Improving matter-wave lensing with multiple Gaussian lenses

Matter-wave lensing, also known as delta-kick cooling (DKC), is a common technique used to prepare narrow momentum distributions by applying an optical "lens" to collimate a cloud of atoms while preserving phase-space density. DKC works by using free expansion to allow the position and momentum of atoms to get correlated before using a linear force from a harmonic potential to cancel that momentum and bring the atoms to a near rest. DKC has been used to achieve record-low temperatures in the pico-Kelvin range, resulting in long coherence lengths. The cooling performance in DKC is limited by the ratio of the final and initial sizes of the cloud and the harmonic nature of the kicking potential. One common implementation of DKC in cold-atom experiments involves using red-detuned Gaussian laser beams to apply an attractive, approximate harmonic potential. Often, the non-harmonic part of the potential limits the final size of the cloud after free expansion, which has to be chosen such that the atoms stay near the potential minima. Recently, it has been shown that using an additional repulsive potential can speed up DKC [Phys. Rev. Research 3, 033261 (2021)]. Here, we present results that show how to improve the cooling performance of delta-kick cooling with Gaussian beams. We show that a combination of Gaussian attractive and repulsive potential kicks can be used to extend the region over which the cloud of atoms sees a linear force, resulting in lower final temperatures. This work also extends the close analogy between DKC and optics, where multiple matter-wave lenses can be used to cancel aberrations, much the same way as aberrations are compensated for with a combination of optical lenses.