Abstract
Coherent control of quantum systems has far-reaching implications in quantum engineering. In this context, coherent population trapping (CPT) involving dark resonances 1,2 has played a prominent role, leading to a wealth of major applications including laser cooling of atoms 3 and molecules 4 , optical magnetometry 5 , light storage 6–8 and highly precise atomic clocks 9. Extending CPT methods to individual solid-state quantum systems has been only achieved in cryogenic environments for electron spin impurities 10–17 and superconducting circuits 18–21. Here, we demonstrate efficient CPT of a single nuclear spin in a room temperature solid. To this end, we make use of a three-level system with a Λ-configuration in the microwave domain, which consists of nuclear spin states addressed through their hyperfine coupling to the electron spin of a single nitrogen-vacancy defect in diamond. Dark state pumping requires a relaxation mechanism which, in atomic systems, is simply provided by spontaneous emission. In this work, the relaxation process is externally controlled through incoherent optical pumping and separated in time from consecutive coherent microwave excitations of the nuclear spin Λ-system. Such a pumping scheme with controlled relaxation allows us (i) to monitor the sequential accumulation of population into the dark state and (ii) to reach a new regime of CPT dynamics for which periodic arrays of dark resonances can be observed, owing to multiple constructive interferences. This work offers new prospects for quantum state preparation, information storage in hybrid quantum systems 22,23 and metrology. Nuclear spins in solids have attracted considerable interest over the last years owing to their high level of isolation from the environment, leading to very long coherence times 24–26. The detection and control of individual nuclear spins is usually achieved by exploiting their hy-perfine coupling to an ancillary electronic spin. This can be performed by using a nitrogen-vacancy (NV) defect in diamond 27,28 whose spin triplet ground state (S = 1) can be optically initialized, coherently manipulated with microwave fields and read-out by optical means through its spin-dependent photoluminescence (PL) intensity 29. Experiments making use of the hyperfine coupling between the NV defect and nearby long-lived nuclear spins are now reaching a level of control allowing to realize elaborate quantum information protocols 30–32. Here, we make use of electron spin transitions of a single NV defect to achieve coherent control of a single nuclear spin through CPT under ambient conditions. The spin system considered in this study is depicted in Fig. 1a. The electronic spin of a single NV defect is coupled by hyperfine interaction with a nearby 13 C nuclear spin (I = 1/2). In the m s = 0 (|0 e) electron spin mani-fold, the nuclear spin eigenstates are denoted |↑ and |↓ , corresponding to the nuclear spin projections along the NV defect quantization axis. In the m s = −1 (|−1 e) electron spin manifold, efficient nuclear spin mixing can be obtained by exploiting the anisotropic component of the hyperfine interaction in combination with a magnetic field applied along the NV defect axis 34. The nuclear spin