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Q: Quantenoptik

Q 32: Poster: Laser Spectroscopy

Q 32.6: Poster

Friday, April 6, 2001, 12:30–15:00, AT2

A dark resonance gradiometer for magnetic field measurements in a noisy environment — •M. Stähler, S. Knappe, N. Bartens, C. Affolderbach, and R. Wynands — Institute for Applied Physics, Bonn University, Wegelerstraße 8, D-53115 Bonn, Germany

The sensitive measurement of magnetic fields outside a magnetically shielded enclosure is hampered by fluctuating external magnetic fields. The spatially decaying magnetic field emanating from a well-localized source can be measured with the help of a gradiometer even in the presence of strong magnetic noise. In the difference signal of two sensors, one close to the source, the other one a suitable distance away, most of those disturbances cancel out, leaving only the field produced by the source. In order for this cancellation to work the two sensors must be balanced extremely well. In our case the sensors are thermal alkali vapor cells, illuminated by a bichromatic light field that excites coherent population trapping resonances. Their Zeeman shift is exploited in order to measure weak fields in the pico- and nanotesla range. For a gradiometer, not only must the average magnetic field at each cell be identical, but also the cells must have equal lengths and vapor pressures. We describe several techniques that can be used to balance the signals from the two vapor cells even in the presence of small imperfections in cell geometry or residual background field gradients.