Magnetocardiography in unshielded environment based on optical magnetometry and adaptive noise cancellation

This thesis proposes and demonstrates the concept of a magnetocardiographic system employing an array of optically-pumped quantum magnetometers and an adaptive noise cancellation for heart magnetic field measurement within a magnetically-unshielded environment. Optically-pumped quantum magnetometers are based on the use of the atomic-spin-dependent optical properties of an atomic medium. An Mxconfiguration- based optically-pumped quantum magnetometer employing two sensing cells containing caesium vapour is theoretically described and experimentally developed, and the dependence of its sensitivity and frequency bandwidth upon the light power and the alkali vapour temperature is experimentally demonstrated. Furthermore, the capability of the developed magnetometer of measuring very weak magnetic fields is experimentally demonstrated in a magnetically-unshielded environment. The adaptive noise canceller is based on standard Least-Mean-Squares (LMS) algorithms and on two heuristic optimization techniques, namely, Genetic Algorithms (GA) and Particle Swarm Optimization (PSO). The use of these algorithms is investigated for suppressing the power line generated 50Hz interference and recovering of the weak magnetic heart signals from a much higher electromagnetic environmental noise. Experimental results show that all the algorithms can extract a weak heart signal from a much-stronger magnetic noise, detect the P, QRS, and T heart features and highly suppress the common power line noise component at 50 Hz. Moreover, adaptive noise cancellation based on heuristic algorithms is shown to be more efficient than adaptive noise canceller based on standard or normalised LMS algorithm in heart features detection

Electromagnetic Induction Imaging with Atomic Magnetometers

Electromagnetic induction imaging (EMI) is a technique for non-invasively mapping the passive electromagnetic properties of materials. It involves the active probing of samples with a radio-frequency magnetic field and recording the details of the magnetic field produced by the induced eddy current response. The performance of an EMI system is ultimately determined by the choice of magnetic field sensor used in the measurement. The sensor’s sensitivity, range of operation frequency, and sensing volume are all crucial characteristics when considering the imaging platform’s capabilities. Atomic magnetometers (AMs) – based on the coherent precession of a polarised alkali atomic vapour – currently rate amongst the most sensitive devices for magnetic field measurements. Radio-frequency atomic magnetometers (RF-AMs) are ultra-sensitive detectors of oscillating magnetic fields across a broad range of frequencies. As such, they are ideally suited to EMI applications. This work presents the development of EMI systems based on RF-AMs. The imaging performance and a wide range of applications are experimentally demonstrated. The continuous development of a single-channel rubidium RF-AM is described. The final device operates in unshielded environments and near room temperature with a measured sensitivity of 55 fT/√Hz, a photon-shot noise limit of 10 fT/√Hz, and a linewidth of 36 Hz. Tunability of the device is proven by consistent, narrow-linewidth operation across the kHz – MHz band – operating in magnetic fields significantly greater than previous AM designs. The sensor was developed with a small effective sensor volume, which increases the spatial resolution of the imaging. High-resolution EMI is performed across a broad range of materials. This spans the first EMI images with an RF-AM at 6x107 S/m to low-conductivity, non-metallic samples at 500 S/m. Typically, sample volumes are of a few cm3 and with an imaging resolution around 1 mm. These numbers make EMI with AMs (EMI-AM) suitable for numerous applications. Techniques – including multi-frequency image analysis – are employed to discriminate sample properties. Further work developed novel image reconstruction approaches – based on machine learning – to maximise the amount of information that can be extracted from EMI images. Finally, the potential of biomedical imaging is discussed and its feasibility verified by simulating the application of EMI-AM to imaging the conductivity of the heart

