Many cold atom experiments rely on precise atom number detection, especially in the context of quantum-enhanced metrology where effects at the single particle level are important. Here, we investigate the limits of atom number counting via resonant fluorescence detection for mesoscopic samples of trapped atoms. We characterize the precision of these fluorescence measurements beginning from the single-atom level up to more than one thousand. By investigating the primary noise sources, we obtain single-atom resolution for atom numbers as high as 1200. This capability is an essential prerequisite for future experiments with highly entangled states of mesoscopic atomic ensembles.The Heisenberg uncertainty principle sets a fundamental limit, ∆φ = 1/N , on the precision at which one can determine an interferometric phase φ using N particles [1]. A prerequisite for reaching the Heisenberglimited uncertainty in a real measurement is a particle detector with atom number variance σ 2 N 1, i.e. exact particle counting at the interferometer output. This capability is challenging to realize, particularly for large particle numbers. For example, single-photon detectors suffer from limited quantum efficiency (typically < 95 %), which prohibits resolving photon numbers for N 10 [2]. On the other hand, single atoms can be detected with near-unit efficiency by trapping them and observing their fluorescence [3]. Here, we extend this single-atom counting capability to mesoscopic atom numbers by high accuracy fluorescence measurements.One example where single-atom resolution becomes necessary is spectroscopy with maximally entangled states. Here, it has been shown that Heisenberg-limited precision requires measurement of the parity [4]. Another example is interferometry with spin-squeezed atomic states [5], where experimental results have shown a reduction of atom number variance approaching a level at which single-atom resolution becomes relevant [6]. Similarly, such high resolution atom detection would allow the direct observation of twin atom pairs produced via spin-changing collisions [7][8][9] and enable their use for interferometry at the Heisenberg limit.The most common detection method for neutral atoms is absorption imaging, but the precision of such measurements on mesoscopic ensembles has thus far been limited to the level of a few atoms [10,11]. Single-atom resolution for small atom numbers (N ∼ 10) has been achieved by fluorescence detection of neutral atoms in free space [12] as well as in magneto-optical traps (MOTs) [13][14][15], optical dipole traps [16] and optical cavities [17][18][19]. A recent experiment explored the detection of mesoscopic ensembles of atoms [20] in an optical cavity, and stability at the single-atom level was observed in repeated measurements for effective atom numbers as high as N = 150. In this case, however, accurate determination of the absolute atom number was not possible due to inhomogeneous coupling to the standing-wave probe light. On the other hand, spatially resolved fluorescence me...

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We report on a protocol to achieve full control of the polarization in a nanofiber. The protocol relies on monitoring the light scattered out from a nanofiber by means of two optical systems with 45°camera angle difference. We study the disturbance of the nanofiber refractive index on the radiation of embedded scatterers, and we propose an explanation for the observed reduced scattering contrast of the nanofiber. Thanks to this approach, we demonstrate an accuracy of the polarization control larger than 95%.