Letters to Nature

Nature 405, 923-926 (22 June 2000) | doi:10.1038/35016020; Received 20 January 2000; Accepted 28 April 2000

Hidden symmetries in the energy levels of excitonic 'artificial atoms'

M. Bayer1, O. Stern1, P. Hawrylak1,2, S. Fafard2 & A. Forchel1

  1. Technische Physik, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany
  2. Institute for Microstructural Science, National Research Council of Canada, Ottawa, K1A OR6 , Canada

Correspondence to: M. Bayer1 Correspondence and requests for material should be addressed to M.B. (e-mail: Email: mbayer@physik.uni-wuerzburg.de).

Quantum dots1, 2, 3, 4, 5, 6, 7 or 'artificial atoms' are of fundamental and technological interest—for example, quantum dots8, 9 may form the basis of new generations of lasers. The emission in quantum-dot lasers originates from the recombination of excitonic complexes, so it is important to understand the dot's internal electronic structure (and of fundamental interest to compare this to real atomic structure). Here we investigate artificial electronic structure by injecting optically a controlled number of electrons and holes into an isolated single quantum dot. The charge carriers form complexes that are artificial analogues of hydrogen, helium, lithium, beryllium, boron and carbon excitonic atoms. We observe that electrons and holes occupy the confined electronic shells in characteristic numbers according to the Pauli exclusion principle. In each degenerate shell, collective condensation of the electrons and holes into coherent many-exciton ground states takes place; this phenomenon results from hidden symmetries (the analogue of Hund's rules for real atoms) in the energy function that describes the multi-particle system. Breaking of the hidden symmetries leads to unusual quantum interferences in emission involving excited states.