An Attosecond Entangled Photon Source for Attosecond Spectroscopy

Attosecond pulses are crucial for measuring the electron dynamics of atoms and molecules on the attosecond time scale. Typical attosecond measurement use classical attosecond pulses to probe electron dynamics. Here we discuss progress towards generation of attosecond quantum light pulses in the form of entangled biphotons in the extreme-ultraviolet (XUV) regime. The entangled biphotons will be generated through two-photon decay of the 1s2s ¹S₀ metastable state in helium to the ground state. The total energy spacing is 20.62 eV, resulting in a photon pair correlation time on the order of attoseconds. A vacuum chamber and gas cell were designed and constructed, and a 240-nm laser is focused into the gas cell for a direct four-photon excitation of the metastable state. A single photon detection setup is being built to detect entangled photons from two-photon decay of metastable helium. The setup is composed of a photomultiplier tube and CaF₂ windows. The photomultiplier tube detects the XUV photons, while the CaF₂ windows select a band of photon energies between 8 – 10 eV. A constant fraction discriminator and a time-to-digital converter are then employed to achieve higher signal-to-noise ratio. We discuss plans for applying the attosecond XUV entangled photons in attosecond pump-probe measurements.