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In recent years, the demand for high-speed communication has been increasing. Satellite laser communication is one of the infrastructures that can realize this demand. However, in free-space optics (FSO) that use satellite laser communications, a scintillation occurs when the light passes through the atmosphere, resulting in a burst error. To prevent this, a strong error correction code is required. The polar code, one of such channel coding schemes, has been recently proposed. We have considered applying polar codes into FSO channels and have evaluated its performances through computer simulations. However, the experiments of polar code transmission in an outdoor FSO channel has not been studied. Therefore, in this paper, we conduct an experiment in polar code transmission in a terrestrial FSO channel instead of a satellite FSO as the first step of study, and then analyze the performance from the transmission results.

Photon source is one of the fundamental bricks for quantum information and computation systems. A widely-tunable, highly efficient and spectrally pure photon source would be very useful for such applications. In this paper, we present such a source based on spontaneous parametric down conversion (SPDC) in a periodically poled KTiOPO4 (PPKTP) crystal.

We report the efficient generation of a four-qubit bound entangled Smolin state. We also demonstrate the activation of the bound entanglement; we make the bound entanglement in the Smolin state distillable with the help of an additional Bell pair.

We report the experimental realization of the four-partite bound-entangled state, i.e., Smolin state [1] and demonstrate the ‘superactivation’ of the bound entanglement. Bound entanglement is a class of multipartite entanglement that cannot be distilled into pure entanglement under local operations and classical communication (LOCC). Despite its undistillable property, this class of entanglement has interesting properties that are expected to be useful in quantum information processing (QIP). For instance, in the Smolin state, the bound entanglement can be activated into distillable entanglement when two of the parties get together. The process is called “unlocking” [1], and has been demonstrated experimentally [2]. More importantly, the bound entanglement can be activated with the help of entanglement in other systems [3]. The activation of bound entanglement, i.e., superactivation, plays an essential role in the QIP protocols using bound entangled states (e.g., remote information concentration [4]).

The reduction of de Broglie wavelength is one of the fascinating characteristics of multiphoton quantum interference. It will be applicable to, for instance, super-resolution imaging beyond the classical diffraction limit and ultrasensitive phase measurement beyond the standard quantum limit. Thus far, the reduced de Broglie wavelength has been experimentally demonstrated using visible or near infrared photons [1–3]. However, it is still a challenging task to demonstrate multiphoton quantum interference in a telecom-wavelength region, because of insufficient quantum efficiencies of single-photon detectors working at telecom wavelength. Also, efficient photon sources of multiple photons that are mutually indistinguishable are essential to carry out various experiments including multiphoton quantum interference and quantum information protocols. In this context, we have developed an efficient photon-pair source designed for such multiphoton experiments at telecom wavelength. Making use of this source, we demonstrate the first measurement of reduced de Broglie wavelength of the four-photon state at the telecom wavelength.

We studied blue light-induced near-infrared absorption (BLINIRA) and its temperature dependence in ferroelectric crystals, i.e., MgO-doped stoichiometric-LiNbO3, congruent-LiNbO3, MgO-doped stoichiometric-LiTaO3, and stoichiometric-LiTaO3. We found the BLINIRA decreased with the increase of temperature.

We have developed a high-efficiency source of cross-polarized photon pairs in a 800-nm band via type-II quasi-phase matched spontaneous parametric down-conversion using periodically poled lithium niobate. We obtained 25,000 coincidence counts sec-1 mW-1 of the photon pairs.

The purpose of our study is to establish the technique to measure such weak cross-Kerr nonlinearity induced by a single photon in solid states. The experimental set-up is based on a pump-probe measurement.

We have developed a high-efficiency source of cross-polarized photon pairs via type-II quasi-phase matched spontaneous parametric down-conversion using periodically poled lithium niobate (PPLN). The photon pairs are generated from the PPLN in fifth-order quasi-phase matched condition around 800 nm, which matches the wavelength band of efficient Si avalanche photodiodes. We obtained 25,000 coincidence counts sec-1mW-1of the photon pairs.

We propose and demonstrate a novel scheme to generate a polarization entangled state utilizing a spatial correlation effect in parametric down-conversion. We also demonstrate the violation of Bell's inequality.

Characterization of small nonlinear phase shifts will be a key to the optical quantum information processing. We propose and demonstrate a novel technique to measure the nonlinear phase shifts in a waveguide-type Kerr medium.

The paper reports the most recent experimental result that exhibits a higher degree of entanglement. They used vapor-phase-grown thin single crystal of CuCl. Since CuCl has a large bandgap (/spl sim/3.4 eV), it is suitable to generate photon pairs in the short wavelength region near ultraviolet.

Diffraction-interference patterns of parametric down-converted photons through a transmission grating by both one- and two-photon detection schemes are presented. The measured patterns are explained by the Fourier analysis of the two-photon wave packet, considering the quantum spatial correlation between the constituent photons. Spatial diffraction-interference patterns were recorded by a two-photon detector consisting of two avalanche photodiodes. For comparison, diffraction-interference patterns of Ti:sapphire laser are also observed using the same apparatus (a uncorrelated case).

In terms of the concept of "photonic de Broglie wave" in multiphoton states, we demonstrate biphoton interference experiments using a Mach-Zehnder interferometer. We have observed interferometric oscillations corresponding to the photonic de Broglie wavelength of the biphoton.