Optically Controlled Switching of the Charge State of a Single Nitrogen-Vacancy Center in Diamond at Cryogenic Temperatures

In this Letter, the photoinduced switching of the single nitrogen-vacancy (NV) center between two different charge states, negative (${\mathrm{NV}}^{-}$) and neutral (${\mathrm{NV}}^0$), is studied under resonant excitation at liquid helium temperature. We show that resonant conversion of ${\mathrm{NV}}^0$ to ${\mathrm{NV}}^{-}$ significantly improves spectral stability of the ${\mathrm{NV}}^{-}$ defect and allows high fidelity initialization of the spin qubit. Based on density functional theory calculations a novel mechanism involving an Auger ionization of ${\mathrm{NV}}^{-}$ and charge transfer of an electron from the valence band to ${\mathrm{NV}}^0$ is discussed. This study provides further insight into the charge dynamics of the NV center, which is relevant for quantum information processing based on an ${\mathrm{NV}}^{-}$ defect in diamond.

Figure 1

Energy-level diagram of the NV defect in the (a) negative and (b) neutral charge state. (c) Photoionization rate of ${\mathrm{NV}}^{-}$ as a function of resonant laser power. Each point is the number of photoionization events extracted at the specified laser power of the ${\mathrm{NV}}^{-}$ laser. The inset shows the three-level diagram used for a fit model.

Figure 2

(a) PLE spectrum of ${\mathrm{NV}}^0$ defect with ZPL centered around 575 nm and corresponding schematic representation of the experiment indicating positions of excitation and “repump” laser on the PL spectra and detection (gray shaded region). The spectrum (i) is observed only when photoionizing laser pulses at the ${\mathrm{NV}}^{-}$ resonance are applied (i*). No fluorescence is detected (ii) when the photoionizing laser is off resonant to the ${\mathrm{NV}}^{-}$ transitions (ii*). (b) The effect of fluorescence recovery on ${\mathrm{NV}}^{-}$ transitions (i), while an additional laser is sweeping over ${\mathrm{NV}}^0$ ZPL (i*). The PLE spectrum of ${\mathrm{NV}}^{-}$ is absent (ii) if the second laser is shifted away from the ${\mathrm{NV}}^0$ resonance (ii*).

Figure 3

PLE spectra of ${\mathrm{NV}}^{-}$ with two different types of repumping: (a) conventional way, when at the beginning of each laser frequency sweep over ${\mathrm{NV}}^{-}$ transitions a 1 mW green laser pulse is applied and (b) resonant repumping, when one laser is swept across ZPL of the ${\mathrm{NV}}^0$ transition with power of 100 nW, before the red laser sweep.

Figure 4

Schematic picture of the charge conversion process for negative and neutral states of the NV defect. (a) ${\mathrm{NV}}^{-}$ to ${\mathrm{NV}}^0$ conversion involves two photons and an Auger process that releases enough energy to detach an electron from the defect. (b) ${\mathrm{NV}}^{-}$ recovering occurs in two steps: first, an electron is excited from the $a_1$ to the $e$ orbital and then an additional electron is transferred form the deep lying $a_1$ orbital to the vacant place on the $a_1$ orbital in the band gap. The deep hole migrates away from the NV center leaving it in the negative charge state. The position of defect levels changes upon different occupation of states shown schematically for both processes.