Yoshiki Sakuma

Single-Photon Generation in the 1.55-µm Optical-Fiber Band from an InAs/InP Quantum Dot

We first succeeded in generating single-photon pulses in the C-band (1.55-µm band: the highest transmittance in optical telecommunication bands) from a single InAs/InP quantum dot. The quantum dot with 1546.1-nm exciton emission was prepared by controlling the growth conditions. A well-designed mesa structure presented efficient injection of the emitted photons into a single-mode optical fiber. A Hanbury-Brown and Twiss measurement has proved that the photons through the fiber were single photons. We also performed to transmit single-photon pulses through 30-km optical fiber. This preliminary trial is a milestone toward quantum telecommunication using ideal single photons.

NOVEL INGAAS/GAAS QUANTUM DOT STRUCTURES FORMED IN TETRAHEDRAL-SHAPED RECESSES ON (111)B GAAS SUBSTRATE USING METALORGANIC VAPOR PHASE EPITAXY

We report a novel GaAs/InGaAs/GaAs quantum dot structure formed in a tetrahedral‐shaped recess (TSR) patterned on a (111)B GaAs substrate with anisotropic chemical etching. The pseudomorphic heterostructure shows two clear photoluminescence peaks which are attributed to an In anisotropic incorporation on (111)B compared to (111)A. Cathodoluminescence at a lower energy peak with InGaAs of 2.5 nm shows a bright image at the bottom of TSRs which indicates the local minimum in potential energy at the bottom of TSR.

channel strained-SiGe p-MOSFET with enhanced hole mobility and lower parasitic resistance

Employment of <100> channel direction in a strained-Si/sub 0.8/Ge/sub 0.2/ p-MOSFET has demonstrated the substantial amount of hole mobility enhancement as large as 25% and parasitic resistance reduction of 20% compared to a <110> strained-Si/sub 0.8/Ge/sub 0.2/ Channel p-MOSFET, which already has an advantage in mobility and the threshold voltage roll-off characteristic over the Si p-MOSFET. This result indicates that the <100> strained SiGe channel p-MOSFET is a promising and practical candidate for realizing high-speed CMOS devices under low-voltage operation.

Symmetric quantum dots as efficient sources of highly entangled photons: Violation of Bell's inequality without spectral and temporal filtering

An ideal emitter of entangled photon pairs combines the perfect symmetry of an atom with the convenient electrical trigger of light sources based on semiconductor quantum dots. Our source consists of strain-free GaAs dots self-assembled on a triangular symmetric (111)A surface. The emitted photons reveal a fidelity to the Bell state as high as 86(±2)% without postselection. We show a violation of Bells inequality by more than five times the standard deviation, a prerequisite to test a quantum cryptography channel for eavesdropping. Due to the strict nonlocal nature the source can be used for real quantum processing without any postprocessing. The remaining decoherence channel of the photon source is ascribed to random charge and nuclear spin fluctuations in and near the dot.

Fabrication of ZnO nanoparticles in SiO2 by ion implantation combined with thermal oxidation

Zinc-oxide (ZnO) nanoparticles (NPs) are fabricated in silica glasses (SiO2) by implantation of Zn+ ions of 60 keV up to 1.0×1017ions∕cm2 and following thermal oxidation. After the oxidation at 700 °C for 1 h, the absorption in the visible region due to Zn metallic NPs disappears and a new absorption edge due to ZnO appears at ∼3.25eV. Cross-sectional transmission electron microscopy confirms the formation of ZnO NPs of 5–10 nm in diameter within the near-surface region of ∼80nm thick and larger ZnO NPs on the surface. Under He–Cd laser excitation at λ=325nm, an exciton luminescence peak centered at 375 nm with FWHM of 113 meV was observed at room temperature.

Site-controlled photoluminescence at telecommunication wavelength from InAs∕InP quantum dots

We fabricated ordered InAs∕InP quantum-dot (QD) arrays using atomic-force-microscopic oxidation, wet etching, and regrowth by metalorganic chemical vapor deposition. The QDs exhibit single-dot photoluminescence peaking at wavelengths ranging from 1.22 to 1.45μm, mostly matching the telecommunication band of optical fibers. The site dependence of single peaks indicates the site controllability of single-dot light emitters, which might be useful in quantum information processing.

Zn and ZnO nanoparticles fabricated by ion implantation combined with thermal oxidation, and the defect-free luminescence

Silica glass implanted with Zn ions of 60keV to 1.0×1017ions∕cm2 was annealed in oxygen gas to form ZnO nanoparticles (NPs). In as-implanted state, the implanted Zn atoms form Zn metallic NPs inside of the silica. After annealing at 600°C, ZnO NPs form on the surface, while Zn metallic NPs still remain in the deep region. At 700°C, most of Zn atoms move to the surface to form the droplet-shaped ZnO NPs which show two photoluminescence bands, i.e., an exciton band at 375nm and a defect band at ∼500nm. The defect band almost disappears in the samples annealed at 600°C, which include both ZnO NPs and Zn NPs.

An optical horn structure for single-photon source using quantum dots at telecommunication wavelengtha)

We succeeded in efficiently generating single-photon pulses from an InAs/InP quantum dot at a wavelength of 1.5 μm. Our optical structure, named a single photon horn, can propagate over 95% photon pulses in InP substrate. We extracted the photon pulses through an anti-reflection coating on a substrate, and then we injected them into an objective lens. Total extraction efficiency from the quantum dot to the lens reached ∼11%, which was estimated using a photon correlation measurement. Furthermore we directly observed the single-photon pulse width ∼1.6 ns as an exciton lifetime in the quantum dot, which opens up the possibility of operating the single photon horn over 100 MHz.

Pulsed jet epitaxy of III–V compounds

An atomic layer epitaxial technique called pulsed-jet epitaxy has been developed for III–V compound crystals using metalorganic and hydride sources. The growth rate was clearly self-limited under a wide range of growth conditions. Epitaxial layers of high purity without carbon contamination could be grown. Fine patterns below 1.0μm were selectively grown with good morphology. A uniform epitaxial layer grown over a 2-inch wafer had a thickness variation within 1.0%. Strained-superlattices (GaAs)m(GaP)n were grown and exhibited an ideal self-limiting mechanism at 500°C. Structural analysis using X-ray rocking curve and Raman scattering measurements showed that superlattice growth was completely controlled within one atomic layer even for the monolayer superlattice (GaAs)1(GaP)1. Optical studies by photoluminescence and reflectance measurements suggest that the monolayer superlattice had a direct energy-band structure.

Quantum key distribution over 120 km using ultrahigh purity single-photon source and superconducting single-photon detectors

Advances in single-photon sources (SPSs) and single-photon detectors (SPDs) promise unique applications in the field of quantum information technology. In this paper, we report long-distance quantum key distribution (QKD) by using state-of-the-art devices: a quantum-dot SPS (QD SPS) emitting a photon in the telecom band of 1.5 μm and a superconducting nanowire SPD (SNSPD). At the distance of 100 km, we obtained the maximal secure key rate of 27.6 bps without using decoy states, which is at least threefold larger than the rate obtained in the previously reported 50-km-long QKD experiment. We also succeeded in transmitting secure keys at the rate of 0.307 bps over 120 km. This is the longest QKD distance yet reported by using known true SPSs. The ultralow multiphoton emissions of our SPS and ultralow dark count of the SNSPD contributed to this result. The experimental results demonstrate the potential applicability of QD SPSs to practical telecom QKD networks.