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We demonstrate magneto-optical double resonance of a single NV center in diamond for quantum media conversion between a photon and an electron or a nuclear spin via resonant photo-absorption.

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.

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.

Deterministic quantum state transfer from a photonic qubit to an electron spin qubit is crucial for building all semiconductor quantum repeaters. We present the coherent state preparation of electron spins imprinted from the photon polarization.

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.

We present photon-spin quantum-state-transfer (QST) approach. GaAs/AlGaAs-QW with g~0 showed nonlinear B-field dependence of spin precession, suggesting photon-to-nuclei polarization transfer. 2DEG with quantum-point-contact on InGaAs/InP showed telecomwavelength-sensitive quantum transport. Efficient/high-fidelity QST conditions were also found.