Abstract
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 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 $^{40}$K atoms with an average classification fidelity of 95 \%.
Highlights
In recent years, ultracold atoms have emerged as a unique and powerful platform to experimentally study few-body physics
Collecting fluorescence light while laser cooling and trapping the atoms in a magneto-optical trap (MOT) has proven to be a reliable and well-established method to measure atom numbers with single-particle resolution [5,6,7,8,9,10,11,12,13,14,15] and is especially suited for experiments studying few atoms confined in optical tweezers, where in situ imaging would lead to photoassociation losses
Common techniques for achieving this include the use of large magnetic field gradients to compress the MOT and thereby spatially concentrate the fluorescence signal, using optics with high numerical aperture to maximize the number of collected photons, as well as
Summary
Ultracold atoms have emerged as a unique and powerful platform to experimentally study few-body physics. Detrimental stray-light collection is suppressed as far as possible by employing laser-cooling beams with small diameters, avoiding the presence of unwanted scattering surfaces close to atoms, and spatial filtering of the fluorescence signal Using these techniques, single-atom resolution for atom numbers in excess of 300 has been achieved [13,14]. D1 fluorescence photons are generated by a D1 pumping beam whose size and beam path have been chosen to avoid scattering surfaces and thereby minimize the amount of stray light that is generated This scheme allows us to perform single-atom counting of up to 17 atoms with an average classification fidelity of 95% in an experimental setup featuring a high-resolution microscope objective placed inside the vacuum chamber very close to the atoms. This is achieved by placing two ultranarrow 780 nm bandpass filters [23] that are angle tuned to 766.7 nm in the beam path between the tapered amplifier and the fibers
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