Locking electron spins into magnetic resonance by electron-nuclear feedback

Quantum information processing requires accurate coherent control of quantum-mechanical two-level systems, but is hampered in practice by their coupling to an uncontrolled environment. For electron spins in III-V quantum dots, the random environment is mostly given by the nuclear spins in the quantum-dot host material; they collectively act on the electron spin through the hyperfine interaction, much like a random magnetic field. Here we show that the same hyperfine interaction can be harnessed such that partial control of the normally uncontrolled environment becomes possible. In particular, we observe that the electron-spin-resonance frequency remains locked to the frequency of an applied microwave magnetic field, even when the external magnetic field or the excitation frequency are changed. The nuclear field thereby adjusts itself such that the electron-spin-resonance condition remains satisfied. General theoretical arguments indicate that this spin-resonance locking might be accompanied by a significant reduction of the randomness in the nuclear field.