Hybrid 3He and alkali magnetometry techniques for fundamental physics experiments

Abstract

Since the 1950s, experiments searching for the neutron electric dipole moment have increased in sensitivity by six orders of magnitude. Yet, the current experimental sensitivity lies well above that predicted from standard model calculations. Corrections from physics beyond the standard model increase the predicted size of the nEDM, resulting in the nextgeneration of experiments hovering tantalisingly close to the detection sensitivity, to either verify or quash standard model extensions. One such experiment, n2EDM, currently in the commissioning stage at the Paul Scherrer Institute in Switzerland, hopes to measure the nEDM with a sensitivity improvement over its predecessor. This thesis details the design, measurement and analysis of hybrid 3He/alkali magnetometers for both accurate and precise magnetic field readings at low and zero fields, crucial for the development of ever-more sensitive fundamental physics searches. The work exploits the precise nature of alkali (87Rb and 133Cs) magnetometry techniques to measure the simultaneous Larmor precession of polarised 3He, avoiding light shifts commonly associated with optically pumped magnetometers. The first part of this work demonstrates the use of a hybrid 3He/87Rb magnetometer for the minimisation of remnant fields in a magnetically shielded environment, resulting in absolute low-field readings below 5 pT. Furthermore, the hybrid magnetometer was shown to be applicable for the reduction of gradient fields by optimising the effective 3He T2 time. This new technique represents a convenient and consistent way to achieve a near-zero magnetic field environment for precision experiments. Second, a new measurement technique was demonstrated using a hybrid 3He/133Cs magnetometer in a 1 µT field, placing experimental limits on the coupling of axion-like dark matter to nucleons. The measurement excluded axion coupling strength to 2:5 x 10-1 GeV-1 for axion masses between 10-20 to 10-17 eV at the 95% level. With the resulting exclusion region weaker than previous measurements, possible improvements to the apparatus should drive the development of a low-cost long-term axion interaction search.