Senior Research Scientist, NTT Basic Research Laboratories

Electrons play important roles in many systems. Nanotechnology allowed us to manipulate single electrons one by one and provided unique systems in which dynamic behaviour of single electrons can be studied. In this work, “Single-electron dynamics in semiconductor quantum dots,” single electron dynamics were successfully tailored and measured precisely in a relatively short timescale.

Single-electron ammeter: A quantum dot is a small conductive island, in which tuneable numbers of electrons occupy discrete energy levels. Electrons transport through the dot one by one in the single-electron tunnelling regime. Each tunnelling event can be monitored by a sensitive charge detector. We have successfully developed a single-electron ammeter that can measure extremely small current on the order of atto-amperes with a single-electron resolution. The single-electron ammeter can be applied to various fields that require high sensitivity and high accuracy in electrical current.

Semiconductor charge qubit: A double quantum dot provides a tuneable quantum two-level system that can be used as a quantum bit (qubit) for quantum computing. We have successfully demonstrated full one qubit operations with rotation and phase-shift gate in a semiconductor double quantum dot. By applying a voltage pulse, coherent oscillations of a single electron between the two dots were successfully induced. Furthermore, two-qubit operations including controlled-NOT gate and swap gate have been successfully demonstrated.

Long spin lifetime: An electron spin in a quantum dot is also attractive as a qubit for its long spin relaxation time. We have developed an electrical pump and-probe scheme to investigate the long spin lifetime. The scheme is now widely used to measure spin lifetime in various quantum dots.

The series of works on single-electron dynamics in semiconductor quantum dots has stimulated subsequent developments in nano-electronics and quantum computing.