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 technologies^{1,2,3,4,5,6,7}. 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 dots^8,9,10. Theoretical models have attributed this limit to formation of coherent ‘dark’ nuclear spin states^11,12,13 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 coherence^4,5. Moreover, GaAs hyperfine material constants are measured here experimentally for the first time.

电子和核自旋的深冷等效于实现接近100％的极化度，并且是固态量子信息技术^{1,2,3,4,5,6,7}的关键要求。当金刚石²和SiC（参考文献3）中单个核自旋的极化达到99％或更高时，量子点^8,9,10中的核被限制为50-65％。理论模型将此限制归因于相干“暗”核自旋态的形成^11,12,13但是缺乏实验验证，尤其是由于偏振度测量的准确性较差。在这里，我们使用一种新的方法来测量GaAs / AlGaAs量子点中的核极化，该新方法是通过利用射频脉冲操纵核自旋态而实现的。观察到高达80％的极化，这是迄今为止量子点中光学冷却的最高记录。该值仍然不受核相干效应的限制。相反，我们发现光学冷却的原子核在经典的自旋温度框架^14中得到了很好的描述。我们的发现开辟了一条向量子点电子自旋量子位进一步发展的途径，其中使用介观核自旋集合体的深冷来实现长的量子位相干^4,5。此外，GaAs超细材料常数是首次在此进行实验测量。