Communication in quantum networks suffers notoriously from photon loss. Resulting errors can be mitigated with a suitable measurement herald at the receiving node. However, waiting for a herald and communicating the measurement result back to the sender in a repeat-until-success strategy makes the protocol slow and prone to errors from false heralds such as detector dark counts. Here, we implement an entanglement herald at the sending node by employing a cascaded two-photon emission of a single atom into two optical fiber cavities: the polarization of one photon is entangled with the spin of the atom, and the second photon heralds 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.

Photon-pair sources are widely used in quantum optics and quantum information experiments. Despite their broad deployment, there has not yet been an on-demand implementation with efficient into-fiber photon generation and high single-photon purity. Here we report on such a source based on a single atom with three energy levels in ladder configuration and coupled to two optical fiber cavities. We efficiently generate photon pairs with an in-fiber emission efficiency of eta pair = 16(1)% and study their temporal correlation properties. We simulate theoretically a regime with strong atom-cavity coupling and find that photons are directly emitted from the ground state, i.e., without atomic population in any intermediate state. We propose a scenario to observe such a double-vacuum-stimulated effect experimentally. (c) 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

One of the biggest challenges in experimental quantum information is to sustain the fragile superposition state of a qubit(1). Long lifetimes can be achieved for material qubit carriers as memories(2), at least in principle, but not for propagating photons that are rapidly lost by absorption, diffraction or scattering(3). The loss problem can be mitigated with a nondestructive photonic qubit detector that heralds the photon without destroying the encoded qubit. Such a detector is envisioned to facilitate protocols in which distributed tasks depend on the successful dissemination of photonic qubits(4,5), improve loss-sensitive qubit measurements(6,7) and enable certain quantum key distribution attacks(8). Here we demonstrate such a detector based on a single atom in two crossed fibre-based optical resonators, one for qubit-insensitive atom-photon coupling and the other for atomic-state detection(9). We achieve a nondestructive detection efficiency upon qubit survival of 79 +/- 3 per cent and a photon survival probability of 31 +/- 1 per cent, and we preserve the qubit information with a fidelity of 96.2 +/- 0.3 per cent. To illustrate the potential of our detector, we show that it can, with the current parameters, improve the rate and fidelity of long-distance entanglement and quantum state distribution compared to previous methods, provide resource optimization via qubit amplification and enable detection-loophole-free Bell tests.