We demonstrate cavity sideband cooling of a single collective motional mode of an atomic ensemble down to a mean phonon occupation number n min = 2.0 +0.9 −0.3 . Both n min and the observed cooling rate are in good agreement with an optomechanical model. The cooling rate constant is proportional to the total photon scattering rate by the ensemble, demonstrating the cooperative character of the light-emission-induced cooling process. We deduce fundamental limits to cavity-cooling either the collective mode or, sympathetically, the single-atom degrees of freedom.

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We report on a simple, compact, and robust 780 nm distributed Bragg reflector laser with subkilohertz intrinsic linewidth. An external cavity with optical path length of 3.6 m, implemented with an optical fiber, reduces the laser frequency noise by several orders of magnitude. At frequencies above 100 kHz the frequency noise spectral density is reduced by over 33 dB, resulting in an intrinsic Lorentzian linewidth of 300 Hz. The remaining lowfrequency noise is easily removed by stabilization to an external reference cavity. We further characterize the influence of feedback power and current variation on the intrinsic linewidth. In this Letter, we present a tunable, long-externalcavity DBR laser with a 3000-fold reduction of its intrinsic linewidth to 300 Hz. This is achieved by implementing a 3.6 m long external cavity using an optical fiber. The system exhibits noise suppression of over 33 dB at frequencies of 100 kHz and above. Low-frequency noise due to mechanical or thermal fluctuations in the feedback path can be controlled by stabilizing the laser to a reference cavity using an active servo loop. Feedback-induced mode instability [16] is avoided by operating within a 13 dB wide range of feedback power.We use a 780 nm DBR laser (Photodigm PH780DBR120T8-S) with 120 mW maximum output power as our source. The laser diode has a front facet reflectivity close to 1%, effective gain region length of 1.8 mm, and DBR reflectivity of 60% (all values are nominal values provided by manufacturer). The diode is temperature stabilized to slightly below room temperature. Throughout our measurements, we operated the laser at 100 mA (16 mW total output power). A beam splitter deflects 10% of the laser power into a 2 m long polarization-maintaining optical fiber that constitutes the feedback path (Fig. 1). An aperture is added before the angled fiber to block unwanted backreflection from the fiber tip. A mirror mounted on a piezoelectric transducer (PZT) reflects the light back into the fiber, and a quarterwave plate in combination with a polarizing beam splitter is used to adjust the feedback power. A maximum −30 dB fractional power can be reflected back into the laser. (Henceforth we denote the fractional feedback power incident on the laser collimator as p, which may be less than the fractional power that is mode matched into the active region of the laser diode.) The externalcavity laser is set up on a 12 00 × 18 00 aluminum baseplate and enclosed by a plastic case to reduce environmental perturbations.In general, the effect of external cavity optical feedback depends on the feedback power, cavity length, and the phase of the feedback light, as summarized by Tkach Fig. 1. Schematic setup for the long-external-cavity laser and the frequency noise characterization. M, mirror; AP, aperture; PM fiber, polarization-maintaining fiber; QWP, quarter-wave plate; BS, beam splitter; PBS, polarization beam splitter; APD, avalanche photodiode; PZT, piezoelectric transducer.