Hidehiko Kamada

Strong photoluminescence emission at room temperature of strained InGaAs quantum disks (200–30 nm diameter) self‐organized on GaAs (311)B substrates

We have recently found that quantum‐box‐like structures are formed during spontaneous reorganization of a sequence of AlGaAs and strained InGaAs epitaxial films grown on GaAs (311)B substrates by metalorganic vapor‐phase epitaxy into InGaAs islands (disks) buried beneath AlGaAs. The size of the disks is directly controlled by the In content in the range 200–30 nm. Strong photoluminescence (PL) efficiency at room temperature is observed in these strained quantum disks. Even for the 30 nm disk the radiative efficiency is not reduced compared to the reference (100) quantum well. The PL spectra are characterized by narrow linewidth and well resolved exciton resonances in excitation spectroscopy.

SPIN RELAXATION OF EXCITONS IN ZERO-DIMENSIONAL INGAAS QUANTUM DISKS

We report the observation of spin relaxation of excitons in zero-dimensional semiconductor nanostructures. The spin relaxation is measured in InGaAs quantum disks by using a polarization dependent time-resolved photoluminescence method. The spin relaxation time in a zero-dimensional quantum disk is as long as 0.9 ns at 4 K, which is almost twice as long as the radiative recombination lifetime and is considerably longer than that in quantum wells. The temperature dependence of the spin relaxation time suggests the importance of exciton–acoustic phonon interaction.

Efficient entanglement distribution over 200 kilometers.

Here we report the first demonstration of entanglement distribution over a record distance of 200 km which is of sufficient fidelity to realize secure communication. In contrast to previous entanglement distribution schemes, we use detection elements based on practical avalanche photodiodes (APDs) operating in a self-differencing mode. These APDs are low-cost, compact and easy to operate requiring only electrical cooling to achieve high single photon detection efficiency. The self-differencing APDs in combination with a reliable parametric down-conversion source demonstrate that entanglement distribution over ultra-long distances has become both possible and practical. Consequently the outlook is extremely promising for real world entanglement-based communication between distantly separated parties.

Long-distance distribution of time-bin entangled photon pairs over 100 km using frequency up-conversion detectors.

We report an experimental demonstration of the distribution of time-bin entangled photon pairs over 100 km of optical fiber. In our experiment, 1.5-mum non-degenerated time-bin entangled photon pairs were generated with a periodically poled lithium niobate (PPLN) waveguide by using the parametric down conversion process. Combining this approach with ultra-low-loss filters to eliminate the pump light and separate signal and idler photons, we obtained an efficient entangled photon pair source. To detect the photons, we used single-photon detectors based on frequency up-conversion. These detectors operated in a non-gated mode so that we could use a pulse stream of time correlated entangled photon pairs at a high repetition frequency (1 GHz). Using these elements, we distributed time-bin entangled photon pairs over 100 km of dispersion shifted fiber and performed a two-photon interference experiment. We obtained a coincidence fringe of 81.6% visibility without subtracting any background noise, such as accidental coincidence or dark count, which was good enough to violate Bells inequality. Thus, we successfully distributed time-bin entangled photon pairs over 100 km.

Long-distance entanglement-based quantum key distribution over optical fiber.

We report the first entanglement-based quantum key distribution (QKD) experiment over a 100-km optical fiber. We used superconducting single photon detectors based on NbN nanowires that provide high-speed single photon detection for the 1.5-mum telecom band, an efficient entangled photon pair source that consists of a fiber coupled periodically poled lithium niobate waveguide and ultra low loss filters, and planar lightwave circuit Mach-Zehnder interferometers (MZIs) with ultra stable operation. These characteristics enabled us to perform an entanglement-based QKD experiment over a 100-km optical fiber. In the experiment, which lasted approximately 8 hours, we successfully generated a 16 kbit sifted key with a quantum bit error rate of 6.9 % at a rate of 0.59 bits per second, from which we were able to distill a 3.9 kbit secure key.

