Low-loss optical coupling structures between photonic crystal waveguides and channel waveguides were investigated. It was emphasized that impedance matching of guided modes of those waveguides, as well as field-profile matching, was essential to achieving the low-loss optical coupling. We developed an impedance matching theory for Bloch waves, and applied it to designing the low-loss optical coupling structures. It was demonstrated that the optical coupling loss between a photonic crystal waveguide and a Si-channel waveguide was reduced to as low as 0.7 dB by introducing an interface structure for impedance matching between the two waveguides.

A compact and stable optical sampling measurement system with a temporal resolution of 2 ps has been developed. External-cavity mode-locked laser-diode (EC-MLLD) modules, which directly generate coherent 2-ps optical pulses, were used as the optical sampling pulse sources. Real-time measurement of the recovery dynamics in semiconductor saturable absorber devices has been achieved by optical sampling combined with the pump-probe method. An EC-MLLD module was also utilized for simple sub-harmonic all-optical clock recovery based on the synchronization of the mode-locking operation by optical-pulse injection. Optical sampling measurement of 160-Gbit/s return-to-zero signals incorporating all-optical clock recovery has been demonstrated.

A Josephson parametric oscillator (JPO) is an interesting system from the viewpoint of quantum optics because it has two stable self-oscillating states and can deterministically generate quantum cat states. A theoretical proposal has been made to operate a network of multiple JPOs as a quantum annealer, which can solve adiabatically combinatorial optimization problems at high speed. Proof-of-concept experiments have been actively conducted for application to quantum computations. This article provides a review of the mechanism of JPOs and their application as a quantum annealer.