Quantum vector magnetometry based on the Voigt effect

Magnetism plays a fundamental role in nature existing in almost every conceivable domain of the physical world ranging from stars and planets to our brains and hearts. Some magnetic phenomena are easy to observe and measure, whilst others are small and subtle making them almost impossible to detect. Developments in precision magnetometry, utilising the quantum nature of atoms has given access to the world of small and subtle enabling the understanding of such signals as well as pushing the boundaries of knowledge of the fundamental forces of nature. In this thesis we describe theoretical and experimental methods to dispersively detect all three vector components of an external magnetic field using radio-frequency dressed alkali atoms based on the Voigt effect. To achieve this, we measure the linear birefringence of the radio frequency dressed atomic medium via polarization homodyning. This gives rise to modulated polarization signals at the first and second harmonic of the dressing frequency. The vector components of the external magnetic field are mapped onto the quadratures of these harmonics. Our scheme requires only one frequency of modulation and has a simple single axis beam geometry making it advantageous for miniature magnetic field sensors. Furthermore, we extend our Voigt effect scheme to dressed microwave spectroscopy and show that it can be used to infer the prepared state populations paving the way towards partial quantum state tomography

A high performance active noise control system for magnetic fields

We present a system for active noise control of environmental magnetic fields based on a filtered-x least mean squares algorithm. The system consists of a sensor that detects the ambient field noise and an error sensor that measures the signal of interest contaminated with the noise. These signals are fed to an adaptive algorithm that constructs a physical anti-noise signal canceling the local magnetic field noise. The proposed system achieves a maximum of 35 dB root-mean-square noise suppression in the DC-1 kHz band and 55 and 50 dB amplitude suppression of 50 and 150 Hz AC line noise, respectively, for all three axial directions of the magnetic vector field

Floquet description of optically pumped magnetometers

We present theoretical description of Voigt and Faraday effect based optically pumped magnetometers using the Floquet expansion. Our analysis describes the spin-operator dynamics of the first, $\hat{F}(t)$, and second, $\hat{F}^2(t)$, order moments and takes into account of different pumping profiles and decoherence effects. We find that the theoretical results are in good agreement with the experimental demonstrations over a wide range of fields and pumping conditions. Finally, the theoretical analysis presented here is generalized and can be extended to different magnetometry schemes with arbitrary pumping profiles and multiple radio-frequency fields

Voigt-effect-based three-dimensional vector magnetometer

We describe a method to dispersively detect all three vector components of an external magnetic field using alkali atoms based on the Voigt effect. Our method relies on measuring the linear birefringence of the radio frequency dressed atomic medium via polarization homodyning. This gives rise to modulated polarization signals at the first and second harmonic of the dressing frequency. The vector components of the external magnetic field are mapped onto the quadratures of these harmonics. We find that our scheme can be utilised in both cold and hot atomic gases to detect such external fields in shielded and unshielded environments. In the shielded hot vapour case we achieve field sensitivities in the pT √ Hz range for all 3 vector components, using pump-probe cycles with 125 Hz repetition rate, and limited by the short coherence time of the cell. Finally, our scheme has a simple single axis beam geometry making it advantageous for miniature magnetic field sensors

Dispersive detection of radio-frequency-dressed states

We introduce amethod to dispersively detect alkali-metal atoms in radio-frequency-dressed states. In particular, we use dressed detection tomeasure populations and population differences of atoms prepared in their clock states. Linear birefringence of the atomic medium enables atom number detection via polarization homodyning, a form of common path interferometry. In order to achieve low technical noise levels, we perform optical sideband detection after adiabatic transformation of bare states into dressed states. The balanced homodyne signal then oscillates independently of field fluctuations at twice the dressing frequency, thus allowing for robust, phase-locked detection that circumvents low-frequency noise. Using probe pulses of two optical frequencies, we can detect both clock states simultaneously and obtain population difference as well as the total atom number. The scheme also allows for difference measurements by direct subtraction of the homodyne signals at the balanced detector, which should technically enable quantum noise limited measurements with prospects for the preparation of spin squeezed states. The method extends to other Zeeman sublevels and can be employed in a range of atomic clock schemes, atom interferometers, and other experiments using dressed atoms

