As various systems and networks in our society grow larger and more complex, analysis and optimization of such systems are becoming increasingly important. Such tasks are classified as combinatorial optimization problems, which are generally difficult to solve with current digital computers. It is well known that combinatorial optimization problems can be converted to ground-state-search problems of the Ising model, a theoretical model for the interacting spins.
Recently, several approaches to find solutions to the Ising model using artificial spin systems have been studied intensively. A coherent Ising machine (CIM) is one of such systems in which degenerate optical parametric oscillators (DOPO) are used as artificial spins. By using a long-distance (typically 1 km) fiber cavity that contains a phase sensitive amplifier based on a periodically poled lithium niobate waveguide, we can generate thousands of DOPO pulses multiplexed in time domain. Since a DOPO phase only takes either 0 or p at above threshold, we can stably express an Ising spin with a DOPO by allocating phase 0 (p) as spin up (down). The “spin-spin interaction” can be implemented by using a measurement-feedback scheme, with which we can effectively realize mutual injection of lights among thousands of DOPO pulses.
The networked DOPOs are most likely to oscillate at a phase configuration that best stabilize the whole network, which gives the solution to the given Ising problem. Based on this scheme, we realized a CIM with fully-coupled 2000 DOPO pulses, by which we could find good solutions to 2000-node combinatorial optimization problems in less than 100 microseconds. In the talk, I will detail the basic principle and the experimental details of the CIM, as well as our effort for finding its applications.
Biography: Hiroki Takesue received his B.E., M.E., and Ph.D. degrees in engineering science from Osaka University, Japan, in 1994, 1996, and 2002, respectively. He joined NTT Access Network Systems Laboratories, Nippon Telegraph and Telephone Corporation in 1996, where he worked on wavelength-division multiplexing access networks. In 2003, he moved to NTT Basic Research Laboratories. Since then, he has been engaged in research on quantum communications and non-von Neumann computation based on nonlinear optics. He is currently a Senior Distinguished Researcher of NTT and the group leader of Quantum Optical State Control Research Group.
He received several awards including the ITU-T Kaleidoscope Conference Best Paper Award (2nd place) in 2008, the Commendation for Science and Technology by the Minister of Education, Culture, Sports, Science and Technology of Japan (the Young Scientists’ Prize) in 2010, and Nishina Memorial Award in 2017. He was a visiting scholar at Stanford University, Stanford, CA from 2003 to 2004, and a guest researcher at the National Institute of Standards and Technology (NIST), Boulder, CO in 2014. He is a member of IEEE and the Japan Society of Applied Physics.