  A universal mechanism of ultrafast two-electron orbital swap is discovered
through two-photon sequential double ionization of Li. After a $1s$ electron in
Li is ionized by absorbing an EUV photon, the other two bound electrons located
on two different shells have either parallel or antiparallel spin orientations.
In the latter case, these two electrons are in the superposition of the singlet
and triplet states with different energies, forming a quantum beat and giving
rise to the two-electron orbital swap with a period of several hundred
attoseconds. The orbital swap mechanism can be used to manipulate the spin
polarization of photoelectron pairs by conceiving the attosecond-pump
attosecond-probe strategy, and thus serves as a knob to control spin-resolved
multielectron ultrafast dynamics.
