Knowing how fast a quantum process can be reveals the ultimate limits to information processing. The brachistochrone problem for two-level quantum systems—the fastest path connecting two quantum states—has been long known. These solutions, however, are generally not applicable to larger quantum systems. In Phys. Rev. X 11, 011035 (2021), we experimentally demonstrate a shortest-duration quantum process that fundamentally cannot be reduced to two-level dynamics.

We carry out fast coherent transport of an atomic wave packet over a distance 15 times its size using an optical conveyor belt. Our measurements of the transport fidelity sharply resolve the transition from a quantum-controllable to a quantum-noncontrollable process as the time is shortened, thus revealing the existence of a minimum duration—a quantum speed limit. Based on a geometric approach to quantum state dynamics, we provide a close lower bound on the minimum process duration beyond the two-level-system paradigm.

These results shed light upon a fundamental speed limit of quantum state dynamics. Identifying quantum processes of the shortest duration is important in quantum sensing and quantum computing.