We have demonstrated the surface plasmon-polariton interference using a long-range surface plasmon-polariton
waveguide coupler. A clear interference fringe with the visibility of 87 % was observed. The coupling ratio of the
waveguide coupler was estimated to be 64:36 by two-photon interference experiments.
We present the latest results on two kinds of photon detectors: single photon detectors (SPDs) and photon number resolving detector (PNRD). We developed high speed and low noise SPDs using superconducting nano-wire (abbreviated by SNSPD) and semiconductor (InGaAs) avalanche photodiode (APD). The SNSPD system has totally four channels all of which have the detection eciency higher than 16% at 100Hz dark count rate. The InGaAs APD system also has four channels and the best performance is represented by the after-pulse probability of 0.61%, the dark count probability of 0.71×10-6 (~1kHz), and the detection eciency of 10.9%. Both systems were applied to wavelength division multiplexing quantum key distribution (WDM-QKD) operated at 1.2GHz repetition rate in a eld environment. The PNRD is made of superconducting transition edge sensor. It was applied to the implementation of quantum receiver which could beat the homodyne limit of the bit error rate of binary coherent states. We discuss future perspective of quantum communications with those photon detection technologies, including multi-user QKD networks and low-power high capacity communications.
A transition edge sensor (TES) is one of superconducting photon detectors, which has a photon number resolving
ability in light pulses. The TES device is a kind of calorimeters operated at an extremely low temperature, and
the energy of the photons is measured as a resistance change in a superconducting transition region of the TES.
The advantages of the TESs are an excellent energy resolution and a high quantum efficiency. However a response
speed is limited due to slow thermal recovery time. To overcome this, we fabricated new TES devices which are
based on a titanium superconductor. The critical temperature of our titanium films is around 410 mK, which
greatly improves the thermal recovery time. The observed decay time constant of response signals to the light
pulses is around several hundreds of ns, that make it possible to operate the devices at a counting rate over 1
MHz. The photon number resolving power is 0.35 eV(FWHM) for a 5 μm size device even at the high operating
temperature. The system quantum efficiency is 65 % by embedding the TES films in an optical structure with
a high reflection dielectric mirror and an anti-reflection coatings fabricated by an ion beam assisted sputtering
method. These features are very promising for high speed photon number resolving applications in the quantum