Development of a single beam SERF magnetometer using caesium atoms for medical applications

This thesis describes the design and implementation of a compact zero-field optically pumped magnetometer for human biomagetic measurements. This project aimed to achieve lower operating temperatures and a higher sensor bandwidth than current commercial rubidium-based equivalent sensors. Through careful selection of the sensing alkali, caesium, and all constituent components of the sensor package design, both of these aims are achieved. All of the required systems and components for a single-beam zero-field magnetometer are discussed, including a high efficiency cell heating and monitoring system, multi-axis field control and the optical detection scheme. Through full understanding and development of these systems, miniaturised and microfabricated versions are developed that facilitate the construction of a sensor package with external dimensions of 25 × 25 × 50 mm3. A number of machine learning tools are developed and applied to directly optimise the sensor’s sensitivity through control of the appropriate operational parameters, yielding a factor of five improvement. These techniques also enabled the investigation of the effect of nitrogen buffer gas pressure on the sensor’s measured sensitivity, demonstrating a linear increase in sensitivity with increasing pressure. The prototype sensor demonstrated a significant advancement in terms of bandwidth achieving a linear frequency response up to ' 900 Hz. The external package temperature of the sensor for prolonged timescales (> 1 hour) maintained a skin-safe temperature (< 41 ◦C), with a biomagnetic field level sensitivity, 90 fT/√ Hz, and compact package footprint, less than a square inch. A practical measurement of the magnetic field of a cardiac signal successfully demonstrates the sensor as a suitable biomagnetic measurement tool.This thesis describes the design and implementation of a compact zero-field optically pumped magnetometer for human biomagetic measurements. This project aimed to achieve lower operating temperatures and a higher sensor bandwidth than current commercial rubidium-based equivalent sensors. Through careful selection of the sensing alkali, caesium, and all constituent components of the sensor package design, both of these aims are achieved. All of the required systems and components for a single-beam zero-field magnetometer are discussed, including a high efficiency cell heating and monitoring system, multi-axis field control and the optical detection scheme. Through full understanding and development of these systems, miniaturised and microfabricated versions are developed that facilitate the construction of a sensor package with external dimensions of 25 × 25 × 50 mm3. A number of machine learning tools are developed and applied to directly optimise the sensor’s sensitivity through control of the appropriate operational parameters, yielding a factor of five improvement. These techniques also enabled the investigation of the effect of nitrogen buffer gas pressure on the sensor’s measured sensitivity, demonstrating a linear increase in sensitivity with increasing pressure. The prototype sensor demonstrated a significant advancement in terms of bandwidth achieving a linear frequency response up to ' 900 Hz. The external package temperature of the sensor for prolonged timescales (> 1 hour) maintained a skin-safe temperature (< 41 ◦C), with a biomagnetic field level sensitivity, 90 fT/√ Hz, and compact package footprint, less than a square inch. A practical measurement of the magnetic field of a cardiac signal successfully demonstrates the sensor as a suitable biomagnetic measurement tool

