Highlighted research:

1. Nano-probing of exciton/biexciton wavefunctions (Phys. Rev. Lett. 91, 177401 (2003))

 Frequency-resolved NSOM(Near-field Scanning Optical Microscopy) was applied to monitor the spatial profile of luminescence from the exciton/biexciton in a single GaAs quantum dot, revealing the more extended spatial profile of the exciton luminescence than that of the biexciton luminescence. Our group contributed the theoretical interpretation and clarified the basic physics. This work was highlighted in Physics Today Nov. issue, p.14 (2003) and Physical Review Focus, Oct. 22, 2003(The Travels of An Exciton).

2. Coherent control of excitons in a semiconductor quantum dot (Phys. Rev. Lett. 87, 246401 (2001); Phys. Rev. Lett. 88, 087401 (2002))

The ability to control the quantum state coherently is the basic ingredient in the quantum information processing. We proposed the coherent quantum control of exciton states in an InGaAs quantum dot by using a phase-locked pulse pair. Detailed features of the exciton Rabi oscillation were predicted when the intensity, detuning and the relative delay of the pulse pair were varied. These predictions were immediately confirmed experimentally by the group of Univ. of Texas and the NTT Basic Research Laboratory, independently.

3. Quantum dynamics in the electron-nuclei coupled spin system (Phys. Rev. B 77, 115304 (2008))

We predicted the significant back-action due to the hyperfine interaction between the electron spin and the nuclear spin bath when the electron spin state is measured repetitively, in which the measurement of the electron spin affects the quantum state of the nuclear spins and then the modified nuclear spin state influences the electron spin state and the outcome of the next measurement of the electron spin state. More concretely, we predicted the phenomenon of bunching of the measurement outcome; for the case of two electrons in a quantum dot, it would be more probable to observe the electron spins in the singlet state than in the triplet state, for example.

4. Quantum correlation (Bell) measurement between two electrons (J. Opt. Soc. Am. B 27, pp. A46-A62 (2010))

In the scheme of quantum repeater, the primary elements are the quantum state transfer between a photon and an electron and the entanglement swapping through the Bell (correlation) measurement between two electrons which are created through the quantum state transfer from two photons. We proposed an optical method to measure the spin state of two electrons based on the Faraday or Kerr rotation. Here we employ a linearly polarized off-resonant probe light and measure the orientation of the transmitted (reflected) light. Thus the method can be non-destructive.

5. Quantum state transfer between the Bloch sphere and the Poincare sphere (Nature 457, 702 (2009))

 The electron spin is considered as a most promising entity for the quantum information processing, while the photon is considered as a most suitable information carrier. In the quantum repeater the quantum state transfer between the electron spin and the photon should be carried out with high fidelity. This quantum state transfer was demonstrated by the group of the Tohoku Univ. using the light-hole exciton transition in a GaAs quantum well. They succeeded in performing the complete mapping between the Bloch sphere of the electron spin and the Poincare sphere of the photon polarization. Our group contributed the theoretical background to the experiments.