Single and entangled photons play a key role in quantum information and communication technology. In this review, concepts and progress in the generation of single and entangled photons are presented. Particularly, recent developments of semiconductor sources for such nonclassical photons are discussed.

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We review the generation of entangled photon pairs at telecommunication wavelengths using a periodically poled lithium niobate (PPLN) waveguide. The characteristics of photon pairs generated using a type II PPLN waveguide are described in detail. We also briefly explain the principle of high speed single-photondetection at 1550nm using an InGaAs/InP avalanche photodiode operated with a sine wave gating.

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I review the recent progress on single-photon detectors developed for detecting 1550-nm photons. Thermoelectrically cooled single-photon detectors based on indium-gallium-arsenide avalanche photodiodes are simple to operate and are desirable for practical applications. Methods for reducing the dark count rate and after-pulsing probability are discussed. I also briefly explain the concepts of upconversion-based singlephoton detectors and ultra-low-noise superconducting single-photon detectors.

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Quantum computation using photons is reviewed. In the first part, the basic idea of optical quantum gates using linear optical element, and the principle to enhance the success probability of the gate, with recent progress of one-way optical quantum computation using cluster state, is explained. In the second part, the effort to develop highly-efficient optical quantum gates realizing strong non-linear interaction between photons is reviewed.

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Quantum Key Distribution (QKD) systems based on single photon transmission can generate unconditionally secure common key between remote users. Improvement of QKD performance, particularly on key generation rate, has been required to meet current network traffic. High key generation rate is realized by increasing photon detection rate and by reducing error rate. Therefore, a high-speed QKD system should be equipped with low loss receivers with high visibility, highly efficient photon detectors with small dark count probability, and a stable clock synchronization system with low stray light to the quantum signals. A solution for these issues are given by employing planar lightwave circuit interferometers, single photon detection circuits and modules, and clock synchronization based on Wavelength Division Multiplexing (WDM) technique.

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Recent developments towards photon-atom quantum interface are described. The quantum interface is realized by reversible time evolution of an atom-cavity coupled system. Proof of principle experiments have been demonstrated with neutral atoms recently. With trapped ions, the interface has not been realized yet, although single photon generation with tailored waveforms has been achieved in an ion-cavity system, in which a same calcium ion has been kept localized in a cavity for more than 90 minutes. Strategies towards the photon-ion quantum interface are described.

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Technique for synthesis of the ZnO nanowires by Nanoparticle-Assisted Pulsed-Laser Deposition (NAPLD) and the characterization of their optical property are described. Vertically-aligned ZnO nanowires were successfully grown on sapphire substrates by NAPLD without any catalyst. The surface density of the nanowires on the substrate was controlled by changing the laser irradiation conditions, such as an energy and a repetition rate. When a nanowire was excited by the third harmonic of a Q-switched Nd: YAG laser, the stimulated emission due to a micro-cavity effect in single nanowire was observed with a lasing threshold of about 400 kW/cm², indicating high crystalinity of ZnO nanowires.

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