Squeezed-ligh-enhanced magnetometry in a high density atomic vapor

(English) This thesis describes experiments that employ squeezed light to improve the performance of a sensitive optically-pumped magnetometer (OPM). The squeezed light source employs parametric amplification of vacuum fluctuations to produce squeezed vacuum and polarization-squeezed light tunable around the Rb D1 line. The OPM employs Bell-Bloom optical pumping of a high density vapor (with atom number density 10^{13}) and paramagnetic Faraday rotation, also on the Rb D1 line. The setup allows convenient switching from probing with laser light to probing with polarization-squeezed light, to study the use of the latter in atomic magnetometry. The magnetometer shows sub-pT/Hz^{1/2} sensitivity, limited by quantum noise; spin projection noise at low frequencies (<100Hz) and photon shot noise at high frequencies. Probing with polarization squeezed light suppresses the photon shot noise by 2dB, limited by the available squeezing and optical losses in passing through the vapor. This shot-noise suppression improves the high-frequency sensitivity and increases the measurement bandwidth, with no observed loss of sensitivity at any frequency. This result confirms experimentally the expected evasion of measurement back-action noise in the Bell-Bloom magnetometer. The thesis also develops a physical model to explain the observed spin dynamics of the Bell-Bloom magnetometer. The model describes the combined spin and optical polarization dynamics using Bloch equations with stochastic drive and detection noise terms. A perturbative approach and Fourier methods are then used to obtain analytic expressions for the magnetometer's frequency response, spin projection noise and photon shot noise. The role of measurement back-action emerges from a study of this model. As polarization squeezing reduces optical noise in the detected Stokes parameter, the accompanying ellipticity anti-squeezing is shunted into the unmeasured spin component. The thesis also reports a study of squeezed-light-enhanced magnetometry at a range of atomic densities, from 2.18 10^{12} atoms/cm3 to 1.13 10^{13} atoms/cm3 . Operating with fixed conditions of optical pumping, the signal amplitude, instrument noise spectrum and magnetic resonance width are measured as a function of atomic number density, for both laser- and squeezed-light probing. The equivalent magnetic noise spectra are then calculated. In the photon-shot-noise-limited portion of the spectrum, the squeezed light probing improves the magnetometer's sensitivity and measurement bandwidth for the full range of atomic density values. In particular, the laser-probed magnetometer shows a sensitivity optimum at n ~ 6 10 ^{12} atoms/cm3, and the squeezed-light-probed magnetometer surpasses this sensitivity. The thesis concludes with a discussion of the potential of stronger optical squeezing to enhance the instrument's sensitivity in different portions of the spectrum. Using the theory model we estimate the enhancement of the equivalent magnetic noise spectrum for 2 dB , 5.6 dB and perfect squeezing (zero noise in the detected polarization component) at the input to the atomic medium.(Català) Aqueta tesi descriu la millora d’un magnetòmetre de bombeig òptic (OPM) mitjançant l’ús d’estats de llum amb incertesa comprimida (squeezed states). S’usa amplificació paramètrica per a comprimir la incertesa de la font de llum. En concret, es comprimeix la incertesa de l’estat de buit quàntic, com també de la polarització òptica, amb la possibilitar d’ajustar la longitud d’ona al voltant de la transició atòmica D1 de 87Rb. L’OPM usa bombeig òptic Bell-Bloom de vapors d’alta densitat (amb densitats atòmiques properes a 1013) i rotació de Faraday, també al voltant de la transició atòmica D1 de 87Rb. L’aparell experimental permet canviar de mostreig amb llum coherent làser a mostreig amb llum de polarització comprimida, amb la finalitat d’avaluar el seu impacte en la sensitivitat del magnetòmetre. El magnetòmetre té una sensitivitat de sub-pT{ ?Hz , principalment limitada per soroll quàntic; soroll de projecció de spin a baixes freqüències (À 100 Hz) i soroll de quantització fotònica a altes freqüències. L’ús d’estats de llum amb polarització comprimida permet reduir el soroll fotònic en „ 2 dB, limitat per la compressió disponible i les pèrdues en travessar el vapor atòmic. La supressió del soroll fotònic augmenta l’amplada de banda del sistema amb l’avantatge de no perdre sensitivitat a cap banda de freqüència. Els resultats experimentals confirmen l’esperada supressió de retroalimentació de soroll en magnetòmetres de Bell-Bloom. La tesi també estudia el model teòric darrere les dinàmiques de spin en un magnetòmetre de tipus Bell-Bloom. El model descriu la combinació de les dinàmiques de spin i de la polarització òptica mitjançant equacions de Bloch forcades estocàsticament i amb termes de soroll de detecció. Es treballa en el límit pertorbatiu on mitjançant mètodes de Fourier s’obtenen expressions analítiques de la resposta en freqüència del magnetòmetre, dels sorolls de projecció de spin i del soroll de quantització fotònica. El rol de la retroalimentació de soroll també s’extrau d’aquest model. En concret, s’observa que la compressió en polarització redueix el soroll en els paràmetres de Stokes detectats, mentre els paràmetres de spin no mesurats experimenten una expansió de la seva incertesa (anti-squeezing). La tesi estudia magnetòmetres òptics de llum amb incertesa comprimida per a densitats entre 2.18 ˆ 1012 atoms{cm3 i 1.13 ˆ 1013 atoms{cm3. Es mesuren l’amplitud de senyal, l’espectre de soroll i l’amplada de la ressonància magnètica en funció de la densitat atòmica, per a un bombeig òptic constant i per a ambdós tipus de mostreig òptic (llum coherent i llum de polarització comprimida). A continuació, es calculen els espectres de soroll equivalents. En la part d’espectre on domina el soroll de quantització fotònica, s’observa que l’ús de llum de polarització comprimida millora la sensitivitat del magnetòmetre al llarg de tot el rang de densitats atòmics. En concret, la sensitivitat del magnetòmetre amb mostreig coherent és òptima per a n « 6ˆ1012 atoms{cm3 i es demostra una millora amb l’ús de mostreig amb llum comprimida. Es conclou amb una discussió sobre l’efecte de compressions més severes en la sensitivitat del magnetòmetre. Mitjançant el model teòric s’estima la millora en la sensitivitat per a compressions de 2 dB, 5.6 dB i “compressió perfecta” a l’entrada del medi atòmicPostprint (published version

