Matter waves at their quantum speed limit
Andrea Alberti, Universität Bonn
Group meeting via video conference (Zoom)
Tuesday November 23rd, 9:00 (MEZ)
Due to the Covid-19 pandemic, we are still holding our group seminars and journal clubs via video conference.This procedure enables us to continue our research, enhance discussions and exchange important information.
How fast can a quantum system evolve between two states? This question is not only important for its basic nature, but it has far-reaching implications on future quantum technologies. There are two well-known limits on the maximum evolution rate, named after their discoverers—Mandelstam–Tamm and Margolus–Levitin. Despite their fundamental character, only the Mandelstam–Tamm limit has been so far investigated and exclusively in effective two-level systems. In this seminar, I will report on a recent experimental study  where we put both limits to the test in a multi-level system. The experiment consists in following with high temporal resolution the quantum dynamics of a matter wave sliding down an optical potential. Our measurements reveal a crossover between the two quantum speed limits, depending on the energy distribution of the quantum state. We find a striking difference between a two-level and a multi-level system—excitations of a multi-level system do not saturate the speed limit but produce a small, universal deviation from it. In the second part of this seminar, I will address a related question, what is the fastest route—the quantum brachistochrone—to transport an atom between distant states. We demonstrate  coherent transport of an atomic matter wave over a distance of 15 times its size in the shortest possible time. Owing to the large separation between the two sites, the two quantum speed limits above fail to capture the relevant time scale. In contrast, we show that quantum optimal control provides solutions to the quantum brachistochrone problem. Our results, establishing quantum speed limits beyond the simple two-level system, are important to understand the ultimate performance of quantum computing devices and related advanced quantum technologies.
 G. Ness, M. R. Lam, W. Alt, D. Meschede, Y. Sagi, and A. Alberti, “Observing quantum-speed-limit crossover with matter wave
interferometry,” arXiv:2104.05638 [quant-ph], Sci. Adv., in press (2021)
 M. R. Lam, N. Peter, T. Groh, W. Alt, C. Robens, D. Meschede, A. Negretti, S. Montangero, T. Calarco, and A. Alberti, “Demonstration
of Quantum Brachistochrones between Distant States of an Atom,” Phys. Rev. X 11, 011035 (2021)
If you would like to join our group seminars via Zoom, please contact us for more information.