Confluence of resonant laser excitation and bidirectional quantum-dot nuclear-spin polarization

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. In semiconductor quantum dots, interactions between the confined electrons and the surrounding reservoir of nuclear spins limit the attainable electron-spin coherence. But the nuclear-spin reservoir can also take a constructive role, as it facilitates the locking of the optical quantum-dot resonance to the changing frequency of an external driving laser, as an experiment now demonstrates.