const c = circuit(4);
c.h(0); c.cx(0, 1);
c.h(2); c.cx(2, 3);
// Bell-state measurement on the repeater qubits: undo the entangler.
c.cx(1, 2);
c.h(1);
// Feed-forward corrections onto Bob's qubit (deferred-measurement form):
c.cx(2, 3);   // X correction conditioned on q2
c.cz(1, 3);   // Z correction conditioned on q1
c.measure();
return c;