Entanglement swapping

Entanglement swapping is a quantum protocol that enables two particles to become entangled without ever directly interacting. It works by taking two independent entangled pairs — Alice-Photon1 and Bob-Photon2 — and performing a joint measurement on Photons 1 and 2. This measurement projects Alice's and Bob's remaining particles into an entangled state, effectively 'swapping' the entanglement across the pairs.

The protocol requires a Bell State Measurement and classical communication to complete, so it does not violate locality. Entanglement swapping is the core mechanism of quantum repeaters and is essential for building long-distance entanglement in a Quantum internet. Without it, the fragility of quantum states would limit entanglement distribution to distances too short for practical networking.

Entanglement swapping demonstrates that entanglement is a relational property of the measurement process, not a pre-existing bond between particles. The particles are not 'connected' in any spatial sense; they are correlated because the measurement that produced the correlation was performed.