Photophysics of the SnV Center in Diamond

For many applications in the field of quantum information processing stationary qubits are required, providing long-lived spin coherence and suitable level schemes for coherent control and efficient optical read out. Color centers in diamond, more specifically the group-IV-vacancy centers, have emerged as promising candidates among solid state qubits. They exhibit favorable properties such as individually addressable spins with long coherence times and bright emission of single, close to transform limited photons. Recent experiments have shown that the negatively charged tin-vacancy center (SnV) [1] combines long spin coherence times at conveniently achievable cryogenic temperatures (&gt;1K) [2,3] with truly lifetime-limited transition linewidths down to 20 MHz [1].<br/>However, exploiting these properties requires active stabilization of the charge state, as the SnV center upon resonant laser illumination can be easily ionized to its double negative charge state (SnV²⁻) which is optically inactive [3]. To prevent ionization, illumination with a second light field in the blue spectral range is required. We find that the charge stabilization requires the presence of additional defects in the vicinity of the SnV center such as di-vacancies [3] in the bulk or sp² defects on the surface [4].<br/>Here, we propose a simple rate equation model for the SnV center photophysics that includes this ionization process. We apply the model to an extensive set of measurements on different SnV centers, along with a thorough characterization of the efficiency of our measurement setup. We conclude that a charge-stabilized SnV⁻ center is a nearly ideal single photon source in terms of quantum efficiency, since we can describe any reduced photon rate by ionization to the optically inactive SnV²⁻ charge state without assuming other non-radiative decay channels. We further discuss the occurrence of fluctuating background emission, its suppression via thermal oxidation in air atmosphere and the role of surface defects in the charge transition cycle.<br/><br/><b>References</b><br/>[1] T. Iwasaki <i>et al., Phys. Rev. Lett. </i><b>119 </b>(2017)<i>, </i>253601; J. Görlitz <i>et al., New J. Phys.</i> <b>22 </b>(2020), 013048.<br/>[2] R. Debroux <i>et al., Phys. </i><i>Rev. X</i> <b>11</b> (2021), 041041<br/>[3] J. Görlitz <i>et al., npj Quantum Inf </i><b>8</b> (2022), 45.<br/>[4] A. Stacey et al., <i>Adv. Mater. Interfaces</i> <b>6</b> (2019), 1801449.