Nuclear spins are a natural choice for various applications requiring long relaxation times, due to their highly localized wave-function which causes a small interaction with the environment. Examples for such applications are vast; among them are bio-sensing using NMR, different kinds of metrology and building blocks for quantum computers. Still, polarization of nuclear spins poses a challenge on every application that involves manipulation of nuclear spin. A method of polarization of single nuclear spins in diamond was suggested in ‎[1] and demonstrated with nuclear spins of ¹⁵N, ¹⁴N and ¹³C ‎[1, 2] . The method, applicable at room temperature, is based on the transfer of the polarization of electronic spins of NV^- color centers to the nuclear spin at the excited-state level anti-crossing (ESLAC) of the center. However, this method was demonstrated only for single spins, while the applicability of this method to ensemble is not trivial due to local effects such as strain. Ensemble polarization would be beneficial for obtaining a high signal to noise ratio in precision measurements based on nuclear spins, or in initializing and manipulating collective quantum memories.

In this work we demonstrate a polarization of a dense ensemble of nuclear spins in diamond by applying this method. Fortunately, the strong coupling between the orbital spin in the excited state to lattice phonons, averages out the spin-orbit and strain terms in the Hamiltonian ‎[3]. Thus, the nuclear spin polarization method can be implemented even with high strain HPHT diamonds.  As a result, we demonstrate a polarization of at least 85% of the ¹⁴N nuclear spins related to the NV centers, as well as a high degree of polarization of the proximal ¹³C nuclear spins. We also study the dependence of the nuclear polarization on the magnetic field in the vicinity of the ESLAC, showing a clear effect of cross-relaxation with substitutional nitrogen centers.

[1]    V. Jacques et al., “Dynamic Polarization of Single Nuclear Spins by Optical Pumping of Nitrogen-Vacancy Color Centers in Diamond at Room Temperature”, Physics Review Letters 102, 057403 (2009).

[2]    B. Smeltzer et al., “Robust Control of Individual Nuclear Spins in Diamond”, Physics Review A 80, 050302 (2009).

[3] L. J. Rogers et al., “Time-Averaging within the Excited State of the Nitrogen-Vacancy Centre in Diamond”, New Journal of Physics 11, 063007 (2009).