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Japanese Journal of Applied Physics

Measurement of a Weak Magnetic Field Using Cold Atoms

Takayuki Kurosu, Yasuhiro Fukuyama, Kentaro Abe, Y. Koga

Published May 15, 2002• 4 citations
Publisher

Abstract

Laser-cooled atoms were utilized to precisely measure a weak magnetic field. The technique is based on the linear Zeeman shift of the hyperfine transition of cesium atom and allows high-resolution measurement in the presence of a field gradient. The method was applied to evaluate the C-field of an atomic fountain frequency standard, in which a field-sensitive transition was probed by Ramsey resonance. The relationship between the measurement time and the resolution is also discussed.

DOI
10.1143/jjap.41.l586

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Laser-cooled atoms were utilized to precisely measure a weak magnetic field. The technique is based on the linear Zeeman shift of the hyperfine transition of cesium atom and allows high-resolution measurement in the presence of a field gradient. The method was applied to evaluate the C-field of an atomic fountain frequency standard, in which a field-sensitive transition was probed by Ramsey resonance. The relationship between the measurement time and the resolution is also discussed.
Sub-Doppler laser-cooled cesium-133 atoms are utilized as quantum sensors to achieve precise mapping of magnetic fields across a region in ultra-high vacuum (UHV), with a spatial resolution of 1 cm and a sensitivity of approximately 550 pT/√Hz, enabling accurate measurements within the nanotesla [nT] range. The cold cesium-133 atoms used for magnetic field measurements in this paper are a key component of the cesium fountain frequency standard at CSIR-NPL, which contributes to both timekeeping and magnetic sensing. The results show magnetic field fluctuations within 1 nT with a spatial resolution of 1 cm. The uncertainty in these measurements is of the order of 1.24 × 10−16, ensuring reliable and precise spatially resolved magnetic field mapping.
The spin-exchange frequency shift due to cold collisions between ground-state atoms appears to be the systematic effect which could limit the accuracy of a laser-cooled cesium fountain operating as a frequency standard. We have investigated this shift for atoms distributed among all the Zeeman substates of either the F = 4 or F = 3 hyperfine level of the 6 S 1/2 ground state of cesium and for the case where most of the atoms are prepared in F = 3, M F = 0 sublevel. The measured relative frequency shifts are respectively − 1.0·10 −21 , − 0.6·10 −21 and − 2.0·10 −21 (at/cm 3 ) −1 . These results agree with the measurements performed by Gibble and Chu at higher atom densities with atoms prepared in F = 3, M F = 0 sublevel. However, the precision of our frequency measurements is much better and is only limited at 2·10 −15 by the H-maser used as a reference oscillator. Finally, we show that the relative frequency shift due to cold collisions can be reduced to 10 −15 , while preserving a short-term frequency stability of about 10 −13 τ −1/2 .
Neutral atom trapping and laser cooling has opened the way to large prospects for the present generation of atomic frequency standards [1].A cold cesium fountain device has been built for the first time at the Observatoire de Paris and is now operating since 1994.Its frequency stability, 4 10 -14  -1/2 where  is the integration time, has been the best one ever measured for a while.This letter shows how accurate can be the measurement of low static magnetic fields within the fountain device and the variations along the longitudinal and transverse axis.Thus the device is no more operating as a clock but constitutes a very precise magnetometer.
It was anticipated that laser cooling of atoms to 2 {mu}K would improve the accuracy and resolution of laser and microwave spectroscopies by 2 to 3 orders of magnitude. In state-of-the-art room temperature Cs frequency standards, the 9 GHz ground state hyperfine transition is measured with a fractional accuracy of 3 x 10{sup {minus}14}. Since all of the formerly known systematic effects that limit the accuracy increase either linearly or as some high power of the atom`s velocity, the accuracy of a laser cooled Cs standard should be {approx} 10{sup {minus}16}. However, at {mu}K temperatures, the de Broglie wavelengths of the atoms are much larger than the scale of the interatomic potential so that collision cross sections can be as large as {lambda}{sub dB}{sup 2}/2{pi} {approx} 10{sup {minus}10} cm{sup 2}. Here, collisional frequency shifts in a laser-cooled Cs fountain are measured to be {minus}1.4 x 10{sup {minus}12} corresponding to a cross section of 0.5 x 10{sup {minus}10} cm{sup 2}. The author also expects to report experimental results showing a potentially strong magnetic field dependence of these cross sections for fields smaller than 0.2 G. The field of ultra-cold collisions will also be reviewed.
Nanofibers have many promising applications because of their advantages of high power density and ultralow saturated light intensity. We present here a Zeeman shift of the Doppler-broadened cesium D 2 transition using a tapered optical nanofiber in the presence of a magnetic field. When a weak magnetic field is parallel to the propagating light in the nanofiber, the Zeeman shift rates for different circularly polarized spectra are observed. For the σ + component, the typical linear Zeeman shift rates of F = 3 and F = 4 ground-state cesium atoms are measured to be 3.10( ±0.19 ) MHz/G and 3.91( ±0.16 ) MHz/G. For the σ − component, the values are measured to be −2.81( ± 0.25) MHz/G, and −0.78( ± 0.28) MHz/G. The Zeeman shift using the tapered nanofiber can help to develop magnetometers to measure the magnetic field at the narrow local region and the dispersive signal to lock laser frequency .
We present a method for magnetic field measurement by the weak magnetic-sensitive Zeeman splitting and a method for measuring the magnetic field vector by probing the line strength of stimulated Raman transitions. Compared with the traditional measurement, the contrast of the weak magnetic-sensitive resonance signal is improved significantly for gradient field. We also present a dynamical compensation method to smooth the spatial fluctuation of the magnetic field in the fountain clock. A homogeneous field with a 0.4 nT peak-to-peak value is realized compared to 5nT under the static magnetic field.