Abstract
Resonant laser scattering along with photon correlation measurements established the atom-like character of quantum dots. Here, we show that for a wide range of experimental parameters it is impossible to isolate elementary quantum-dot excitations from a strong influence of nuclear spins; the absorption lineshapes at magnetic fields exceeding 1 T indicate that the nuclear spins get polarized by an amount that ensures locking of the quantum-dot resonance to the incident laser frequency. In stark contrast to earlier experiments, this nuclear-spin polarization is bidirectional, allowing the combined electron–nuclear-spin system to track the changes in laser frequency dynamically on both sides of the resonance. This unexpected feature stems from a competition between two spin-pumping processes that attempt to polarize nuclear spins in opposite directions. We find that the confluence of laser excitation and nuclear-spin polarization suppresses the fluctuations in resonant absorption. A master-equation analysis suggests narrowing of the nuclear-spin distribution, pointing to applications in quantum information processing.
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Acknowledgements
We thank S. Fält for growing samples B and C. We also acknowledge many useful discussions with H. Türeci, J. Taylor, G. Giedke, M. Rudner and L. Levitov. This work was supported by NCCR Quantum Photonics (NCCR QP), research instruments of the Swiss National Science Foundation (SNSF), and by an ERC Advanced Investigator Grant (A.I.). The work carried out in Cambridge was supported by QIP IRC and EPSRC grant No EP/G000883/1. D.S. and W.W. would like to thank the Deutsche Forschungsgemeinschaft (DFG) and the Bundesministerium fuer Bildung und Forschung (BMBF) for financial support.
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C.L. and A.H. carried out the experiments on samples A and C. Y.Z. and A.N.V. carried out the experiments on sample B. A.B., D.S. and W.W. grew the samples. A.I., along with C.L. and A.H., developed the model that explained the experimental observations. C.L., I.C. and A.I. did the theoretical analysis. P.M., M.K., J.D. and M.A. carried out earlier experiments and actively participated in discussions. A.H., M.A. and A.I. planned the experiments.
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Latta, C., Högele, A., Zhao, Y. et al. Confluence of resonant laser excitation and bidirectional quantum-dot nuclear-spin polarization. Nature Phys 5, 758–763 (2009). https://doi.org/10.1038/nphys1363
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DOI: https://doi.org/10.1038/nphys1363
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