  We propose the generation of attosecond entangled bi-photons in the
extreme-ultraviolet regime by two-photon decay of a metastable atomic state as
a source similar to spontaneous parametric down-conversion photons. The 1s2s
$^1S_0$ metastable state in helium decays to the ground state by emission of
two energy-time entangled photons with a photon bandwidth equal to the total
energy spacing of 20.62 eV. This results in a pair correlation time in the
attosecond regime making these entangled photons a highly suitable source for
attosecond pump-probe experiments. The bi-photon generation rate from a direct
four photon excitation of helium at 240 nm is calculated and used to assess
some feasible schemes to generate these bi-photons. Possible applications of
entangled bi-photons in attosecond time scale experiments, and a discussion of
their potential to reach the zeptosecond regime are presented.

Quantum Physics