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arXiv:2009.07569

Sensitive BEC magnetometry with optimized probing

Naota Sekiguchi, Aki Torii, Hiroyuki Toda, Ryohei Kuramoto, Daiki Fukuda, Hirano, Takuya, Kosuke Shibata

Published Sep 16, 2020• 2 citations
arXivHTMLPDF

Abstract

An improved spatial magnetometer using a spinor Bose-Einstein condensate of $^{87}$Rb atoms is realized utilizing newly developed two-polarization phase contrast imaging. The optical shot noise is suppressed by carefully choosing the probe parameters. We attain a dc-magnetic field sensitivity of 7.7 ${\rm pT/\sqrt{Hz}}$ over a measurement area of 28 ${\rm μm^{2}}$. The attained sensitivity per unit area is superior to that for other modern low-frequency magnetometers with micrometer-order spatial resolution. This result is a promising step for realizing quantum-enhanced magnetometry surpassing classical methods.

DOI
10.48550/arxiv.2009.07569

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An improved spatial magnetometer using a spinor Bose-Einstein condensate of 87^{87}87Rb atoms is realized utilizing newly developed two-polarization phase contrast imaging. The optical shot noise is suppressed by carefully choosing the probe parameters. We attain a dc-magnetic field sensitivity of 7.7 pT/Hz{\rm pT/\sqrt{Hz}}pT/Hz​ over a measurement area of 28 μm2{\rm μm^{2}}μm2. The attained sensitivity per unit area is superior to that for other modern low-frequency magnetometers with micrometer-order spatial resolution. This result is a promising step for realizing quantum-enhanced magnetometry surpassing classical methods.
We demonstrate a sensitive spatially resolved magnetometer using a spinor Bose-Einstein condensate (BEC) of 87Rb^{87}\mathrm{Rb}87Rb atoms with increased probe fluence. By performing a probe parameter search using a two-polarization phase contrast imaging technique, we find that the probe-induced atom loss, a major constraint in optical magnetometry with high-density gas, can be reduced. We attain a sensitivity of 5.4(6) mrad for the phase measurement of Larmor precession using a measurement area of $1.4\ifmmode\times\else\texttimes\fi{}{10}^{2}\phantom{\rule{3.33333pt}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{2}$ with a probe fluence of $4.1\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{3.33333pt}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{\ensuremath{-}2}$. A dc magnetic field sensitivity of 5.0(4)pT/Hz5.0(4)\phantom{\rule{3.33333pt}{0ex}}\mathrm{pT}/\sqrt{\mathrm{Hz}}5.0(4)pT/Hz​ is attained with a probe fluence of $3.9\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{\ensuremath{-}2}$. The results should contribute to further enhancement of the sensitivity of a BEC magnetometer.
We demonstrate a precise magnetic microscope based on direct imaging of the Larmor precession of a 87Rb^{87}\mathrm{Rb}87Rb spinor Bose-Einstein condensate. This magnetometer attains a field sensitivity of 8.3  pT/Hz1/28.3\text{ }\text{ }\mathrm{pT}/{\mathrm{Hz}}^{1/2}8.3  pT/Hz1/2 over a measurement area of $120\text{ }\text{ }\ensuremath{\mu}{\mathrm{m}}^{2}$, an improvement over the low-frequency field sensitivity of modern SQUID magnetometers. The achieved phase sensitivity is close to the atom shot-noise limit, estimated as 0.15  pT/Hz1/20.15\text{ }\text{ }\mathrm{pT}/{\mathrm{Hz}}^{1/2}0.15  pT/Hz1/2 for a unity duty cycle measurement, suggesting the possibilities of spatially resolved spin-squeezed magnetometry. This magnetometer marks a significant application of degenerate atomic gases to metrology.
Polarization-dependent phase-contrast imaging is used to resolve the spatial magnetization profile of an optically trapped ultracold gas. This probe is applied to Larmor precession of degenerate and nondegenerate spin-1 87Rb gases. Transverse magnetization of the Bose-Einstein condensate persists for the condensate lifetime, with a spatial response to magnetic field inhomogeneities consistent with a mean-field model of interactions. In comparison, the magnetization of the non-condensed gas decoheres rapidly. Rotational symmetry implies that the Larmor frequency of a spinor condensate be density independent, and thus suitable for precise magnetometry with high spatial resolution.
