Hyperpolarisation Dynamics: Asymptotic Polarisation

Abstract: For applications of solid state quantum computing and quantum simulations, high fidelity initialisation of thermally mixed electronic and nuclear spin qubits is essential. Whereas electronic spins can readily be initialised optically to high fidelity, initialisation of the nuclear spins requires alternative approaches, such as dynamic nuclear polarisation (DNP) via the electronic spin. Pulse-based DNP methods, such as PulsePol, are already widely utilised. By means of repeated application of PulsePol sequences, interspersed with re-initialisation of the electronic spin, high levels of nuclear polarisation -- termed hyperpolarisation -- have been achieved. From theoretical analysis of these protocols perfect nuclear initialisation is expected; however, in practice, saturation below $\sim$ 95$\%$ is seen in experiment. We develop an analytical model to describe hyperpolarisation dynamics, predicting non-maximal nuclear polarisation saturation in the asymptotic limit for realistic nuclear spin clusters. We argue that perfect initialisation of a typical nuclear cluster using this method may not, in general, be possible even with an arbitrarily large number of repetitions.