Radio-frequency atomic magnetometry with a rubidium Bose-Einstein condensate

This thesis details progress in radio-frequency atomic magnetometry with ultracold rubidium atoms. Motivations and context are first covered, before an introduction of the main concepts required to understand the underlying physics is given. At first, a cold atom magnetometer is designed, built and characterised. Consistent 20 µK atoms are produced. Radio-frequency (RF) atomic magnetometry (AM) is performed by placing the atoms in a bias magnetic field and generating coherent precession with an external AC field. A noise floor at 330 pT/√Hz defines the sensor’s sensitivity, with a range of applications. RF-AM is then performed with a Bose-Einstein condensate (BEC). The 20 µK atoms are loaded into a magnetic trap, where RF evaporation increases their phase space density (PSD = nλ^3dB, n is the density and λdB is the thermal de Broglie wavelength of the atoms). Next, atoms are transferred into a hybrid dipole trap, collecting in a dimple created at the intersect of two high power laser beams. Production and stabilisation of these beams is described, which are focused down to a 75 µm beam waist at the trap position with a total power of 7 W. Optimisation of the evaporation process in both traps leads to consistent BEC production. A pure condensate with 4x10^4 atoms at 25 nK is reported. Radio-frequency magnetometry is performed at various probe volumes. With systematic optimisation a best AC sensitivity of 24 pT/√Hz with 3.4 × 10^8 atoms in the magnetic trap before evaporation is achieved. This is extended to the BEC with 4 × 10^4 atoms, where an AC sensitivity of 84 nT/√Hz and DC sensitivity of 14 nT/√Hz is reported, bringing previously achieved atomic magnetometry into the micrometer regime. A trade-off must be considered due to reduction in sensitivity at lower probe volumes. Volumes between 1.4×10−7 m^3 and 1.6×10−14 m^3 can be accessed, highlighting the sensors adaptability and tunability for different applications. The results are contextualised in the background of previously achieved magnetometers of various types. Finally, proof-of-concept electromagnetic induction imaging (EMI) measurements are made to confirm the sensor’s viability for high resolution imaging

Quantum Sensors for Electromagnetic Induction Imaging: from Atomic Vapours to Bose-Einstein Condensates