This thesis presents advancements in radio-frequency atomic magnetometry utilising cold atoms as sensors. While cold atom magnetometers demonstrate lower absolute sensitivity compared to conventional vapour cell systems due to reduced atom numbers, they offer exceptional spatial resolution at the micrometre scale. The experimental apparatus features a cold atom production system capable of reaching quantum degeneracy through a refined cooling sequence that involves optical molasses, magnetic trapping and evaporation, and optical evaporative cooling in the dipole trap, culminating in a 87Rb Bose-Einstein condensate. This work details the development and characterisation of magnetometer set-ups with progressively enhanced performance. The first magnetometer utilised atoms in the optical molasses phase as a sensor, achieving an AC sensitivity of 330 pT/√Hz with approximately 108 atoms at 20 μK. Comprehensive magnetometry optimisation methods were developed to enhance sensitivity. Following laboratory interruptions due to the COVID-19 pandemic, system rebuilding incorporated significant upgrades, including a new probe laser and gradient compensation systems. The improved magnetometer prototype achieved a substantially enhanced sensitivity of 0.72±0.33 pT/√Hz, approaching the fundamental sensitivity limit of the system. Using atoms in the magnetic trap as a sensor, a sensitivity of 1.85 pT/√Hz was demonstrated. The system was verified to be photon-shot noise limited. Investigations into a double-pass probe configuration showed that, while the measured sensitivity was similar to that of the single-pass (4.3 pT/√Hz compared to 3.1 pT/√Hz at a probe power of 6.4 μW measured at the detector), the rotation angle increased by a factor of 2.2, with significantly reduced noise levels. These findings suggest that multi-pass strategies hold promise for enhanced performance in future implementations. In addition, the first signal detection from a BEC magnetometer, achieving 4.5 nT/√Hz with only 95,000 atoms at 35 nK was demonstrated.
We demonstrate detection of a weak alternate-current magnetic field by application of the spin-echo technique to F=2F=2F=2 Bose-Einstein condensates. A magnetic field sensitivity of 12 pT/Hz/\sqrt{\mathrm{Hz}}/Hz​ is attained for an atom number of $5\ifmmode\times\else\texttimes\fi{}{10}^{3}$ at a spatial resolution of 100 $\ensuremath{\mu}$m2{}^{2}2. Our observations indicate magnetic field fluctuations synchronous with the power supply line frequency. We show that this noise is greatly suppressed by application of a reverse phase magnetic field. Our technique is useful in order to create a stable magnetic field environment, which is an important requirement for atomic experiments which require a weak bias magnetic field.
The development of an experimental apparatus to produce Bose-Einstein condensates (BECs) of ⁸⁷Rb atoms and their application to magnetometry are discussed. Optical detection of atomic Larmor precession is a widely explored method for high-sensitivity measurements of magnetic fields. In this context, short laser/atom interaction time, atomic thermal diffusion and decoherence effects are among the main limitations. In this thesis, we overcome such problems by using spin-polarised ⁸⁷Rb ultra-cold atoms as the sensing element. After the atoms are polarised, a resonant pulse of radio-frequency excites Larmor precession, which is sensitive to external magnetic fields. By measuring the perturbations of the radio-frequency induced spin precession, information on the magnetic fields of interest. This is achieved by monitoring the polarisation plane’s rotation of a linearly polarised resonant laser probe. In the first part of this thesis, the building and optimisation of a laser-cooling set up to obtain a BEC in a hybrid trap is reported. In order to achieve the Phase Space Density (PSD) required for BEC, several different stages of trapping and cooling are necessary. Each phase has been implemented and optimised. The first step consists in the magneto-optical trap (MOT). Here a velocity dependent damping force and a spatially dependent confining force give the largest changes in PSD. Then atoms are loaded into a hybrid trap obtained by overlapping a quadrupole magnetic potential and a far detuned optical crossed dipole trap. The final stage for the condensation consists of forced evaporative cooling, both via magnetic and optical evaporation. In the second part of the thesis, a general overview of the principles of optical atomic magnetometry is provided and the advantages of using ultra-cold atoms with respect to conventional thermal vapours are discussed. The implementation, operation and a preliminary characterisation of the ultra-cold atom magnetometer are described along with the preliminary results collected. Finally, a plan for future improvements of its sensitivity is presented.