Haruki Kiyama

Single photoelectron detection after selective excitation of electron heavy-hole and electron light-hole pairs in double quantum dots

We demonstrate the real-time detection of single photogenerated electrons in two different lateral double quantum dots made in AlGaAs/GaAs/AlGaAs quantum wells having a thin or a thick AlGaAs barrier layer. The observed incident laser power and photon energy dependences of the photoelectron detection efficiency both indicate that the trapped photoelectrons are, for the thin barrier sample, predominantly photogenerated in the buffer layer followed by tunneling into one of the two dots, whereas for the thick barrier sample they are directly photogenerated in the well. For the latter, single photoelectron detection after selective excitation of the heavy and light hole state in the dot is well resolved. This ensures the applicability of our quantum well-based quantum dot systems for the coherent transfer from single photon polarization to single electron spin states.

Nonreciprocal Directional Dichroism in Ferroelectric Nd2Ti2O7

The nonreciprocal directional dichroism derived from the optical magnetoelectric effect has been investigated for the f – f transitions of Nd 3+ ion in a ferroelectric Nd 2 Ti 2 O 7 single crystal under magnetic field, in which both space-inversion and time-reversal symmetries are viewed as simultaneously broken. The magnitude of nonreciprocity in each absorption band of Nd 3+ was semi-quantitatively elucidated by taking account of the interference between electric dipole and magnetic dipole transitions.

Spin-dependent current through a quantum dot from spin-polarized nonequilibrium quantum Hall edge channels

We report selective injection of both spin-up and spin-down single electrons into a quantum dot (QD) from spin-polarized nonequilibrium quantum Hall edge channels (ECs) generated by selective transmission of spin-resolved ECs using a surface gate placed at a distance from the QD. We change the spin polarization of nonequilibrium ECs by changing the bias voltages applied to different source Ohmic contacts. The efficiency of spin-up electron injection reaches 0.5, which is approximately 0.2 higher than that induced by spin-dependent tunnel coupling between QD and ECs. On the other hand, the efficiency of spin-down electron injection reaches 0.4. In addition, we rectify the underestimation of the efficiency of spin filtering for equilibrium ECs by numerically subtracting the contribution of the excited states in the QD. The obtained spin-filtering efficiency is higher than that evaluated from the raw experimental data and increases with magnetic field as expected with the increase in the spatial separation between ECs.

Single-electron charge sensing in self-assembled quantum dots

Measuring single-electron charge is one of the most fundamental quantum technologies. Charge sensing, which is an ingredient for the measurement of single spins or single photons, has been already developed for semiconductor gate-defined quantum dots, leading to intensive studies on the physics and the applications of single-electron charge, single-electron spin and photon–electron quantum interface. However, the technology has not yet been realized for self-assembled quantum dots despite their fascinating transport phenomena and outstanding optical functionalities. In this paper, we report charge sensing experiments in self-assembled quantum dots. We choose two adjacent dots, and fabricate source and drain electrodes on each dot, in which either dot works as a charge sensor for the other target dot. The sensor dot current significantly changes when the number of electrons in the target dot changes by one, demonstrating single-electron charge sensing. We have also demonstrated real-time detection of single-electron tunnelling events. This charge sensing technique will be an important step towards combining efficient electrical readout of single-electron with intriguing quantum transport physics or advanced optical and photonic technologies developed for self-assembled quantum dots.

Conversion from Single Photon to Single Electron Spin Using Electrically Controllable Quantum Dots

Polarization is a fundamental property of light and could provide various solutions to the development of secure optical communications with high capacity and high speed. In particular, the coheren...

Design of bull’s eye structures on gate-defined lateral quantum dots

Quantum repeaters are required for realizing long-distance quantum communication. The quantum repeater consists of a quantum memory to store quantum information and an interface between photonic flying qubits and the memory qubits. Electron spins in gate-defined quantum dots (QDs), which have a relatively long coherence time and high electrical tunability, are promising candidates for such memory qubits because the fundamental technologies of detecting and manipulating single photoelectron spins have been established. The remaining challenge for the realization of quantum repeaters is an efficient coupling between photons and electron spins in the QDs. In this study, we discuss the enhancement of the transmission and the maintenance of the incident light polarization through bulls eye structures on gate-defined QDs on the basis of electromagnetic field simulations.

Angular momentum conversion from single photons to single electron spins in a lateral double quantum dot

In gate-defined quantum dots (QDs), coherent manipulation of electron spins and two-qubit gate operations have been achieved, verifying their suitability to the scalable qubits for quantum computations. If the spin states in such gate-defined QDs could couple to the photon states coherently the ability of the gate-defined QDs would be considerably extended to a long distance quantum communications. Here we demonstrate that the angular momentum of the circularly polarized single photon can be transferred to the electron spin in a double QD. This result confirms that the photon can be coupled to the spin degree of freedom in the gate-defined QDs.

Development of a Numerical Algorithm for Identifying Single Photon Detection with a Quantum Dot

We present a numerical identification scheme for single photon detection (SPD) using a lateral GaAs Quantum Dot (QD). Trapping and resetting of the photo‐generated single electrons can be sensed by the quantum point contact (QPC) fabricated near the dot. A step‐like increase of the QPC current can be observed after laser pulse irradiation meaning an electron is released from the dot. This increase could only be observed if a photo‐generated electron had been trapped in the dot. We show that our developed scheme successfully identifies SPD and measures computationally the trapping time of the generated electron. This work will allow us to analyze single photon to electron spin conversion in a statistical manner.