(color online) (a) Molecule of crotonic acid ^[28–30] with four fixed-coupling qubits driven by a global rf-magnetic pulse. (b) The fidelity of transferring an unknown quantum state (by a pair of entangling-disentangling operations) from the first qubit C ₁ to the fourth one C ₄ versus . Here, only the influence from the nearest-resonant transitions with the smallest detunings are considered for the typical Rabi frequencies: .