Driven by the environment

Phys. Rev. Lett. 118, 167204 (2017)

The coherent control of a spin qubit state and its time evolution between two quantum levels is usually achieved by the application of an external a.c. magnetic field whose frequency matches the separation between the energy levels resonantly. Simultaneously, local fluctuations induced by the surrounding electronic environment limit the intrinsic quantum coherence of the qubit — a detrimental condition for the implementation of quantum computation algorithms.

Now, Lillie et al. report on so-called environmentally mediated resonance — a double-resonance process where direct coherent control of the electronic environment of a single qubit is indirectly transferred to the qubit itself. In their proof-of-principle demonstration, the reference qubit is encoded in a single nitrogen–vacancy centre ∼10 nm below the surface of a diamond crystal, while free electrons on the surface are considered as the environment. A static magnetic field ∼1 kOe is used to induce spin resonance processes in free electrons at ∼2.87 GHz while preventing direct resonant absorption by the nitrogen–vacancy centre. By tuning the intensity of the a.c. magnetic field, the researchers match the frequency of the induced Rabi oscillation in free electrons to the characteristic resonant frequency between |m_s = 0〉 and |m_s = −1〉 states in the qubit, providing evidence for its indirect coherent control.