to construct a 10-GHz clock QKD system and thus distribute secure keys over channel loss of 42 dB. In addition, we achieved a 17 kbit/s secure key rate over 105 km of optical fibre, which is two orders of

Conventional single-photon detectors at communication wavelengths suffer from low quantum efficiencies and large dark counts. We present a single-photon detection system, operating at communication wavelengths, based on guided-wave frequency upconversion in a nonlinear crystal with an overall system detection efficiency (upconversion + detection) exceeding 46% at 1.56 microm. This system consists of a fiber-pigtailed reverse-proton-exchanged periodically poled LiNbO3 waveguide device in conjunction with a silicon-based single-photon counting module.

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(–αDWN2)] relative to CIMs [exp(–αCIMN)] 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.

Simulating a network of Ising spins with physical systems is now emerging as a promising approach for solving mathematically intractable problems [1][2][3][4][5]. Here we report a large-scale network of artificial spins based on degenerate optical parametric oscillators (DOPO), paving the way towards a photonic Ising machine capable of solving difficult combinatorial optimization problems. We generated >10,000 time-divisionmultiplexed DOPOs using dual-pump four-wave mixing (FWM) [6,7] in a highly nonlinear fibre (HNLF) placed in a fibre cavity. Using those DOPOs, a one-dimensional (1D) Ising model was simulated by introducing nearest-neighbour optical coupling. We observed the formation of spin domains and found that the domain size diverged near the DOPO threshold, which suggests that the DOPO network can simulate the behaviour of low-temperature Ising spins. Combinatorial optimization problems are becoming increasingly important in our society, for example in applications such as artificial intelligence, drug discovery, optimization of cognitive wireless networks, and analysis of social networks. Many such problems are classified as non-deterministic polynomial time (NP)-hard or NP-complete problems, which are considered to be hard to solve efficiently with modern computers [8]. It is well known that many combinatorial optimization problems can be mapped onto the ground-state-search problems of the Ising Hamiltonian [9] demonstrated a CIM using DOPOs [13]. A DOPO can be utilized as a stable artificial spin because it takes only the 0 or π phase relative to the pump phase [14]. The spinspin interaction can be simply implemented with mutual injections of DOPO lights using delay interferometers. In [13], a spin system composed of four DOPOs was employed for a proof-of-principle CIM experiment. However, to simulate a more complex Ising Hamiltonian to verify the advantages of the CIM over existing methods, we need to implement a CIM with a much larger number of spins. Here we report a large scale network of artificial spins realized with as many as 10,000 time-divisionmultiplexed DOPOs generated via dual-pump FWM in an HNLF placed in a fibre cavity. We successfully simulated the ferro-and anti-ferromagnetic-like behaviour of a 1D Ising spin chain by introducing uni-directional nearest-neighbour coupling between DOPOs. In addition, we observed a formation of domain walls with which we could obtain information on how much the state of the spin network was excited from the ground state. We believe the present result will provide a promising platform on which to realize an efficient machine for solving the Ising model based on the CIM concept. A dimensionless Hamiltonian of an N -spin Ising model without an external magnetic field (Fig. 1 a) is given bywhere J ij is the coupling coefficient between the ith and jth spins, and σ ℓ (ℓ ∈ {i, j}) denotes the z projection of the ℓth spin, which can take ±1 values. The purpose of an Ising machine is to find the ground state of the above Hamiltonian with a given set of J ij us...

We report the generation of polarization entangled photon pairs in the 1550-nm wavelength band using spontaneous four-wave mixing in a dispersion-shifted fiber loop. The use of the fiberloop configuration made it possible to generate polarization entangled states very stably. With accidental coincidences subtracted, we obtained coincidence fringes with >90 % visibilities, and observed a violation of Bell's inequality by seven standard deviations. We also confirmed the preservation of the quantum correlation between the photons even after they had been separated by 20 km of optical fiber.