Single-particle-sensitive imaging of freely propagating ultracold atoms

We present a novel imaging system for ultracold quantum gases in expansion. After release from a confining potential, atoms fall through a sheet of resonant excitation laser light and the emitted fluorescence photons are imaged onto an amplified CCD camera using a high numerical aperture optical system. The imaging system reaches an extraordinary dynamic range, not attainable with conventional absorption imaging. We demonstrate single-atom detection for dilute atomic clouds with high efficiency where at the same time dense Bose–Einstein condensates can be imaged without saturation or distortion. The spatial resolution can reach the sampling limit as given by the 8 μm pixel size in object space. Pulsed operation of the detector allows for slice images, a first step toward a three-dimensional (3D) tomography of the measured object. The scheme can easily be implemented for any atomic species and all optical components are situated outside the vacuum system. As a first application we perform thermometry on rubidium Bose–Einstein condensates created on an atom chip.