  A photoelectron, emitted due to the absorption of light quanta as described
by the photoelectric effect, is often characterized experimentally by a
classical quantity, its momentum. However, since the photoelectron is a quantum
object, its rigorous characterization requires the reconstruction of the
complete quantum state, the photoelectron's density matrix. Here, we use
quantum state tomography to fully characterize photoelectrons emitted from
helium and argon atoms upon absorption of ultrashort, extreme ultraviolet light
pulses. While in helium we measure a pure photoelectronic state, in argon,
spin-orbit interaction induces entanglement between the ion and the
photoelectron, leading to a reduced purity of the photoelectron state. Our work
shows how state tomography gives new insights into the fundamental quantum
aspects of light-induced electronic processes in matter, bridging the fields of
photoelectron spectroscopy and quantum information, and offering new
spectroscopic possibilities for quantum technology.
