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Analyzing atomic noise with a consumer sound card
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Image of Fig. 1.
Fig. 1.

(Color online) (a) Off-resonant linearly polarized laser light is transmitted through a rubidium gas cell. The direction of the linear light polarization acquires a rotation due to the random spin polarization in the sample based on the magneto-optical Faraday effect. The resulting polarization noise is converted to electrical noise which is digitized and spectrally analyzed on a computer using a fast Fourier transform. (b) The black curve shows a noise spectrum including contributions from spin noise, shot noise, electrical noise, and classical 1/f noise. Subtracting a reference spectrum containing only background noise yields the pure spin noise signal. The spikes in the spectrum slightly below 80 kHz result from electromagnetic interference with unknown sources. The noise power above 90 kHz drops due to the limited bandwidth of the sound card.

Image of Fig. 2.
Fig. 2.

Energy states of 87 Rb and 85 Rb.26 The different hyperfine splitting for the two isotopes results in different effective g -factors g * = g F  = 1/2 (87 Rb) and g * = g F  = 1/3 (85Rb). The hyperfine splitting is negligible for the experiment.

Image of Fig. 3.
Fig. 3.

Spin noise spectra at different effective magnetic fields. The spin noise is centered at the Larmor frequency which is proportional to the magnetic field. For clarity, the spectra are shifted vertically by 45 mV2/Hz each.

Image of Fig. 4.
Fig. 4.

(Color online) A detailed fit of the experimental spectra reveals residual 85Rb content in the cell. The presence of 85Rb is verified by a fit containing the sum of two Lorentzian curves. The peak width corresponds to a dephasing time of 36 μs.


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Scitation: Analyzing atomic noise with a consumer sound card