Chip-scale atomic magnetometer based on free-induction-decay

This thesis describes the implementation of an optically pumped caesium magnetometer containing a 1:5mm thick microfabricated vapour cell with nitrogen buffer gas, operating in a free-induction-decay configuration. This allows us to monitor the free Larmor precession of the spin coherent Cs atoms by separating the pump and probe phases in the time domain. A single light pulse can sufficiently polarise the atomic sample;however, synchronous modulation of the light field actively drives the precession and maximises the induced spin coherence. Both amplitude- and frequency-modulation have been adopted producing noise floors of 3.4 pT / √Hz and 15.6 pT/√Hz, respectively,within a Nyquist limited bandwidth of 500 Hz in a bias field comparable to the Earth's (~50 μT). We investigate the magnetometers capability in reproducing time-varying magnetic signals under these conditions, including the reconstruction of a 100 pT perturbation using signal averaging.Additionally, we discuss a novel detection mode based on free-induction-decay that observes the spin precession dynamics in-the-dark using Ramsey-like pulses. This aids in suppressing the systematic effects originating from the light-atom interaction during readout, thus vastly improving the accuracy of the magnetometer whilst maintaining a sensitivity that is competitive with previous implementations. This detection technique was implemented further to measure the spin relaxation properties intrinsic to the sensor head, useful in determining the optimal buffer pressure that extends the spin lifetime and improves the sensor's sensitivity performance.This thesis describes the implementation of an optically pumped caesium magnetometer containing a 1:5mm thick microfabricated vapour cell with nitrogen buffer gas, operating in a free-induction-decay configuration. This allows us to monitor the free Larmor precession of the spin coherent Cs atoms by separating the pump and probe phases in the time domain. A single light pulse can sufficiently polarise the atomic sample;however, synchronous modulation of the light field actively drives the precession and maximises the induced spin coherence. Both amplitude- and frequency-modulation have been adopted producing noise floors of 3.4 pT / √Hz and 15.6 pT/√Hz, respectively,within a Nyquist limited bandwidth of 500 Hz in a bias field comparable to the Earth's (~50 μT). We investigate the magnetometers capability in reproducing time-varying magnetic signals under these conditions, including the reconstruction of a 100 pT perturbation using signal averaging.Additionally, we discuss a novel detection mode based on free-induction-decay that observes the spin precession dynamics in-the-dark using Ramsey-like pulses. This aids in suppressing the systematic effects originating from the light-atom interaction during readout, thus vastly improving the accuracy of the magnetometer whilst maintaining a sensitivity that is competitive with previous implementations. This detection technique was implemented further to measure the spin relaxation properties intrinsic to the sensor head, useful in determining the optimal buffer pressure that extends the spin lifetime and improves the sensor's sensitivity performance

Quantum metrology with nonclassical states of atomic ensembles

Quantum technologies exploit entanglement to revolutionize computing, measurements, and communications. This has stimulated the research in different areas of physics to engineer and manipulate fragile many-particle entangled states. Progress has been particularly rapid for atoms. Thanks to the large and tunable nonlinearities and the well developed techniques for trapping, controlling and counting, many groundbreaking experiments have demonstrated the generation of entangled states of trapped ions, cold and ultracold gases of neutral atoms. Moreover, atoms can couple strongly to external forces and light fields, which makes them ideal for ultra-precise sensing and time keeping. All these factors call for generating non-classical atomic states designed for phase estimation in atomic clocks and atom interferometers, exploiting many-body entanglement to increase the sensitivity of precision measurements. The goal of this article is to review and illustrate the theory and the experiments with atomic ensembles that have demonstrated many-particle entanglement and quantum-enhanced metrology.Comment: 76 pages, 40 figures, 1 table, 603 references. Some figures bitmapped at 300 dpi to reduce file siz

Investigations of weak and dilute magnetic behaviour in organic and inorganic systems

