Category: fundamentals | Difficulty: intermediate | Qubits: 4 | Gates: 10 | Depth: 8
Entanglement swapping extends entanglement over long distances without direct interaction. Two Bell pairs are created: (q[0],q[1]) and (q[2],q[3]). A Bell measurement on q[1] and q[2] (held by an intermediate node) projects q[0] and q[3] into an entangled Bell state, even though they never interacted. Classical communication of the measurement result allows the receiver to apply corrections. This is the basis of quantum repeaters.
q[0] and q[3] in a Bell state; measurement outcomes correlated
The OpenQASM 2.0 circuit is in circuit.qasm.
OPENQASM 2.0;
include "qelib1.inc";
// Entanglement swapping: q[0]-q[1] and q[2]-q[3] Bell pairs
// then Bell measurement on q[1],q[2] entangles q[0] and q[3]
qreg q[4];
creg c[4];
// Create Bell pair 1: q[0] <-> q[1]
h q[0];
cx q[0],q[1];
// Create Bell pair 2: q[2] <-> q[3]
h q[2];
cx q[2],q[3];
// Bell measurement on q[1] and q[2] (intermediate node)
cx q[1],q[2];
h q[1];
measure q[1] -> c[1];
measure q[2] -> c[2];
// Apply corrections to q[3] based on measurement outcomes
if(c[2]==1) x q[3];
if(c[1]==1) z q[3];
measure q[0] -> c[0];
measure q[3] -> c[3];
entanglement-swapping quantum-repeater bell-measurement fundamentals
MIT — part of the OpenQC Algorithm Catalog.