Faraday spectroscopy of atoms confined in a dark optical trap

We demonstrate Faraday spectroscopy with a high duty cycle and sampling rate using atoms confined to a blue-detuned optical trap. Our trap consists of a crossed pair of high-charge-number hollow laser beams, which forms a dark, boxlike potential. We have used this to measure transient magnetic fields in a 500-μm-diameter spot over a 400 ms time window with nearly unit duty cycle at a 500-Hz sampling rate. We use these measurements to quantify and compensate time-varying magnetic fields to ≈10 nT per time sample.