In this thesis, two sensors for electromagnetic induction imaging (EMI) are presented based on radio-frequency atomic magnetometry (RF-AM) in alkali atoms. The first sensor addresses portability and real-world use of EMI with AMs, by housing the major components of the RF-AM within a lightweight, minaturised system that can be mechanically translated. The atomic source was provided by a thermal vapour of 87Rb and was pumped/probed on the D1 line. The performance of the sensor is detailed and an RF sensitivity of dBAC = 19pT/√Hz was achieved. Stability of the device was investigated and potential improvements to the design are discussed. EMI with the sensor is then tested by application to two real-world industrial problems. Through-skin pilot-hole detection in Al strut-skin arrangements and corrosion detection under thermal/electrical insulation. The mechanically translatable RF-AM was able to detect and localise pilot-holes of diameter 16 mm concealed by an Al skin of thickness 0.41 mm with sub-mm precision. For corrosion detection, localisation and depth detection of recesses in an Al plate was achieved when concealed with a 1.5 mm thick piece of rubber acting as an electrical/thermal insulator. The sensor demonstrates key advantages over existing solutions to these challenges in a package that is within the reach of real-world deployment. The second sensor addresses the spatial resolution limitations of thermal vapours, by instead utilising ultra-cold atoms trapped in a tight optical potential, as the atomic source for the RF-AM. Initially an existing 87Rb BEC setup is optimised and characterised. A BEC of 65k atoms is produced via optical evaporation with a final volume of 3.2×10^−8 cm^−3. The BEC RF sensitivity is measured to be 268pT/√Hz with a volumetric sensitivity of 50.2fT/(cm3/Hz). The BEC RF-AM is found not to be limited by the atomic projection noise and a strategy for further improvements is discussed

Neutron Polarimetry with Polarized 3He for the NPDGamma Experiment

Cold neutrons enable the study of the fundamental interactions of matter in low-energy, low-background experiments that complement the efforts of high-energy particle accelerators. Neutrons possess an intrinsic spin, and the polarization of a beam of neutrons defines the degree to which their spins are oriented in a given direction. The NPDGamma experiment uses a polarized beam of cold neutrons to make a high precision measurement, on the order of one part in 100 million, of the parity-violating asymmetry in the angular distribution of emitted gamma-rays from the capture of polarized neutrons on protons. This asymmetry is a result of the hadronic weak interaction (HWI) and is directly proportional to the long-range, weak interaction modeled by the exchange of a pion between two nucleons. The results of the NPDGamma experiment are dependent on the polarization of the neutron beam used in the capture reaction. The neutron polarization is measured using the large spin-dependent neutron capture cross section of polarized 3He to a precision of less than 2%, which does not significantly increase the total error of the measured gamma-ray asymmetry. Reported here is a description of the NPDGamma experiment, the work done to polarize 3He, and the results of the neutron beam polarimetry measurements

A Measurement of Neutron Polarization and Transmission for the nEDM@SNS Experiment

The D.O.E Nuclear Science Advisory Committee Long Range Plan has called for experimental programs to explore fundamental symmetry violations and their implications in nuclear, particle and cosmological physics. The neutron electric dipole moment experiment at the Spallation Neutron Source (nEDM@SNS) aims to search for new physics in the Time-reversal (T) and Charge-Parity (CP) symmetry violating sector by setting a new limit on the nEDM down to a few x 10-28 e·cm using a novel cryogenic technique, which combines the unique properties of polarized Ultracold Neutrons (UCN), polarized 3He, and superfluid 4He. The experiment will employ a cryogenic magnet and magnetic shielding package to provide the magnetic field environment required to achieve the proposed sensitivity. This dissertation describes the design and implementation of a 3He based neutron polarimetry setup at the SNS to measure the monochromatic neutron polarization and transmission losses resulting from passage through the magnetic shielding and cryogenic windows. Results from monochromatic neutron polarization and transmission measurements will be presented

Radio frequency atomic magnetometer for applications in magnetic induction tomography

