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

Attosecond Entangled Photons from Two-Photon Decay of Metastable Atoms: A Source for Attosecond Experiments and Beyond