|M.Sc Student||Shaham Roy|
|Subject||Nuclear Hyperpolarization through Coherent Spin Exchange|
|Department||Department of Physics||Supervisor||Professor Michael Reznikov|
|Full Thesis text|
Spin-polarized nuclei are of major importance for technological applications such as Nuclear Magnetic Resonance, Magnetic Resonance Imaging, as well as application oriented scientific research such as Quantum Information Processing. Due to small nuclear magnetic moments, magnetic field is notoriously ineffective for their polarization. Therefore, different methods of enhancing the polarization beyond the thermally limited one are commonly used. These hyperpolarization methods are based on spin transfer between an easily polarized spin specie, usually electrons, and the target nuclear spin population.
In our work, we discuss a system of a single electron coupled with nuclear spins; the electron is polarized by optical pumping, as is commonly the case for hyperpolarization. Pumping, being necessary to polarize electrons, destroys coherence between the spin species as a side effect. Rapid decoupling slows down the polarization transfer between the two spin species, which is a manifestation of the quantum Zeno effect. We found that the pumping for an efficient hyperpolarization process should be synchronized with the spin transfer process to ensure both efficient spin transfer due to coherence between spin species, and sufficient introduction of angular momentum into the system.
We analyzed the effect of synchronized pumping on different systems with homogeneous and inhomogeneous hyperfine interactions. We found that, for homogeneous systems, dark states are formed, which cannot be polarized any further. For a system with a large number of nuclear spins these states set a polarization limit of the order of N-1/2, where N is the number of nuclear spins. Fortunately, realistic systems are inhomogeneous, and the dark states are only approximately dark. The leakage between these states allows polarization increase, albeit at a slower rate. We found the optimal optical polarization rate to be proportional to N1/2, and that it is beneficial to increase this rate along polarization buildup.
We examined the applicability of the suggested coherent hyperpolarization process to several realistic systems. We found the implementation for gaseous systems with two interacting gas species to be very problematic. This happens since in such systems the interactions between the spin species occur primarily during collisions. It turns out that, for a realistic 3D system, a pair of colliding gas atoms will probably never meet again. As a result, one cannot regard such systems as systems with well-defined coherence between the spin species. For solid state systems, the interaction is permanent, since the spins are confined. When spin flips occur in such systems, preserving coherence of spin transfer by proper optical pumping enhances the polarization rate. We considered in detail two solid state systems: a quantum dot bounding an electron together with nuclear spins of the atoms composing the semiconductor lattice, and a nitrogen-vacancy color center in a diamond coupled to nuclear spins of 13C atoms.