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Applied Physics Letters

A radio-frequency Bose–Einstein condensate magnetometer

Y. Cohen, Benjamin Maddox, Cameron Deans, Luca Marmugi, Ferruccio Renzoni

Published Apr 18, 2022• 10 citations
Publisher

Abstract

We report on a radio frequency magnetometer employing a Bose–Einstein condensate of 87Rb atoms held in a dipole trap. An AC sensitivity of 296 pT/Hz is achieved at a probing volume of 3.2×10−8 cm3, leading to a volume-normalized sensitivity of 53 fT/Hz cm−3. At larger probing volumes with the atoms released from the magnetic trap used in the initial phase of the evaporation sequence, the AC sensitivity is improved to 1.4 pT/Hz, allowing a two-mode approach for applications requiring improved sensitivity. Immediate application in high-resolution electromagnetic induction imaging is expected when compared to performance of other induction-based imaging platforms.

DOI
10.1063/5.0090776

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We report on a radio frequency magnetometer employing a Bose–Einstein condensate of 87Rb atoms held in a dipole trap. An AC sensitivity of 296 pT/Hz is achieved at a probing volume of 3.2×10−8 cm3, leading to a volume-normalized sensitivity of 53 fT/Hz cm−3. At larger probing volumes with the atoms released from the magnetic trap used in the initial phase of the evaporation sequence, the AC sensitivity is improved to 1.4 pT/Hz, allowing a two-mode approach for applications requiring improved sensitivity. Immediate application in high-resolution electromagnetic induction imaging is expected when compared to performance of other induction-based imaging platforms.
The sensitive detection of either static or radio frequency (RF) magnetic fields is essential to many fundamental studies and applications. Here, we demonstrate the operation of a cold-atom-based RF magnetometer in performing 1D and 2D imaging of small metallic objects. It is based on a cold 85Rb atomic sample and operates in an unshielded environment with no active field stabilization. It shows a sensitivity up to 200 pT/Hz in the 5−35 kHz bandwidth range and can resolve a 0.4 mm wide cut in a 0.8 mm thick metallic foil. The characteristics of our system make it a good candidate for applications in civil and industrial surveillance.
Magnetic field sensing utilizing Bose-condensed atomic gases can achieve both high sensitivity and spatial resolution. Employing two-photon transitions from F=1, mF=1→F=2, mF=-1 (F is the quantum number of total atomic angular momentum, mF is the magnetic quantum number of F level) of Bose-condensed 87Rb atomic gases in one-dimensional optical lattice, the precision magnetic field measurement with phase Ramsey interference has been demonstrated. Utilizing single-photon transitions from F=1, mF=1→F=2, mF=0 of Bose-condensed 87Rb atomic gases in a crossed optical dipole trap, we experimentally study frequency domain Ramsey interference, time domain Ramsey interference, and phase domain Ramsey interference. Based on phase Ramsey interference, the sensitivity and spatial resolution of static magnetic field measurement are evaluated. The comparison of these three typical Ramsey interferences reveals that phase Ramsey interference can effectively avoid the fringe distortion in frequency-domain interference and the decoherence effects in time-domain interference, thereby improving the sensitivity and accuracy of the static magnetic field measurement. Utilizing Bose-condensed 87Rb atomic gases with the volume of 1.31±0.03×10-9 cm3 in an unshielded environment, a sensitivity of 130 nT/Hz for static magnetic fields can be achieved. By reducing the preparation time of the atomic gases and increasing the signal interrogation time, the sensitivity has the potential to be further improved.
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 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.
We describe an alkali-metal magnetometer for detection of weak magnetic fields in the radio-frequency (rf) range. High sensitivity is achieved by tuning the Zeeman resonance of alkali atoms to the rf frequency and partially suppressing spin-exchange collisions in the alkali-metal vapor. We demonstrate magnetic field sensitivity of 2 fT/Hz(1/2) at a frequency of 99 kHz with a resonance width of 400 Hz. We also derive a simple analytic expression for the fundamental limit on the sensitivity of the rf magnetometer and show that a sensitivity of about 0.01 fT/Hz(1/2) can be achieved in a practical system with a measurement volume of 200 cm3.
Radio-frequency (RF) atomic magnetometers (AMs) can be used in many applications, such as magnetic resonance imaging and nuclear quadrupole resonance. High-density AMs provide both superior sensitivity and large bandwidth. Previously, high-density potassium AMs were demonstrated, but these magnetometers have various disadvantages, such as high-temperature of operation and bulky design. We demonstrate a rubidium-87 RF AM with 5 fT/Hz1/2 sensitivity (3 fT Hz1/2 probe noise), which is comparable to that of the best potassium magnetometers. Our magnetometer also features a simple fiber-optic design, providing maximum flexibility for magnetic-field measurements.