is also taken as the non-entangled state ${|\uparrow\uparrow\rangle}$. As is evident from the uncontrolled case (bottom curve) , the decoherence time scale without any decoherence suppression is of the order 0.1 in dimensionless units. Turning on the two-qubit UDD control sequence described by Eq. (\[control1\]) for $N=8$, the decoherence (top solid curve) is seen to be greatly suppressed. We have also examined the decoherence suppression using a UDD sequence based on the single-qubit-based intuitive control Hamiltonian $H_{c,\text{single}}$ described by Eq. (\[intu\]). As shown in Fig. 1, $H_{c,\text{single}}$ can only produce unsatisfactory decoherence suppression.

Similar results are obtained in Fig. 2, where we aim to preserve the coherence measure ${\cal P}_{|\Psi\rangle}$ associated with the Bell state defined in Eq. (\[bell\]). Apparently, with the assistance of our two-qubit UDD control sequence, the system is seen to be locked on the Bell state with a fidelity close to unity at all times. Figure 2 also presents the parallel result if the control Hamiltonian is given by $H_{c,\text{single}}$ shown in Eq. (\[intu\]). The drastic oscillation of $F(t)$ in this case indicates that strong population oscillation occurs, thereby demonstrating again the difference between single-qubit decoherence suppression and two-qubit decoherence suppression.

Using the same initial state 