PERFECT SPATIAL ORDERING OF SELF-ORGANIZED INGAAS/ALGAAS BOX-LIKE STRUCTURE ARRAY ON GAAS (311)B SUBSTRATE WITH SILICON NITRIDE DOT ARRAY

The metalorganic vapor phase epitaxial growth of strained InGaAs/AlGaAs box-like structure self-organized on GaAs (311)B substrate was investigated using fine silicon nitride (SiN) dot arrays for improving the controllability of self-organization phenomena. AlGaAs barrier layer grown at 750 °C buries SiN dots, forming novel pentagonally shaped hollows on (311)B substrate due to the (−100) facet growth and lateral growth. The In0.3Ga0.7As layer is preferentially grown in these hollows, then box-like structure is formed in these hollows during the growth interruption. Successive growth of AlGaAs/In0.3Ga0.7As epilayers induces the stacking of box-like structures just on top of the bottom boxes. The pairing probability of bottom and upper boxes is strongly dependent on the SiN dot array pitch, and the perfect spatial ordering of upper box arrays is achieved when the SiN dot pitch is in a range of 250–300 nm. This approach allows the exact positioning of self-formed box structure.

NEAR-FIELD OPTICAL SPECTROSCOPY AND IMAGING OF SINGLE INGAAS/ALGAAS QUANTUM DOTS

We use near-field optical probing at low temperatures (T=5 K) to image and examine the linear and nonlinear luminescence properties of single InGaAs/AlGaAs quantum dots grown on (311)B oriented GaAs substrates. The high spatial resolution of near-field “nanoprobing,” which is typically 200 nm or less, makes the observation of single dots at different locations on the sample possible, even though the spatial density of quantum dots is on the order of 100/μm2. We observe narrow excitonic emission lines at low excitation powers and, with increasing excitation, we observe biexcitonic emission strongly shifted (3 meV) to the low-energy side of the exciton emission.

Efficient and low-noise single-photon detection in 1550 nm communication band by frequency upconversion in periodically poled LiNbO3 waveguides.

We demonstrate 1500 nm band single-photon detection with low dark-count noise and a potentially high efficiency, which may allow long distance and high-bit-rate quantum key distribution. By developing frequency upconversion devices based on periodically poled lithium niobate waveguides, which are specifically designed to use a pump wavelength longer than that of communication-band photons, we completely eliminate the dark-count noise caused by parasitic nonlinear processes in the waveguide. We observed an internal conversion efficiency as high as 40% and demonstrated scaling down to the single photon level while maintaining a background dark-count rate of 10(2)s(-1).

Optical characteristics of single InAs∕InGaAsP∕InP(100) quantum dots emitting at 1.55μm

The authors have studied the emission properties of individual InAs quantum dots (QDs) grown in an InGaAsP matrix on InP(100) by metal-organic vapor-phase epitaxy. Low-temperature microphotoluminescence spectroscopy shows emission from single QDs around 1550nm with a characteristic exciton-biexciton behavior and a biexciton antibinding energy of more than 2meV relative to the exciton. Temperature-dependent measurements reveal negligible optical phonon induced broadening of the exciton line below 50K, and emission from the exciton state clearly persists above 70K. These results are encouraging for the development of a controllable photon source for fiber-based quantum information and cryptography systems.

Dephasing Processes in Self-Organized Strained InGaAs Single-Dots on (311)B-GaAs Substrate

Single-dot photoluminescence measurements are undertaken on a number of individual InGaAs disks spontaneously formed on the GaAs-(311)B face. Well-isolated distinctive narrow single-dot luminescence lines, the narrowest of which is 34 µ eV in FWHM, is measured using a microscope and their evolution with excitation density is examined. Under very low excitation, individual dot luminescence is well approximated by the Lorentzian lineshape. Excitation via the barrier continuum results in very low luminescence saturation density and simultaneous broadening into a non-Lorentzian lineshape. In contrast, excitation resonant with excited states, causes no such broadening, but saturation power is about three orders of magnitude larger than under barrier excitation. Such phenomena are explained by different carrier flows into the dot states. Carrier-carrier scattering is discussed as a primary dephasing process that causes line broadening.