Dynamic nuclear polarization with colour centres in diamond

Abstract

Nuclear magnetic resonance (NMR) spectroscopy and imaging (MRI) are of extraordinary importance in natural and life sciences. The signal observed in these measurement techniques crucially depends on the degree of nuclear spin polarization. Optically-pumped nitrogen-vacancy (NV) centres in diamond show an electron spin polarization exceeding 92% [1], which is several orders of magnitude higher than thermal nuclear spin polarization at conditions typical for NMR measurements. This high electron spin polarization can be transferred to nuclear spins leading to a polarization above the thermal, equilibrium condition, the latter being limited by the Boltzmann distribution. The transfer of optically-pumped NV electron spin polarization to nuclear spins is the main topic of this thesis. The polarization transfer via coherent polarization methods was demonstrated with a bulk sample polarized in a conventional electron paramagnetic resonance (EPR) spectrometer. These experiments were repeated on the single electron spin level and a measurement technique was developed working on the principle of polarization read-out via polarization inversion (PROPI). Using this method different polarization techniques were benchmarked including nuclear orientation via electron locking (NOVEL), the integrated solid effect (ISE), and the PulsePol sequence, which was demonstrated to have high robustness whilst keeping good efficiency. Moreover, NOVEL was utilized to demonstrate transfer of polarization from single, shallow NV centres to nuclear spins in oil molecules on the diamond surface. The described dynamic nuclear polarization (DNP) methods are not the only way to speed up NMR measurements. The matrix completion algorithm was applied to a single NV two-dimensional spectrum in order to accelerate data acquisition, leading to a reconstruction of the spectrum by using only 10% of the original measurement data. The findings presented in this thesis might contribute to goals such as hyperpolarized nano-scale sensing, high-signal-tonoise NMR, allowing small sample sizes or fast data acquisition, and the initialization of a quantum simulator based on nuclear spins around a central electron spin.

[1] G. Waldherr et al., PRL 106, 157601 (2011)