NMR signals detect the effective spin polarisation of the sample in the direction of the applied magnetic field. The polarisation of the sample depends on the difference in the number of nuclear spins that are aligned along and opposite to the fields. Even at the largest magnetic fields used for NMR experiments, this difference in population is pretty small, making the NMR detector look for a small needle in a haystack of ambient noise. The spin polarisation of the sample can be enhanced for certain noble gases like Xenon using laser light. Alkali atoms like Rubidium, are polarised by absorbing laser light and when Xe interacts with polarised Rb atoms, the polarisation is transferred to the Xe molecules.