We demonstrate the use of a hybrid 3He/87 magnetometer to measure absolute magnetic fields in the pT range. The measurements were undertaken by probing time-dependent 3He magnetisation using 87Rb zero-field magnetometers. Measurements were taken to demonstrate the use of the magnetometer in cancelling residual fields within a magnetic shield. It was shown that the absolute field could be reduced to the 10 pT level by using field readings from the magnetometer. Furthermore, the hybrid magnetometer was shown to be applicable for the reduction of gradient fields by optimising the effective 3He T2 time. This procedure represents a convenient and consistent way to provide a near zero magnetic field environment which can be potentially used as a base for generating desired magnetic field configurations for use in precision measurements.
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.
Developed and characterised for the first time is a quantum vector magnetometer using coherent population trapping (CPT) in 87Rb vapour and external magnetic field compensation by 3-axis Helmholtz coils to stabilize the amplitude and spectral position of the CPT resonance excited on a magneto-sensitive (mf ≠ 0) energy level transition. To measure the magnetic field components orthogonal to the magnetometer optical axis, resonance amplitude dependence upon the transverse field strength is used, which has a maximum at zero transverse field. The proposed approach allowed measurement of the external magnetic field vector with a sensitivity exceeding 500 pT/Hz1/2.
Maintaining a near-zero magnetic field condition is a requisite for atomic magnetometer under the spin-exchange relaxation-free (SERF) regime. Therefore, the precise determination of the residual magnetic fields is of great significance. Herein, we proposed an in-situ measurement method of residual magnetic fields in a dual-beam atomic magnetometer based on a transient response of the transverse polarization along the probe beam direction after switching off the pump beam. Proceeding from the Bloch equation, we established a complete evolution expression of the polarization vector under any static residual magnetic field with the pump beam switched off. On this basis, we analyzed the transient response of the transverse polarization by numerical simulation under different situations of residual magnetic fields. Besides, we also considered two possible conditions when the residual magnetic fields are relatively large or small, respectively. Accordingly, we presented an efficient and convenient estimation method of the residual magnetic fields based on certain time characteristics of the transient response. We verified the feasibility of our method by experiment and achieved a triaxial residual magnetic fields measurement within 0.5 s. This method can realize an in situ and real-time measurement of the residual magnetic fields in a fast and convenient way. Furthermore, this method can evaluate the effectiveness of different magnetic field compensation methods without introducing any modulation or extra devices.
Compact atomic magnetometer has become an important approach for noise measurement of magnetic shielding systems as well as biomagnetic measurement. In this study, we propose a novel method that enables time-shard triaxial magnetic field measurements in near-zero magnetic field through a compact high-performance atomic magnetometer with femtotesla sensitivity. With pump–probe orthogonal configuration, the magnetometer can be operated without optical and magnetic modulation. They-axis magnetic field measurement was realized in zero field, whereas thex- andz- axes magnetic fields were measured by the cross-application of the bias magnetic fields operating in near-zero field. The maximum response amplitude and sensitivity with bandwidths of 46.8 Hz (x- axis), 10.5 Hz (y-axis), and 10.9 Hz (z-axis) are achieved under bias magnetic fields of
Bz−offset=8nT and
Bx−offset=2nT. The bandwidth alongx- axis was extended to 232 Hz under
Bz−offset=32nT, which can further be improved by increasing
Bz−offset. The linear interval of measurable magnetic field are between −11.7 to +11.7 nT (x- axis), −0.8 to +0.8 nT (y-axis), and −28.2 to +28.2 nT (z-axis). Forx- andz-axes, the measurable interval can further be improved by increasing
Bz−offsetand
Bx−offset, respectively. The triaxial sensitivities achieved through our method reached 3.1 ± 0.3 fT/Hz1/2(x- axis), 2.0 ± 0.1 fT/Hz1/2(y- axis), and 6.0 ± 0.5 fT/Hz1/2(z- axis). This system is measured in a low-noise ferrite magnetic shield to ensure extra-low magnetic noise condition. Furthermore, the non-modulation triaxial vector operation demonstrated in this study is advantageous for reducing interference and spin-exchange relaxation caused by magnetic field modulation.
We describe the development of a simple atomic magnetometer using 87 Rb vapour suitable for monitoring Earth's magnetic field. The magnetometer is based on time-domain determination of the transient precession frequency of the atomic alignment around the measured field. A sensitivity of 1.5 nT Hz −1/2 is demonstrated by the measurement of the Earth's magnetic field in the laboratory, while for a simulated Earth field inside a magnetic shield a 130 pT Hz −1/2 sensitivity was attained. We discuss the various parameters determining the magnetometer precision and accuracy and predict a sensitivity of 30 pT Hz −1/2 .