Optisch gepumpte Magnetometer mit reduzierter Spin-Austausch-Relaxation im Erdmagnetfeld

Optisch gepumpte Magnetometer (OPM) nutzen den Zeeman-Effekt in (Alkali-)Atomen aus, welcher mittels Elektronenspinresonanz (ESR) in Doppelresonanztechnik optisch detektiert wird (ODMR). Dadurch werden extrem sensitive Magnetfeldsensoren möglich, deren Empfindlichkeit mit denen von supraleitenden Quanteninferenzdetektoren (SQUID) konkurrieren kann, jedoch ohne Kryogenik auskommt. Im Rahmen dieser Arbeit wurde ein neuer Betriebsmodus optisch gepumpter Magnetometer vorgestellt und detailliert untersucht - das Light-Narrowing-Mx-Magnetometer. Es basiert auf selektivem optischem Pumpen der Hyperfeinstruktur des Cäsiumatoms mittels wohlabgestimmten, intensiven Laserlichtes. Die Überlappung der optischen Hyperfeinübergänge der Cäsium-D1-Linie in einer mit Puffergas gefüllten, miniaturisierten Zelle wird genutzt, um unter Verwendung nur eines Lasers eine Besetzungsverteilung im Grundzustand des Ensembles der Cäsium-Atome zu generieren, welche zu einer markanten Verbesserung der Empfindlichkeit des Sensors führt. Diese Verbesserung im neuen LN-Regime gegenüber dem gewöhnlichen Betriebsmodus resultiert einerseits aus der Vergrößerung des magnetischen Resonanzsignals durch Nutzung fast aller Atome des Ensembles für die Signalgeneration und andererseits - vermöge der Reduktion der Leistungsverbreiterung durch das Laserlicht und der Unterdrückung der Spin-Austausch-Relaxation - aus der Verkleinerung der Resonanzbreite. Anwendungen des Sensors sind die hochempfindliche, mobile Vermessung des Erdmagnetfeldes für Fragestellungen der Geophysik, Rohstoffexploration bzw. der archäologischen Prospektion, die medizinische Diagnostik, wie zum Beispiel die Magnetokardiographie (MKG), oder die Detektion magnetischer Bakterien in Lösungen. Auch die zukünftige Verwendung zur Verfolgung magnetisch markierter Wirkstoffe innerhalb eines lebenden Organismus, bei der Magnetorelaxometrie von Nanopartikeln (MNP) oder in der Niederfeld-Kernspinresonanzspektroskopie (ULF-NMR) ist vorstellbar

An optically pumped magnetometer working in the light-shift dispersed Mz mode

We present an optically pumped magnetometer working in a new operational mode— the light-shift dispersed Mz (LSD-Mz) mode. It is realized combining various features; (1) high power off-resonant optical pumping; (2) Mz configuration, where pumping light and magnetic field of interest are oriented parallel to each other; (3) use of small alkali metal vapor cells of identical properties in integrated array structures, where two such cells are pumped by circularly polarized light of opposite helicity; and (4) subtraction of the Mz signals of these two cells. The LSD-Mz magnetometer’s performance depends on the inherent and very complex interplay of input parameters. In order to find the configuration of optimal magnetometer resolution, a sensitivity analysis of the input parameters by means of Latin Hypercube Sampling was carried out. The resulting datasets of the multi-dimensional parameter space exploration were assessed by a subsequent physically reasonable interpretation. Finally, the best shot-noise limited magnetic field resolution was determined within that parameter space. As the result, using two 50 mm3 integrated vapor cells a magnetic field resolution below 10 fT/√Hz at Earth’s magnetic field strength is possible

Suppression of spin-exchange relaxation in tilted magnetic fields within the geophysical range

We present a detailed experimental and theoretical study on the relaxation of spin coherence due to the spin-exchange mechanism arising in the electronic ground states of alkali-metal vapor atoms. As opposed to the well-explored formation of a stretched state in a longitudinal geometry (magnetic field parallel to the laser propagation direction) we employ adapted hyperfine-selective optical pumping in order to suppress spin-exchange relaxation. By comparing measurements of the intrinsic relaxation rate of the spin coherence in the ground state of cesium atoms with detailed density-matrix simulations we show that the relaxation due to spin-exchange collisions can be reduced substantially even in a tilted magnetic field of geomagnetic strength, the major application case of scalar magnetic surveying. This explains the observed striking improvement in sensitivity and further deepens the understanding of the light- narrowed Mx magnetometer, which was presented recently. Additionally, new avenues for investigating the dynamics in alkali-metal atoms governed by the spin-exchange interaction and interacting with arbitrary external fields open up

Quantitative study of optical pumping in the presence of spin-exchange relaxation

We have performed quantitative measurements of the variation of the on-resonance absorption coefficients κ0 of the four hyperfine components of the Cs D1 transition as a function of laser power P, for pumping with linearly and with circularly polarized light. Sublevel populations derived from rate equations assuming isotropic population relaxation (at a rate γ1) yield algebraic κ0(P) dependences that do not reproduce the experimental findings from Cs vapor in a paraffin-coated cell. However, numerical results that consider spin-exchange relaxation (at a rate γse) and isotropic relaxation fit the experimental data perfectly well. The fit parameters, viz., the absolute value of κ0, the optical pumping saturation power Psat, and the ratio γse/γ1, are well described by the experimental conditions and yield absolute values for γ1 and γse. The latter is consistent with the previously published Cs-Cs spin-exchange relaxation cross section

