Generation of entanglement using a short-wavelength seeded free-electron laser
Saikat Nandi, Axel Stenquist, Asimina Papoulia, Edvin Olofsson, L. Badano, Mattias Bertolino, David Busto, Carlo Callegari, Stefanos Carlström, M. Danailov, Philipp V. Demekhin, Michele Di Fraia, P. Johnsson, R. Feifel, Guillaume Gallician, L. Giannessi, Mathieu Gisselbrecht, Michele Manfredda, Michael Meyer, Catalin Miron, Jasper Peschel, Oksana Plekan, Kevin C. Prince, Richard J. Squibb, Marco Zangrando, Felipe Zapata, Shiyang Zhong, Jan Marcus Dahlström
Abstract
Quantum entanglement between the degrees of freedom encountered in the classical world is challenging to observe due to the surrounding environment. To elucidate this issue, we investigate the entanglement generated over ultrafast timescales in a bipartite quantum system comprising two massive particles: a free-moving photoelectron, which expands to a mesoscopic length scale, and a light-dressed atomic ion, which represents a hybrid state of light and matter. Although the photoelectron spectra are measured classically, the entanglement allows us to reveal information about the dressed-state dynamics of the ion and the femtosecond extreme ultraviolet pulses delivered by a seeded free-electron laser. The observed generation of entanglement is interpreted using the time-dependent von Neumann entropy. Our results unveil the potential for using short-wavelength coherent light pulses from free-electron lasers to generate entangled photoelectron and ion systems for studying spooky action at a distance.