AOKI, Takao

ISSN：0031-9015

ISSN：1882-0778

ISSN：0003-6951

ISSN：1882-0778

ISSN：0146-9592

ISSN：1050-2947

ISSN：0003-6951

ISSN：2045-2322

ISSN：1094-4087

ISSN：1094-4087

ISSN：1050-2947

ISSN：1094-4087

ISSN：1943-0655

ISSN：03870200

ISSN：00319007

Outline：© 2015 American Physical Society.We demonstrate an all-fiber cavity quantum electrodynamics system with a trapped single atom in the strong coupling regime. We use a nanofiber Fabry-Perot cavity, that is, an optical nanofiber sandwiched by two fiber-Bragg-grating mirrors. Measurements of the cavity transmission spectrum with a single atom in a state-insensitive nanofiber trap clearly reveal the vacuum Rabi splitting.

ISSN：2189079X

ISSN：2045-2322

Outline：:Single-photon sources are important elements in quantum optics and quantum information science. It is crucial that such sources be able to couple photons emitted from a single quantum emitter to a single propagating mode, preferably to the guided mode of a single-mode optical fiber, with high efficiency. Various photonic devices have been successfully demonstrated to efficiently couple photons from an emitter to a single mode of a cavity or a waveguide. However, efficient coupling of these devices to optical fibers is sometimes challenging. Here we show that up to 38% of photons from an emitter can be directly coupled to a single-mode optical fiber by utilizing the flat tip of a silica nanofiber. With the aid of a metallic mirror, the efficiency can be increased to 76%. The use of a silicon waveguide further increases the efficiency to 87%. This simple device can be applied to various quantum emitters.

ISSN：19430655

Outline：© 2015 IEEE. Secrecy issues of free-space optical links realizing information theoretically secure communications and high transmission rates are discussed. We numerically study secrecy communication rates of optical wiretap channel based on on-off keying (OOK) modulation under typical conditions met in satellite-ground links. It is shown that, under reasonable degraded conditions on a wiretapper, information theoretically secure communications should be possible in a much wider distance range than a range limit of quantum key distribution, enabling secure optical links between geostationary Earth orbit satellites and ground stations with currently available technologies. We also provide the upper bounds on the decoding error probability and the leaked information to estimate a necessary code length for given required levels of performances. This result ensures that a reasonable length of wiretap channel code for our proposed scheme must exist.

ISSN：00036951

Outline：© 2015 AIP Publishing LLC.By combining a tilted-pulse-intensity-front scheme using a LiNbO3 crystal and a chirped-pulse-beating method, we generated a narrowband intense terahertz (THz) pulse, which had a maximum electric field of more than 10 kV/cm at around 2 THz, a bandwidth of ∼50 GHz, and frequency tunability from 0.5 to 2 THz. By performing THz-pump and near-infrared-probe experiments on GaAs quantum wells, we observed that the resonant excitation of the intraexcitonic 1s-2p transition induces a clear and large Autler-Townes splitting. Our time-resolved measurements show that the splitting energy observed in the rising edge region of electric field is larger than in the constant region. This result implies that the splitting energy depends on the time-averaged THz field over the excitonic dephasing time rather than that at the instant of the exciton creation by a probe pulse.

Outline：© 2016 Optical Society of America.We present experimental data on message transmission in a free-space optical (FSO) link at an eye-safe wavelength, using a testbed consisting of one sender and two receiver terminals, where the latter two are a legitimate receiver and an eavesdropper. The testbed allows us to emulate a typical scenario of physical-layer (PHY) security such as satellite-to-ground laser communications. We estimate information-theoretic metrics including secrecy rate, secrecy outage probability, and expected code lengths for given secrecy criteria based on observed channel statistics. We then discuss operation principles of secure message transmission under realistic fading conditions, and provide a guideline on a multi-layer security architecture by combining PHY security and upper-layer (algorithmic) security.

ISSN：00214922

Outline：© 2017 The Japan Society of Applied Physics. Diamond nanowires are fabricated on a bulk, single crystalline diamond near an edge of aluminum coating using inductively coupled plasma reactive ion etching. Two different density areas are simultaneously appeared where the dense area has 9 times higher density than that of the sparse area while keeping the size of nanowires almost uniform in these areas. The nanowire sizes realized in the dense (sparse) area are 858 ± 22nm (876 ± 25nm) in height and 126 ± 6 nm (124 ± 7 nm) in diameter, which is suitable for applications in optical quantum information processing.

Outline：© 2017 The Author(s). Nitrogen-Vacancy (NV) centers in diamond are promising solid-state quantum emitters that can be utilized for photonic quantum applications. Various diamond nanophotonic devices have been fabricated for efficient extraction of single photons emitted from NV centers to a single guided mode. However, for constructing scalable quantum networks, further efficient coupling of single photons to a guided mode of a single-mode fiber (SMF) is indispensable and a difficult challenge. Here, we propose a novel efficient hybrid system between an optical nanofiber and a cylindrical-structured diamond nanowire. The maximum coupling efficiency as high as 75% for the sum of both fiber ends is obtained by numerical simulations. The proposed hybrid system will provide a simple and efficient interface between solid-state quantum emitters and a SMF suitable for constructing scalable quantum networks.

ISSN：0913-5685

Outline：Efficient coupling of photons from single-photon source to the single-mode fiber is requied for quantum information and cryptography technology. We have performed simulations of photon-collecting devices utilizing a nanofiber tip. Up to 38% of light from point dipole source is coupled to the fundamental guided mode of the nanofiber. These devices utilizing a nanofiber tip are promising to achive efficient single-photon sources.

ISSN：2189079X

ISSN：2189079X