Heralded generation of atom-photon entanglement

Reducing inefficiency and infidelity errors in quantum communication operations is essential in order to implement advanced quantum communication schemes. While heralding photonic quantum states at their reception allows to track and reduce propagation and measurement errors, heralding the generation of such states allows to immediately and faithfully react upon source errors. Here we implement a new method to track and mitigate errors in the generation of atom-photon entanglement. The method is based on the cascaded two-photon emission of a single atom into two crossed fiber cavities. The polarization state of one photon is entangled with the spin degree of freedom of the atom, and the second photon heralds the successful entanglement generation. We show that heralding improves the atom-photon entanglement in-fiber efficiency and fidelity to 68(3)% and 87(2)%, respectively. We highlight the potential of our source for noise-limited long-distance quantum communication by extending the range for constant fidelity or, alternatively, increasing the fidelity for a given distance.