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Allowed and forbidden transitions in artificial hydrogen and helium atoms

Nature volume 419, pages 278–281 (2002)Cite this article

Abstract

The strength of radiative transitions in atoms is governed by selection rules that depend on the occupation of atomic orbitals with electrons1. Experiments have shown2,3,4,5 similar electron occupation of the quantized energy levels in semiconductor quantum dots—often described as artificial atoms. But unlike real atoms, the confinement potential of quantum dots is anisotropic, and the electrons can easily couple with phonons of the material6. Here we report electrical pump-and-probe experiments that probe the allowed and ‘forbidden’ transitions between energy levels under phonon emission in quantum dots with one or two electrons (artificial hydrogen and helium atoms). The forbidden transitions are in fact allowed by higher-order processes where electrons flip their spin. We find that the relaxation time is about 200?µs for forbidden transitions, 4 to 5 orders of magnitude longer than for allowed transitions. This indicates that the spin degree of freedom is well separated from the orbital degree of freedom, and that the total spin in the quantum dots is an excellent quantum number. This is an encouraging result for potential applications of quantum dots as basic entities for spin-based quantum information storage.

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Figure 1: Artificial hydrogen and helium atoms.
Figure 2: Relaxation time of a one-electron QD (artificial hydrogen atom).
Figure 3: Relaxation time of a two-electron QD (artificial helium atom).

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Acknowledgements

We thank G. E. W. Bauer, T. Honda, T. Inoshita, A. V. Khaetskii and L. P. Kouwenhoven for discussions and help.

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Authors and Affiliations

  1. NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, 243-0198, Japan

    Toshimasa Fujisawa, David Guy Austing, Yasuhiro Tokura, Yoshiro Hirayama & Seigo Tarucha

  2. Institute for Microstructural Sciences M23A, National Research Council of Canada, Ontario, K1A 0R6, Ottawa, Canada

    David Guy Austing

  3. CREST Interacting Carrier Electron Project, 4-1-8 Honmachi, 331-0012, Kawaguchi, Japan

    Yoshiro Hirayama

  4. University of Tokyo, Bunkyo-ku, 113-0033, Tokyo, Japan

    Seigo Tarucha

  5. ERATO Mesoscopic Correlation Project, 3-1 Morinosato-Wakamiya, Atsugi, 243-0198, Japan

    Seigo Tarucha

Authors
  1. Toshimasa Fujisawa
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  2. David Guy Austing
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  3. Yasuhiro Tokura
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  4. Yoshiro Hirayama
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  5. Seigo Tarucha
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Correspondence to Toshimasa Fujisawa.

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Fujisawa, T., Austing, D., Tokura, Y. et al. Allowed and forbidden transitions in artificial hydrogen and helium atoms. Nature 419, 278–281 (2002). https://doi.org/10.1038/nature00976

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