No abstract available.No abstract available

Atomic magnetometry for nuclear threat reduction applications

The work within this thesis explores the applicability of atomic magnetometry to nuclear threat reduction applications. The scope of nuclear threat reduction is explored in the context of UK Government strategy with research conducted at the Atomic Weapons Establishment. An application space is defined which includes nuclear treaty verification, nuclear forensics and detection science. Within these areas, the requirement to detect shielded nuclear and radiological materials is established and eddy current induction methods are proposed to meet the detection of a sub-set of these materials. Eddy current induction measurements are conventionally met using coil based technologies, however, the sensitivities of these systems increase as a function frequency. Eddy current induction is a frequency dependent measurement, where penetration through an object is reduced at larger values due to the skin-depth effect. These factors combine to be counterproductive for the measurement of objects in thickly shielded configurations. Atomic magnetometers, which measure magnetic fields through the inference of Zeeman splitting within alkali metal vapours, are proposed as an alternative to coil based sensors. Atomic magnetometer technologies benefit from high sensitivities which can be achieved across broad range of frequencies. Two systems are explored within this work, the first focusses on the detection of targets in challenging shielding configurations. These include through high conductivity (aluminium), ferrous (steel) and high density (lead) materials which necessitated low frequency measurements and were achieved using a commercially available atomic magnetometer. The second system examines the detection of smaller unshielded objects with a focus on tuning the excitation frequency to higher values. Tuning the frequency enabled the maximisation of the magnetic field phase response of nuclear materials such as uranium and plutonium. This was achieved by constructing a radio-frequency atomic magnetometer. In the low frequency regime, an imaging system was constructed that allowed raster scan images of high conductivity materials to be obtained behind aluminium shielding up to 63 mm thick at distances > 200 mm. These same objects were also imaged behind steel plates up to 12 mm thick and lead shielding 100 mm thick, where these values represent the maximum thickness tested. These high conductivity objects are of interest for the confirmation of declarations made within treaty verification or for the detection of illicit items in bag/personnel scanning. Lower conductivity materials such as conductivity surrogates for uranium were imaged behind 35 mm of aluminium and the aforementioned material thickness values for steel and lead. In the high frequency regime, a rubidium based radio-frequency magnetometer was constructed, optimised and integrated into an eddy current imaging system. Images of the lower conductivity plutonium surrogate were obtained at frequencies above 20 kHz. Additional work was identified across both the magnetometer and the imaging system to allow the full sensitivity of the system to be exploited. Outside of the imaging function of the system, the frequency of the eddy current excitation field was successfully tuned to enhance the measurement of the uranium and plutonium surrogates over higher conductivity samples such as copper and aluminium

Two pulse per cavity lasers and their applications

Lasers have been around now for over 50 years. Initially it was said that it is an invention without an application. Since then it turned out that lasers are valuable tools for applications like mapping atomic transitions or interferometric measurements. The research presented here will show one type of lasers, a two-pulse-per cavity mode-locked laser, its applications and analyze its practical and theoretical limits. An analysis of a two pulse per cavity mode-locked laser is presented as analogy to a quantum mechanical two level system. It can be shown that our lasers with two independent pulses propagating inside the cavity can be coupled via a scattering medium and will behave exactly the same way as a two level atom driven by a step function electric field. This analogy is providing a new insight into the dynamics of two-pulse coupling in mode-locked ring lasers. The most important application of a mode-locked laser with two intracavity pulses is Intra-cavity Phase Interferometry (IPI). The unique feature of mode-locked lasers, where a pulse to pulse phase shift is converted into a frequency, can be used for very sensitive phase measurements that outperform standard interferometers. Two pulse trains with the same repetition rate from the same cavity and a different phase shift will show a beatnote signal directly proportional to this phase shift when interfered on a detector. It leads to an good signal strength since all modes of the frequency comb contribute to the signal. In this dissertation, I will describe the advancement of IPI with our standard Ti:Sapphire cavities to the measurement of magnetic fields. Also another cavity, Optical Parametric oscillators (OPO) can be used for IPI. I will be showing two different implementations, intra-cavity pumped and extra-cavity pumped and their usefulness to phase interferometry by presenting a new method to measure the non-linear index of a material. In the end, a discussion about the theoretical limit of the sensitivity and resolution is provided for both, the Ti:Sapphire and OPO system