Single-atom counting in a two-color magneto-optical trap

Recording the fluorescence of a magneto-optical trap (MOT) is a standard tool for measuring atom numbers in experiments with ultracold atoms. When trapping few atoms in a small MOT, the emitted fluorescence increases with the atom number in discrete steps, which allows one to measure the atom number with single-particle resolution. Achieving such single-particle resolution requires stringent minimization of stray light from the MOT beams, which is very difficult to achieve in experimental setups that require in-vacuum components close to the atoms. Here, we present a modified scheme that addresses this issue: Instead of collecting the fluorescence on the MOT (D2) transition, we scatter light on an additional probing (D1) transition and collect this fluorescence with a high-resolution microscope while filtering out the intense MOT light. Using this scheme, we are able to reliably distinguish up to 17 K-40 atoms with classification fidelities of similar to 98% for up to 5 atom numbers and fidelities of more than 85% for up to 17 atoms.

Automated summary

A modified scheme is proposed to address the issue of stringent minimization of stray light from the MOT beams, achieving reliable discrimination of up to 17 K-40 atoms with classification fidelities of similar to 98% for up to 5 atom numbers and fidelities of more than 85% for up to 17 atoms.