High-resolution imaging and manipulation of cold atoms through a multimode fiber

A high imaging resolution or a tight focus of a laser beam imposes a certain minimal numerical aperture (NA) of the optical system. Macroscopic high NA lenses constraint strongly the spatial organization of experiments, in particular when imaging or trapping cold atoms inside a vacuum chamber. Multimode fibers, in conjunction with spatial light modulators, offer an interesting alternative to high NA lenses. Indeed, they are flexible optical waveguides with very small transverse dimensions (similar to 100 mu m), and reasonably high NA (up to 0.5). For those reasons, the use of multimode fibers for imaging or laser manipulation purposes has been widely studied in the past years. Here, we transfer these techniques to the field of cold atoms with a multimode fiber forming a compact optical bridge between the inside and the outside of an ultra-high vacuum chamber. We manipulate atoms with laser beams produced through the fiber by digital optical phase conjugation with a spatial light modulator. We are able to transport a small cloud of cold rubidium atoms with a moving optical lattice at about 200 mu m from the fiber tip. We can then load them in small optical tweezers with a waist of 1.2 mu m. By characterizing the propagation of light modes inside the fiber, we numerically invert its transformation and we reconstruct absorption images of the trapped atoms with a resolution of approximately 1 mu m. These results pave the way towards the efficient use of multimode fibers in spatially constrained quantum technology platforms relying on cold atoms.