Detection and Characterisation of Conductive Objects Using Electromagnetic Induction and a Fluxgate Magnetometer

Eddy currents induced in electrically conductive objects can be used to locate metallic objects as well as to assess the properties of materials non-destructively without physical contact. This technique is useful for material identification, such as measuring conductivity and for discriminating whether a sample is magnetic or non-magnetic. In this study, we carried out experiments and numerical simulations for the evaluation of conductive objects. We investigated the frequency dependence of the secondary magnetic field generated by induced eddy currents when a conductive object is placed in a primary oscillating magnetic field. According to electromagnetic theory, conductive objects have different responses at different frequencies. Using a table-top setup consisting of a fluxgate magnetometer and a primary coil generating a magnetic field with frequency up to 1 kHz, we were able to detect aluminium and steel cylinders using the principle of electromagnetic induction. The experimental results were compared to numerical simulations, with good overall agreement. This technique enables the identification and characterisation of objects using their electrical conductivity and magnetic permeability

III-V semiconductor waveguides for photonic functionality at 780 nm

Photonic integrated circuits based on III-V semiconductor polarization-maintaining waveguides were designed and fabricated for the first time for application in a compact cold-atom gravimeter1,2 at an operational wavelength of 780 nm. Compared with optical fiber-based components, semiconductor waveguides achieve very compact guiding of optical signals for both passive functions, such as splitting and recombining, and for active functions, such as switching or modulation. Quantum sensors, which have enhanced sensitivity to a physical parameter as a result of their quantum nature, can be made from quantum gases of ultra-cold atoms. A cloud of ultra-cold atoms may start to exhibit quantum-mechanical properties when it is trapped and cooled using laser cooling in a magneto-optical trap, to reach milli-Kelvin temperatures. The work presented here focuses on the design and fabrication of optical devices for a quantum sensor to measure the acceleration of gravity precisely and accurately. In this case the cloud of ultra-cold atoms consists of rubidium (87Rb) atoms and the sensor exploits the hyperfine structure of the D1 transition, from an outer electronic state of 5 2S ½ to 5 2P3/2 which has an energy of 1.589 eV or 780.241 nm. The short wavelength of operation of the devices dictated stringent requirements on the Molecular Beam Epitaxy (MBE) and device fabrication in terms of anisotropy and smoothness of plasma etch processes, cross-wafer uniformities and alignment tolerances. Initial measurements of the optical loss of the polarization-maintaining waveguide, assuming Fresnel reflection losses only at the facets, suggested a loss of 8 dB cm-1, a loss coefficient, α, of 1.9 (±0.3) cm-1

Towards rotation sensing with a single atomic clock

We discuss a scheme to implement a gyroscopic atom sensor with magnetically trapped ultra-cold atoms. Unlike standard light or matter wave Sagnac interferometers no free wave propagation is used. Interferometer operation is controlled only with static, radio-frequency and microwave magnetic fields, which removes the need for interferometric stability of optical laser beams. Due to the confinement of atoms, the scheme may allow the construction of small scale portable sensors. We discuss the main elements of the scheme and report on recent results and efforts towards its experimental realization

Quantum vector magnetometry based on the Voigt effect

Magnetism plays a fundamental role in nature existing in almost every conceivable domain of the physical world ranging from stars and planets to our brains and hearts. Some magnetic phenomena are easy to observe and measure, whilst others are small and subtle making them almost impossible to detect. Developments in precision magnetometry, utilising the quantum nature of atoms has given access to the world of small and subtle enabling the understanding of such signals as well as pushing the boundaries of knowledge of the fundamental forces of nature. In this thesis we describe theoretical and experimental methods to dispersively detect all three vector components of an external magnetic field using radio-frequency dressed alkali atoms based on the Voigt effect. To achieve this, we measure the linear birefringence of the radio frequency dressed atomic medium via polarization homodyning. This gives rise to modulated polarization signals at the first and second harmonic of the dressing frequency. The vector components of the external magnetic field are mapped onto the quadratures of these harmonics. Our scheme requires only one frequency of modulation and has a simple single axis beam geometry making it advantageous for miniature magnetic field sensors. Furthermore, we extend our Voigt effect scheme to dressed microwave spectroscopy and show that it can be used to infer the prepared state populations paving the way towards partial quantum state tomography