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Measurement of the spin temperature of optically cooled nuclei and GaAs hyperfine constants in GaAs/AlGaAs quantum dots


Deep cooling of electron and nuclear spins is equivalent to achieving polarization degrees close to 100% and is a key requirement in solid-state quantum information technologies1,2,3,4,5,6,7. While polarization of individual nuclear spins in diamond2 and SiC (ref. 3) reaches 99% and beyond, it has been limited to 50–65% for the nuclei in quantum dots8,9,10. Theoretical models have attributed this limit to formation of coherent ‘dark’ nuclear spin states11,12,13 but experimental verification is lacking, especially due to the poor accuracy of polarization degree measurements. Here we measure the nuclear polarization in GaAs/AlGaAs quantum dots with high accuracy using a new approach enabled by manipulation of the nuclear spin states with radiofrequency pulses. Polarizations up to 80% are observed—the highest reported so far for optical cooling in quantum dots. This value is still not limited by nuclear coherence effects. Instead we find that optically cooled nuclei are well described within a classical spin temperature framework14. Our findings unlock a route for further progress towards quantum dot electron spin qubits where deep cooling of the mesoscopic nuclear spin ensemble is used to achieve long qubit coherence4,5. Moreover, GaAs hyperfine material constants are measured here experimentally for the first time.

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Figure 1: Electron and nuclear spins in quantum dots.
Figure 2: Manipulation of the nuclear spin states in quantum dots.
Figure 3: Probing nuclear spin temperature in a quantum dot.


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The authors are grateful to A. Rastelli (JKU Linz), Y. Huo (HFNL Hefei), A. Waeber (TU Munich) and P. Atkinson (CNRS Paris) for fruitful discussions. This work has been supported by the EPSRC Programme Grants EP/J007544/1 and EP/N031776/1. E.A.C. was supported by a University of Sheffield Vice-Chancellor’s Fellowship and a Royal Society University Research Fellowship.

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E.Z., F.D. and O.G.S. designed and grew the samples. E.A.C. and A.U. developed the techniques and conducted the experiments. E.A.C. conceived the project and analysed the data. E.A.C. and M.S.S. wrote the manuscript with input from all authors.

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Correspondence to E. A. Chekhovich.

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The authors declare no competing financial interests.

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Chekhovich, E., Ulhaq, A., Zallo, E. et al. Measurement of the spin temperature of optically cooled nuclei and GaAs hyperfine constants in GaAs/AlGaAs quantum dots. Nature Mater 16, 982–986 (2017).

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