Toshiyuki Miyazawa

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.

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 1.5u2009μm. 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 ∼1.6u2009ns as an exciton lifetime in the quantum dot, which opens up the possibility of operating the single photon horn over 100 MHz.

First Demonstration of Electrically Driven 1.55 µm Single-Photon Generator

We succeeded in demonstrating single-photon generation from a single InAs quantum dot (QD) at a 1.55 µm band by current injection. A p–i–n light-emitting diode (LED), which includes a quantum dot layer, was grown on an n-InP substrate and fabricated into a nano scaled mesa structure with electrodes. Electrical pulses of 80 ps width were injected in order to generate excitons in quantum dots. We directly determined the electroluminescence (EL) and radiative lifetime of a single exciton to be 1.59 ns. Hanbury-Brown and Twiss (HBT)-type photon correlation measurements proved the antibunching behavior of exciton recombination in a current-injected quantum dot at a wavelength of 1551.2 nm. These measurements demonstrate that our QD LEDs are sources of triggered single photons in the C-band by current injection.

Exciton dynamics in current-injected single quantum dot at 1.55μm

We investigate the exciton dynamics in a current-injected single InAs quantum dot (QD) which emits 1.55μm photons. Photon antibunching behavior is observed using a single electroluminescence line of a single QD. The radiative lifetime of this line determined by time-resolved measurement is 1.59ns. The single exciton recombination time agrees with the lifetime calculated with an eight-band kp model. We examine a high drive rate operation of the device by changing the delay time between two electrical pulses. These results demonstrate that our device has the potential to achieve telecommunication band subgigahertz single-photon emission with electrical pulses.

Single-photon emission at 1.5 μm from an InAs/InP quantum dot with highly suppressed multi-photon emission probabilities

We have demonstrated highly pure single-photon emissions from an InAs/InP quantum dot at the wavelength of 1.5u2009μm. By applying quasi-resonant excitation, one exciton is deterministically generated in an excited state, which then relaxes to the exciton ground state before recombining to emit a single photon. The photon-correlation function of the emission from the exciton ground state exhibits a record g(2)(0) value of (4.4u2009±u20090.2)u2009×u200910−4 measured using high-performance super-conducting single-photon detectors, without any background subtraction. This single-photon source with extremely low multi-photon emission probability paves the way to realize long distance quantum key distribution and low error-rate quantum computation.

Development of Electrically Driven Single-Quantum-Dot Device at Optical Fiber Bands

We succeeded in observing the electroluminescence and Stark shift of a single InAs/GaAs quantum dot in the O-band (O-band is a 1.3 µm band which has the lowest dispersion characteristics in optical fiber bands). In order to access a single quantum dot, we fabricated a p–i–n diode containing one quantum dot layer with a small ohmic contact area. The electroluminescence of a single exciton (λ=1321.6 nm) and biexciton (λ=1322.3 nm) were clearly observed at the center of the O-band at 7 K. This result is the longest wavelength attained up to now. The Stark shift of single quantum dots was also observed at around 1.32 µm at 7 K. These results are promising for the realization of electrically driven single-photon emitters at optical fiber bands.

Single-Photon Generator for Optical Telecommunication Wavelength

We report on the generation of single-photon pulses from a single InAs/InP quantum dot in telecommunication bands (1.3-1.55 µm: higher transmittance through an optical fiber). First we prepared InAs quantum dots on InP (0 0 1) substrates in a low-pressure MOCVD by using a so-called InP double-cap procedure. The quantum dots have well-controlled photo emission wavelength in the telecommunication bands. We also developed a single-photon emitter in which quantum dots were embedded. Numerical simulation designed the emitter to realize efficient injection of the emitted photons into a single-mode optical fiber. Using a Hanbury-Brown and Twiss technique has proved that the photons through the fiber were single photons.

Two-Photon Control of Biexciton Population in Telecommunication-Band Quantum Dot

Rabi oscillations of telecommunication-band excitons and biexcitons have successfully been observed by the photocurrent spectroscopy in a single InAs/GaAs quantum dot. We show the excitonic Rabi oscillations up to the rotation angle of 8π as well as the biexciton Rabi oscillation up to 2π. The decoherence time of both the telecommunication-band exciton and biexciton is much longer than the excitaion pulse-duration of 40 ps. The results demonstrate that the telecommunication-band exciton and biexciton system is promising for exciton-based-quantum information devices compatible with optical fiber networks.

Electric field modulation of exciton recombination in InAs/GaAs quantum dots emitting at 1.3μm

Changing the electric field applied to InAs quantum dots embedded in a p-i-n diode was found to modulate the radiative recombination rate of excitons in the dots. The quantum dots were capped with a strain-reducing layer to realize 1.3u2002μm photoemission and a large dipole moment to the exciton states. The exciton states in a quantum dot were investigated by measuring the quantum-confined Stark shift for various applied electric fields and were compared with the theoretical electron and hole wave functions calculated using an eight-band k⋅p model. When the absolute value of the applied electric field was reduced from −82.4u2002kV/cm to 0, the radiative recombination rate increased from 0.88 to 1.11u2002ns−1. Comparison of the experimental rate with the calculated one revealed that the increase in the radiative recombination rate was due to a decrease in the overlap integral between the electrons and holes. These optical characteristics of InAs quantum dots are especially important for developing optical devices tha...

Change in electrical resistivity due to icosahedral phase precipitation in Zr70Pd20Ni10 and Zr65Al7.5Cu7.5Ni10Ag10 glasses

The glass-to-icosahedral phase transformation in Zr70Pd20Ni10 and Zr65Al7.5Cu7.5Ni10Ag10 glasses was examined by the electrical resistivity measurement performed with a heating rate of 0.67 K/s. The resistivity increased with the promotion of icosahedral precipitation in Zr70Pd20Ni10 glass. On the other hand, Zr65Al7.5Cu7.5Ni10Ag10 glass exhibited the decrement of the resistivity according to the evolution of icosahedral phase. The latter was qualitatively explained by the drop of the resistivity of supercooled liquid phase due to the transfer of oxide atoms into the icosahedral phase. Also, the low temperature resistivity experiment showed that the conductivity of glassy and icosahedral phases might obey the weak localization model of conduction electrons.