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 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.

Figures (3)

• Figure 1: The photon correlation function $\langle vac|{E}(t_2){E}(t_1)|2ph\rangle$ (up to a constant factor) as a function of time difference $(t_2-t_1)$, which indicates the correlation time is around $1.93\times 10^{-16}$$sec$.
• Figure 2: (a) Generation of entangled bi-photons in the XUV via two-photon decay of the $1s2s$$^1S_0$ state excited by four-photon excitation using a broad band 240 nm laser. (b) Two-step sequential excitation of the $1s2s$ state via the $1s2p$ state using a high photon flux helium lamp and a 2059 nm coupling laser. (c) The SCRAP technique to populate the $1s2s$ state using a multiphoton pump pulse and a Stark shifting pulse which enable rapid adiabatic passage and ionization suppression by LICS (LICS not shown). The estimated bi-photon generation rate is also shown for each scheme in (a) - (c). (d) Proposed experimental scheme to generate XUV entangled photons and utilize them in an attosecond pump-probe photoionization experiment. (e) An attosecond pump-probe photoionization scheme in molecules using entangled bi-photons.
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