Motomu Takatsu

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.

Transmission Experiment of Quantum Keys over 50 km Using High-Performance Quantum-Dot Single-Photon Source at 1.5 µm Wavelength

We have developed a high-performance single-photon source (SPS) operating at 1.5 µm wavelength. The source is an InAs/InP quantum dot with a horn-shaped nanostructure. A resonant excitation to the p-shell state helps achieve a single-photon efficiency of 5.8% after coupling into a single-mode fiber with a second-order correlation value of g(2)(0)~0.055. The performance of the source has been assessed by integrating it into a conventional quantum key distribution system. We have successfully transmitted secure keys over a 50 km commercial fiber, exceeding the previously reported range for an SPS operating below 1.3 µm.

Quantum functional devices for advanced electronics

Abstract Recent research in semiconductor device technology seems to be focused on reducing the cost and power dissipation of traditional Si CMOS integrated circuits, rather than developing new and advanced semiconductor devices. We believe however, that devices enter the nanometer-scale regime in the next century, where quantum mechanical effects play an important role in the devices function; therefore, it is important to continue basic research into the physics and technology of nanometer scale structures and device applications in order to cultivate “nanoelectronics”. This paper reviews our research activities on quantum functional devices and discusses our future research direction.

A full adder using resonant-tunneling hot electron transistors (RHETs)

Full adders are demonstrated using InGaAs-In(AlGa)As RHETs. The RHETs emitter and base electrodes were self-aligned using a SiO/sub 2/ sidewall and angled beam ion milling. The common-base current gain was about 0.9 and the emitter current peak-to-valley ratio was 10. The RHET full adder was constructed using a three-input exclusive-OR logic gate and a three-input majority logic gate. The authors confirmed normal operation of the full adder at 77 K. Only seven RHETs were needed for the full adder, about one-quarter of bipolar transistors that would have been required. >

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.

Few-Electron Molecular States and Their Transitions in a Single InAs Quantum Dot Molecule

We study electronic configurations in a single pair of vertically coupled self-assembled InAs quantum dots, holding just a few electrons. By comparing the experimental data of nonlinear single-electron transport spectra in a magnetic field with many-body calculations, we identify the spin and orbital configurations to confirm the formation of molecular states by filling both the quantum mechanically coupled symmetric and antisymmetric states. Filling of the antisymmetric states is less favored with increasing magnetic field, and this leads to various magnetic field induced transitions in the molecular states.

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 1.5 μm. 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 (4.4 ± 0.2) × 10−4 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.

Electron waves through quantum point contacts

This paper reviews the authors work on the experimental and theoretical analyses of the angular distribution of electrons injected through a single quantum point contact. They observed double peaks in the distribution with the point contact quantized in two modes. The authors calculation of the distribution using a Fraunhofer diffraction approximation through a quantized single slit agreed well with results. In this calculation they use a Greens function in weak magnetic fields and constructed mirror images there. They also investigated the possibility of observing interference in the angular distribution between the first and second modes, and found that the interference terms of the angular distribution were cancelled due to system symmetry. Another possible way to observe interference is due to electron waves through double point contacts. To measure this interference, the authors developed submicron air bridges to make double point contacts with independently controlled widths. The authors measured the controlled additivity of the conductance for four point contacts.