Muon spin relaxation is an ideal tool with which to study dilute magnetic systems, coupled with tailored bulk magnetic susceptibility measurements it is possible to examine previously unobserved magnetic exchange interactions. Investigations into non-stoichiometric LaCo03 reveals evidence of magnetic excitons in transition metal oxides for the first time. Moreover, data is presented that supports the concept of the defect driven excitons interacting with the stoichiometric LaCo03 which is known to undergo a thermally driven spin state transition. The data suggest the occurrence of more than one possible magnetic interaction of the excitons. Hole doped La1-xSrxCo03 is of interest as it is known to be magnetically and electronically phase separated; by a direct analogy with magnetic excitons it is suggested that the Sr rich ferromagnetic clusters interact with the pure LaCo03 below the metal insulator transition (x = 0.18 ) . It is suggested that it is this interaction observed for the first time that enables the rich phase diagram of La1_xSrxCo03 . The persistent photoconductivity effect on the spin glass transition in the elilute magnetic semiconductor Ccl0.85Mn0.15 Te:In has been investigated using low temperature magnetic susceptibility measurements and for the first time muon spectroscopy. Muon measurements on an Al eloped sample clearly show the spin glass transition, however the presence of the DX centre, which causes PPC when doping with In donors, perturbs the muon response. Particular attention is paid to possibility of the DX centre trapping muonium and preventing the detection of the spin glass transition. PPC does not induce a change in the muon response, however continuous illumination of the sample allows the observation of the spin glass transition, suggesting the presence of multiple DX centres, moreover the centre is found to be diamagnetic. The search for magnetic ordering at room temperature in an organic material has generally neglected polymers. PANiCNQ combines a fully conjugated nitrogen containing backbone with molecular charge transfer side groups. This combination gives rise to a stable polymer with a high density of localised spins, which are expected to give rise to coupling. Magnetic measurements suggest that the polymer is ferri- or ferro- magnetic with a Curie temperature of over 350 K, and a maximum saturation magnetization of 0.1 JT-1 kg-1 . Magnetic force microscopy images support this picture of room temperature magnetic order by providing evidence for domain wall formation and motion

Bio-magnetic imaging using highly sensitive quantum magnetometers

This thesis describes the experimental results of highly sensitive quantum magnetometers, known as optically pumped magnetometers (OPMs), for bio-magnetic measurements. First, I show that commercially available OPMs can achieve enhanced spatio-temporal resolution compared to superconducting quantum interference devices (SQUIDs) when recording visual-evoked fields, using a standard visual paradigm. The improved resolution enabled the observation of novel neuronal findings in healthy participants. As OPMs were not originally developed for this neurophysiological application, the technology is not optimised for it, and the product design limits the final performance. In our lab, we developed a modular OPM sensor to surpass these limitations in terms of bandwidth and scalability. I tested the performance of this prototype for bio-magnetic measurements, and validated its use through a series of alpha-rhythm experiments over the visual cortex. Finally, I designed an experiment to explore the possibility of measuring the spinal cord evoked response using both the modular and the commercially available OPMs. Using peripheral nerve stimulations, I demonstrate the importance of the broader bandwidth for spinal cord measurements compared to the brain ones, and thus highlight the suitability of the modular OPM sensor for more sensitive measurements. Preliminary in-vivo spinal cord data using the commercial OPMs show simultaneous evoked fields of the brain and spinal cord

Ultrafast Laser-induced Magnetisation Dynamics: Gilbert damping of Metal and Half-metal

In this thesis, the magnetic damping in different metals and half-metals is studied using an all-optical pump-probe method: the time-resolved magneto-optic Kerr effect (TR-MOKE), combined with several advanced characterization techniques, such as angle-resolved photoemission spectroscopy (APRES) and X-ray magnetic circular dichroism (XMCD). In Fe/Cr/GaAs heterostructures, the uniaxial magnetic anisotropy (UMA) and the magnetic damping have been studied as a function of the thickness of Cr interlayer. The UMA is attributed to the Fe-GaAs chemical bonding at the interface via increasing the orbit moments of Fe atoms. The Cr interlayer with increasing thickness blocks the Fe-GaAs bonding and the UMA gradually. The magnetic damping shows a dramatic drop when Cr interlayer is deposited, even with only 0.5 ML. The results have indicated that the UMA and the damping originate from different mechanisms although the Fe/GaAs interface plays an important role in both phenomena. The correlation between magnetic damping and the electronic structures have been investigated in Co2FeAl Heusler alloy. By varying the growth temperature, both the magnetic damping and the electronic structure have been found affected significantly. It is experimentally demonstrated that the low damping constant originates from a low density of state (DOS) at Fermi level. A “shoulder-like” peak in the vicinity of Fermi level in the energy distribution curve would enhance the DOS and the damping. A pair of half-metallic Fe3O4 films, with and without structural defects, have been compared in order to investigate the effect of defects in magnetic damping. The enhancement of the intrinsic damping 0.063 in defect-free Fe3O4 sample is attributed to the emergence of the perpendicular standing spin wave (PSSW), while the magnetic damping in the as-grown sample 0.039 is lower because the defects would scatter the spin wave or magnon and the energy transfer channel from the uniform precession to the PSSW does not exist