Entangled Photon Pairs Generated from Atomic Three-Level Systems and their Cross Correlation Functions

Photon pairs generated from electronic transitions in atomic three-level systems are now considered as a highly promising quantum state for constructing qubit, because of easy on-demand generation and low photon­photon interaction [1]. While polarization-entangled biphotonic states have been successfully demonstrated by using diamond-shaped four level systems with degenerated intermediates states in atoms and quantum dots and verified through the violation of Bell inequality [2,3], quantifying spectrally entangled photon pairs in experiments is still hardly available even with state-of-art techniques. In this theoretical study, we investigated the possibility of the cross correlation function of two photons as an observable for verifying the entanglement. To calculate the cross photon density-density correlation function and the degree of entanglement, Heisenberg's quantum kinetic equations of motion for single electron interacting with photons and two coherent pump fields are derived within the framework of density matrix formalism. In figure 1 (a), the energy configuration of a three-level light emitter is depicted with specially focused observables such as photon number densities $\langle C_{k}^{\dagger} C_k \rangle$ and $\langle C_{q}^{\dagger} C_q \rangle$ and their normalized cross correlation function $g^{(2)}_{12}(t)= \langle c_{k}^{\dagger}c_{q}^{\dagger} c_{q}c_{k} \rangle / \langle c_{k}^{\dagger} c_{k} \rangle \langle c_{q}^{\dagger} c_q \rangle$ . We compared (2) the temporal behavior of $g^{(2)}_{12}(t)$ and the entanglement criterion D(t) introduced by Duan et al. [4] and find peculiar similarities between them for coherently generated photon pairs, as presented in figure (b).