Dynamic nuclear polarization of ensemble of nuclear spins in diamond
Ran Fischer1, Andrey Jarmola2, Pauli Kehayias2, Dmitry Budker2, 3
1Department of Physics, Technion – Israel Institute of Technology, Haifa 32000, Israel
2Department of Physics, University of California, Berkeley, California 94720-7300, USA
3Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
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  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.
In this work we demonstrate a polarization of a dense ensemble of nuclear spins in diamond by applying this method. Thanks to the strong coupling between the orbital spin in the excited state to lattice phonons, which averages out the spin-orbit and strain terms in the Hamiltonian , 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.
This work may further enable room temperature polarization of bulk nuclear spins in diamond by means of spin-diffusion, making it relevant for NMR and other applications in which high nuclear spin polarization is necessary.
 V. Jacques et al., "Dynamic Polarization of Single Nuclear Spins by Optical Pumping of Nitrogen-Vacancy Color Centers in Diamond at Room Temperature", PRL 102, 057403 (2009).
 B. Smeltzer et al., "Robust Control of Individual Nuclear Spins in Diamond", PRA 80, 050302 (2009).
 L. J. Rogers et al., "Time-Averaging within the Excited State of the Nitrogen-Vacancy Centre in Diamond", NJOP 11, 063007 (2009).