Quantum correlations in the two-photon decay of few-electron ions

A theoretical study of the polarization entanglement of two photons emitted in the decay of metastable ionic states is performed within the framework of density-matrix theory and second-order perturbative approach. Particular attention is paid to relativistic and nondipole effects that become important for medium- and high-Z ions. To analyze these effects, the degree of entanglement is evaluated both in the dipole approximation and within rigorous relativistic theory. Detailed calculations are performed for the two-photon 2s1/2→1s1/2 transition in hydrogenlike ions as well as for the 1s1/2 2s1/2 ¹S₀→1s1/2² ¹S₀, 1s1/2 2s1/2 ³S₁→1s1/2² ¹S₀, and 1s1/2 2p1/2 ³P₀→1s1/2² ¹S₀ transitions in heliumlike ions.