Generation and characterization of bound entanglement in optical qubits

For efficient quantum information processing (QIP), pure and strong entanglement in qubits has been thought to be indispensable. However, it was recently pointed out that bound entangled states, which involve undistillable entanglement between qubits, also have great potential for QIP. In this paper, we report the efficient generation of a four-qubit bound entangled state referred to as the Smolin state using photon polarization qubits. We observed the unlockable bound entanglement which is the specific property of the Smolin state.