OPM magnetorelaxometry in the presence of a DC bias field

Spatial quantitative information about magnetic nanoparticle (MNP) distributions is a prerequisite for biomedical applications like magnetic hyperthermia and magnetic drug targeting. This information can be gathered by means of magnetorelaxometry (MRX) imaging, where the relaxation of previously aligned MNP’s magnetic moments is measured by sensitive magnetometers and an inverse problem is solved. To remove or minimize the magnetic shielding in which MRX imaging is carried out today, the knowledge of the influence of background magnetic fields on the MNP’s relaxation is a prerequisite. We show MRX measurements using an intensity-modulated optically pumped magnetometer (OPM) in background magnetic fields of up to 100μT. We show that the relaxation parameters alter or may be intentionally altered significantly by applying static fields parallel or antiparallel to the MNP’s alignment direction. Further, not only the relaxation process of the MNP’s magnetic moments could be measured with OPM, but also their alignment due to the MRX excitation field. © 2020, The Author(s)

Searching for axion stars and QQ-balls with a terrestrial magnetometer network

Light (pseudo-)scalar fields are promising candidates to be the dark matter in the Universe. Under certain initial conditions in the early Universe and/or with certain types of self-interactions, they can form compact dark-matter objects such as axion stars or Q-balls. Direct encounters with such objects can be searched for by using a global network of atomic magnetometers. It is shown that for a range of masses and radii not ruled out by existing observations, the terrestrial encounter rate with axion stars or Q-balls can be sufficiently high (at least once per year) for a detection. Furthermore, it is shown that a global network of atomic magnetometers is sufficiently sensitive to pseudoscalar couplings to atomic spins so that a transit through an axion star or Q-ball could be detected over a broad range of unexplored parameter space

Characterization of the global network of optical magnetometers to search for exotic physics (GNOME)

The Global Network of Optical Magnetometers to search for Exotic physics (GNOME) is a network of geographically separated, time-synchronized, optically pumped atomic magnetometers that is being used to search for correlated transient signals heralding exotic physics. The GNOME is sensitive to nuclear- and electron-spin couplings to exotic fields from astrophysical sources such as compact dark-matter objects (for example, axion stars and domain walls). Properties of the GNOME sensors such as sensitivity, bandwidth, and noise characteristics are studied in the present work, and features of the network’s operation (e.g., data acquisition, format, storage, and diagnostics) are described. Characterization of the GNOME is a key prerequisite to searches for and identification of exotic physics signatures

Orientational dependence of optically detected magnetic resonance signals in laser-driven atomic magnetometers

We have investigated the dependence of lock-in-demodulated Mx-magnetometer signals on the orientation of the static magnetic field B0 of interest. Magnetic resonance spectra for 2400 discrete orientations of B0 covering a 4π solid angle have been recorded by a PC-controlled steering and data acquisition system. Off-line fits by previously derived lineshape functions allow us to extract the relevant resonance parameters (shape, amplitude, width, and phase) and to represent their dependence on the orientation of B0 with respect to the laser beam propagation direction. We have performed this study for two distinct Mx-magnetometer configurations, in which the rf- field is either parallel or perpendicular to the light propagation direction. The results confirm well the algebraic theoretical model functions. We suggest that small discrepancies are related to hitherto uninvestigated atomic alignment contributions

Analysis method for detecting topological defect dark matter with a global magnetometer network

The Global Network of Optical Magnetometers for Exotic physics searches (GNOME) is a network of time-synchronized, geographically separated, optically pumped atomic magnetometers that is being used to search for correlated transient signals heralding exotic physics. GNOME is sensitive to exotic couplings of atomic spins to certain classes of dark matter candidates, such as axions. This work presents a data analysis procedure to search for axion dark matter in the form of topological defects: specifically, walls separating domains of discrete degenerate vacua in the axion field. An axion domain wall crossing the Earth creates a distinctive signal pattern in the network that can be distinguished from random noise. The reliability of the analysis procedure and the sensitivity of the GNOME to domain-wall crossings are studied using simulated data

Search for topological defect dark matter with a global network of optical magnetometers

Ultralight bosons such as axion-like particles are viable candidates for dark matter. They can form stable, macroscopic field configurations in the form of topological defects that could concentrate the dark matter density into many distinct, compact spatial regions that are small compared with the Galaxy but much larger than the Earth. Here we report the results of the search for transient signals from the domain walls of axion-like particles by using the global network of optical magnetometers for exotic (GNOME) physics searches. We search the data, consisting of correlated measurements from optical atomic magnetometers located in laboratories all over the world, for patterns of signals propagating through the network consistent with domain walls. The analysis of these data from a continuous month-long operation of GNOME finds no statistically significant signals, thus placing experimental constraints on such dark matter scenarios