Observation of critical spin dressing

It has long been proposed that spin dressing could be employed to realize a highly effective helium-3 nuclear precession co-magnetometer for a neutron electric dipole moment (nEDM) search. The proposal is to apply an intense, continuous, and far off-resonant oscillating magnetic field, called a dressing field, in such a way that the apparent Larmor precession frequencies of the helium-3 and the neutron are modified. Under appropriateconditions a desirable situation known as critical spin dressing (CSD) is anticipated: the neutron and the helium-3 nucleus (or more generally, any two spin species) are expected to behave as if they had the same gyromagnetic ratio and hence should precess at the same rate in a static magnetic field. Spin dressing has been studied in the context of the neutron, helium-3, and a variety of other systems. Critical spin dressing, however, has not previously been demonstrated. In this thesis I report the first experimental demonstration of pulsed CSD in which simultaneous spin dressing of 1H and 19F nuclei is achieved and studied. I also demonstrate that CSD can be performed using variety of different dressing field waveforms, a consideration that until now has received little or no attention. Examples of parameters studied include the role of phase and amplitude modulation on spin dressing. Of particular note is a significant improvement in reproducibility achieved by alternating the phase of successive cycles of the dressing field waveform by pi radians. Such innovations may prove useful in an eventual nEDM search where demands on precession stability are anticipated to be extreme. To enable my study of CSD I developed a simple and robust apparatus. The central innovation was the first use of Magneto-Impedance (MI) sensors to detect weak magnetic fields associated with the precession of nuclear magnetic moments. The thesis thus begins with summaries of experiments to characterise and validate the use of MI sensors for ultra-low field (ULF) nuclear magnetic resonance. I then describe a refined version of the ULF NMR apparatus, and the manner in which it is used to investigate CSD