Fig. 2: State preparation.

a The system is initially prepared in a separable state where qubit Q₁ is prepared in the excited state while all the other qubits Q_j of the quantum chain are in the ground state; external noise ξ(t) is applied to the central spin Q₃. b Quantum circuits used in the tomographic measurement of the two-qubit density matrix ρ₁₅ of the two end spins. c The measured real part of the density matrix ρ₁₅ of the synchronized edge qubits at time Jt = 3π corresponds to a maximally entangled mixed state.