Noise spectroscopy with large clouds of cold atoms

Abstract

Noise measurement is a powerful tool to investigate many phenomena from laser characterization to quantum behavior of light. In this paper, we report on intensity noise measurements obtained when a laser beam is transmitted through a large cloud of cold atoms. While this measurement could possibly investigate complex processes such as the influence of atomic motion, one is first limited by the conversion of the intrinsic laser frequency noise to intensity noise via the atomic resonance. This conversion is studied here in details. We show that, while experimental intensity noise spectra collapse onto the same curve at low Fourier frequencies, some differences appear at higher frequencies when the probe beam is detuned from the center of the resonance line. A simple model, based on a mean-field approach, which corresponds to describing the atomic cloud by a dielectric susceptibility, is sufficient to understand the main features. Using this model, the noise spectra allow extracting some quantitative informations on the laser noise as well as on the atomic sample.

Mollow triplet in cold atoms

“…We will first focus on the sidebands. The first contribution comes from the laser linewidth n D = 3 MHz L , measured with a beat-note technique between this laser and a laser with a much smaller linewidth, as explained in [28]. As said before, this linewidth has no effect on the carrier because we use the same laser for the LO and the laser beams A and B.…”

“…The experimental setup is described in Refs. [24,26]. A cold atomic cloud is provided by loading 85 Rb atoms in a MOT.…”

Diffusing-wave spectroscopy of cold atoms in ballistic motion

“…This quantity is inferred from the measurement of the transmission of a small probe beam going through the center of the atomic cloud as a function of the frequency detuning on the |3 → |4 D 2 hyperfine transition as shown on Fig. 1b (frequency ω 0 , wavelength λ = 780.24 nm, linewidth Γ/2π = 6.07 MHz) [30].…”

“…This probe beam is delivered by a distributed-feedback (DFB) laser, amplified by a tapered amplifier. Although it is known that DFB lasers have strong frequency noise [30,31], it does not affect our measurements of intensity correlations [32]. The duration of each probe pulse is 100 µs, corresponding to a time-window where the intensity correlations are recorded, separated by a 50-µs pulse of repumper and 50 µs of free expansion.…”

Sub-kHz-linewidth VECSELs for cold atom experiments

“…Alternatively, the laser linewidth can be estimated by reconstructing the shape of the optical field via auto-correlation and the Wiener-Khintchine theorem. This technique is used to estimate laser linewidth relative to a suitable reference when no other laser is available or where a self-heterodyne measurement is impractical [28,37,38]. A linewidth of (125 ± 2) Hz is estimated [see Fig.…”