### Superactivation of Bound Entanglement  
The four two-qubit maximally entangled Bell states are given as [1]  
\[|\Phi^{\pm}\rangle=\frac{|00\rangle\pm|11\rangle}{\sqrt{2}},\ \ \ |\Psi^{\pm} \rangle=\frac{|01\rangle\pm|10\rangle}{\sqrt{2}}. \tag{1}\]  
For simplicity, we consider the following assignment \(|\Psi_{1}\rangle=|\Phi^{+}\rangle\), \(|\Psi_{2}\rangle=|\Phi^{-}\rangle\), \(|\Psi_{3}\rangle=|\Psi^{+}\rangle\), and \(|\Psi_{4}\rangle=|\Psi^{-}\rangle\). In their work on the family of bound entangled states that can be activated, [PERSON] and [PERSON] presented a four-party state that [PERSON] (A) and [PERSON] (B) are given one of the four Bell states, but they do not know which one [4]. [PERSON] (C) and [PERSON] (D) are also given the same state, and they do not know which one either. This state can be written as  
\[\rho_{ABCD}=\frac{1}{4}\sum_{i=0}^{3}|\Psi_{i}\rangle_{AB}\langle\Psi_{i}| \otimes|\Psi_{i}\rangle_{CD}\langle\Psi_{i}|. \tag{2}\]  
Because neither pair knows which state they possess, they have a maximally mixed state that they cannot use to realize any quantum task such as teleportation. If [PERSON] and [PERSON] can bring their qubits together, due to the orthogonality of Bell states, they can perform a Bell measurement to find out which Bell state they are given. Receiving the measurement result over a classical channel, [PERSON] and [PERSON] learn which Bell state they have. Therefore, their state is now a maximally entangled Bell state. However, in accordance with the notion of quantum networks, if parties remain spatially separated, no entanglement can be distilled from \(\rho_{ABCD}\)[4]. Because this state is entangled but nondistillable, it is a BE state.  
Considering two such BE states shared among five parties, [PERSON] et al. showed that bound entanglement can be superactivated [13]. As illustrated in Figure 1a, the first state is shared among [PERSON] and [PERSON] and [PERSON] and [PERSON], while the second state is shared among [PERSON] and [PERSON] and [PERSON] and [PERSON] (E). Therefore, parties [PERSON], [PERSON], and [PERSON] each have a system of two qubits, while parties [PERSON] and [PERSON] each have a system of one qubit. Denoting the first system as \(\rho_{A_{1}C_{1}B_{1}D}\) and the second system as \(\rho_{A_{2}B_{2}C_{2}E}\), the joint system is  
\[\rho_{init}:=\rho_{A_{1}C_{1}B_{1}D}\otimes\rho_{A_{2}B_{2}C_{2}E}, \tag{3}\]  
which we rewrite for a more intuitive representation considering the qubits held by each party as  
\[\rho:=\rho_{A_{1}A_{2}B_{1}B_{2}C_{1}C_{2}DE}. \tag{4}\]  
Because four-qubit bound entangled states are separable, no entanglement can be distilled from this eight-qubit system \(\rho\), i.e., \(\rho\) is also bound entanglement. However, it was shown in [13] that it can be superactivated by the following simple teleportation procedure.  
1. [PERSON] teleports \(A_{2}\) to [PERSON] by using the \(A_{1}C_{1}\) pair.
2. [PERSON] teleports \(B_{2}\) to [PERSON] by using the \(B_{1}D_{1}\) pair.
3. [PERSON] performs a Bell measurement on the two qubits he now possesses, and communicates the result to [PERSON] and [PERSON].  
Because the \(A_{1}C_{1}\) and \(B_{1}D_{1}\) states are unknown, each teleportation process adds a factor of a Pauli operator to the teleported qubit. However, these factors cancel each other, leaving a known EPR pair between [PERSON] and [PERSON], as shown in Figure 1b. In summary, a two-qubit maximally entangled Bell state can be distilled from an eight-qubit bound entangled state distributed to a quantum network of five nodes through a teleportation mechanism. This process is recognized as the superactivation of bound entanglement, and it was experimentally demonstrated by [PERSON] et al. in a continuous-variable optical system [14]. Four-qubit bound entangled Smolin state, and its entanglement unlocking were also experimentally demonstrated [15].