Single-atom billiards in an ultracold bath with energy-tunable collisions and threshold-sensitive detection

Transport processes at the microscopic scale can differ strongly from their macroscopic counterparts, particularly when the environment is not a well-defined surface but an unordered thermal medium. Ultracold quantum gases offer a high degree of controllability and directly observable dynamics, thereby providing a versatile platform to study such transport processes under precisely controllable conditions. Here, we realize an energy-tunable single-atom collider by transporting individual cesium atoms at controlled velocity through an ultracold rubidium bath. The constant lattice velocity sets the directed contribution to the collision-energy distribution, while the lattice depth provides a tunable energy threshold for collision-induced transport disruption. By independently varying these parameters together with bath density and temperature, we separate the number of collision events from their probability to disrupt transport. Across a regime with only a few expected collisions per passage, the inferred single-collision ejection probability rises from small values to near unity as the collision energy exceeds the lattice confinement scale. The experiment thus converts microscopic collisional energy transfer into a binary single-atom observable and provides a controlled link between individual scattering events and emergent dissipative transport.