entanglementswapping

Entanglement swapping is a quantum information protocol that entangles two particles that have not directly interacted. The setup uses two entangled pairs, typically labeled A–B and C–D. A joint measurement on the middle qubits B and C, performed in the Bell basis, projects the outer qubits A and D into an entangled state even though A and D never interacted.

After the Bell-state measurement on B and C, the overall state collapses so that A and D become entangled. The specific entangled state of A and D depends on the measurement outcome of B and C; the result is classically communicated to the parties holding A and D, who may apply local operations to obtain a desired Bell state. The process does not enable faster-than-light signaling; classical communication of the measurement result is required to know or adjust the entangled state.

Entanglement swapping is a foundational tool for quantum communication networks, enabling the linking of short-range entangled segments into longer ones. It is central to quantum repeaters, which aim to distribute entanglement over long distances by chaining swapping operations while mitigating loss and decoherence. In practice, swapping is commonly demonstrated with photons using spontaneous parametric down-conversion, where the Bell-state measurement is typically probabilistic and relies on linear optics.

Key practical considerations include the probabilistic nature of Bell-state measurements, detector efficiency, and photon loss. Fidelity and success rates are affected by imperfect operations and environmental decoherence. To scale swapping into large networks, quantum memories are often envisioned to store intermediate entangled states while subsequent links are established.

Entanglement swapping has been demonstrated in multiple photonic experiments and remains a core technique in the development of quantum communication and networking technologies.