Attosecond entangled photons from two-photon decay of metastable atoms: A source for attosecond experiments and beyond

Abstract

We propose the generation of attosecond entangled biphotons 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 ^{1}S_{0} metastable state in helium decays to the ground state by the 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 biphoton 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 biphotons. Possible applications of entangled biphotons in attosecond timescale experiments, and a discussion of their potential to reach the zeptosecond regime are presented.