Dynamic Polarization of Single Nuclear Spins by Optical Pumping of Nitrogen-Vacancy Color Centers in Diamond at Room Temperature

We report a versatile method to polarize single nuclear spins in diamond, based on optical pumping of a single nitrogen-vacancy (NV) defect and mediated by a level anticrossing in its excited state. A nuclear-spin polarization higher than 98% is achieved at room temperature for the ${}^{15}\mathrm{N}$ nuclear spin associated with the NV center, corresponding to $\mu \mathrm{K}$ effective nuclear-spin temperature. We then show simultaneous initialization of two nuclear spins in the vicinity of a NV defect. Such robust control of nuclear-spin states is a key ingredient for further scaling up of nuclear-spin based quantum registers in diamond.

Figure 1

(a) Atomic structure of the NV defect in diamond. (b) Simplified energy-level diagram showing the ground state hyperfine structure associated with ${}^{15}\mathrm{N}$ nuclear-spin states $|\uparrow ⟩$ and $|\downarrow ⟩$. Levels associated to electron spin state $m_s=+1$ are not shown. Optical transitions are used to polarize and read out the electron spin state. (c) ODMR spectra recorded at different magnitudes of a magnetic field applied along the NV symmetry axis ([111] crystal axis). Nuclear-spin polarization is observed at the excited-state LAC ($B\approx 500\mathrm{G}$). Solid lines are data fitting using Lorentzian functions. Identical results are obtained for transitions from $m_s=0$ to the $m_s=+1$ manifold (data not shown). (d),(e) Selective Rabi nutation measurements using microwave pulses at frequency ${\nu }_{\downarrow }$ [blue (or dark gray) points] and ${\nu }_{\uparrow }$ [red (or gray) points]. The experiment is performed for $B=40\mathrm{G}$ (d) and $B=500\mathrm{G}$ (e). Solid lines are data fitting using cosine functions.

Figure 2

Nuclear-spin polarization mechanism. (a) Eigenstates of the excited-state Hamiltonian described by Eq. (2) showing a LAC at $B_{\mathrm{LAC}}\approx 500\mathrm{G}$. This simulation is performed using $D_{\mathrm{es}}=-1420\mathrm{MHz}$ and ${\mathcal{A}}_{\mathrm{es}}=+60\mathrm{MHz}$. The inset shows the evolution of parameters $\alpha$, $\beta$, and $4{\alpha }^2{\beta }^2$ as a function of the magnetic field magnitude. (b) Far from LAC, no nuclear-spin polarization is achieved as optical transitions (${}^{3}A\to {}^{3}E$) are fully nuclear-spin conserving (black arrows). (c) At LAC, precession at frequency $\Omega$ between excited-state sublevels $|0,\uparrow ⟩$ and $|+1,\downarrow ⟩$ can lead to nuclear-spin flip, which can be transferred to the ground state through nonradiative intersystem crossing (curved arrow).

Figure 3

(a) Nuclear-spin polarization $\mathcal{P}$ as a function of the magnetic field magnitude, while keeping its orientation along the NV symmetry axis. $\mathcal{P}$ is estimated by recording ODMR spectra and using Eq. (1). The solid line is data fitting using Eq. (6) with $k_{\mathrm{eq}}^0/\Gamma$ as single adjustable parameter ($k_{\mathrm{eq}}^0/\Gamma =0.009\pm 0.001$). (b) Nuclear-spin polarization $\mathcal{P}$ as a function of the magnetic field angle, its magnitude being fixed at $B=472\mathrm{G}$. The solid line is a guide for eyes.

Figure 4

ODMR spectra recorded for a single NV center with a ${}^{13}\mathrm{C}$ on the first coordination shell. At small magnetic field magnitude ($B=60\mathrm{G}$), four ESR lines are observed, each line being associated to a given orientation $|{}^{15}\mathrm{N}_{\downarrow \mathrm{or}\uparrow },{}^{13}\mathrm{C}_{\downarrow \mathrm{or}\uparrow }⟩$ of the nuclear spins. Around the LAC ($B=470\mathrm{G}$), ${}^{15}\mathrm{N}$ and ${}^{13}\mathrm{C}$ nuclear spins are both polarized. Note that the width of the ESR line is bigger for the measurement performed close to the LAC because of power broadening. However, such width would be small enough to resolve the 3 MHz hyperfine splitting related to ${}^{15}\mathrm{N}$ (see black dot lines). Red (or gray) solid lines are data fitting using Lorentzian functions.