Short answer: Entanglement swapping is a process in which two particles that have never directly interacted become entangled, by means of a joint measurement on one particle from each of two entangled pairs. It is the mechanism behind quantum repeaters. A 2004 arXiv paper reports what it calls the first experimental realization over large distances in optical fibres, with two photons separated by more than two kilometres of fibre.
Bell states and the measurement
An explainer on the site postquantum.com describes Bell states, also called EPR pairs, as the fundamental two-qubit entangled states that serve as building blocks for distributed entanglement. It says the four maximally entangled two-qubit states form an orthonormal basis, and that entangled photon pairs are a common physical instantiation, often generated through spontaneous parametric down-conversion or similar nonlinear optical processes.
How swapping works
A blog post from Aliro, a quantum networking company, so a vendor's explanation, describes the protocol this way: given two pairs of entangled qubits, A1-A2 and B1-B2, the entanglement can be transferred to A2-B2 by doing a joint measurement, known as a Bell state measurement, on A1-B1. The post says the Bell state in which A1-B1 is measured is the entangled state into which A2-B2 collapses.
The postquantum.com explainer gives the same idea with three nodes in a line. Alice and an intermediate node share one entangled pair, the intermediate node and Charlie share another, and the intermediate node holds one qubit from each pair. It defines entanglement swapping as the process by which two particles that have never interacted become entangled, by means of intermediate entangled pairs and a joint measurement at a relay node.
Why it matters for long distances
The Aliro post says that quantum information is generally transmitted using photons, which experience exponential loss with the length of optical fibre. It says that classically, long links get around loss by using error correction techniques that involve cloning the classical data, for example a repeater that copies and retransmits a signal, but that quantum data cannot be cloned, so a different approach is needed. It says a long-distance entangled pair can be created by doing multiple entanglement swaps on a chain of shorter-distance entangled pairs.
The postquantum.com explainer calls entanglement swapping the fundamental mechanism behind quantum repeaters that allow multi-hop quantum communication, and says it extends entanglement beyond the direct transmission range without trusting intermediate devices. These are explanatory descriptions. This site covers repeaters in a separate article.
An experiment
The 2004 arXiv paper, titled "Long distance entanglement swapping with photons from separated sources", reports the first experimental realization of entanglement swapping over large distances in optical fibres. It says two photons separated by more than two kilometres of fibre were entangled, although they never directly interacted. The authors used two pairs of time-bin entangled qubits created in spatially separated sources and carried by photons at telecommunication wavelengths. A partial Bell state measurement was performed with one photon from each pair, projecting the two remaining photons, formerly independent, onto an entangled state. They report a visibility high enough to violate a Bell inequality after both photons had each travelled through 1.1 km of optical fibre.
What this does not tell you
This article draws on two explainers, one of them from a vendor, and one 2004 experiment paper. It does not describe any current network or product, and the experiment result is a laboratory demonstration from 2004.
FAQ
What is entanglement swapping?
A process by which two particles that have never interacted become entangled, through a joint measurement on one particle from each of two entangled pairs.
What is a Bell state measurement?
A joint measurement on two qubits that projects them onto one of the Bell states. Its outcome determines the state into which the other two qubits collapse, according to the Aliro post.
Why is it linked to quantum repeaters?
The postquantum.com explainer calls it the fundamental mechanism behind quantum repeaters, which allow multi-hop quantum communication.
Has it been shown in experiments?
A 2004 arXiv paper reports an experiment with two photons separated by more than two kilometres of optical fibre, with visibility high enough to violate a Bell inequality.
Does the swapped pair ever interact directly?
No. The experiment paper stresses the two photons never directly interacted.
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