The norms of Bloch vectors and classification of four-qudits quantum states
Ming Li, Zong Wang, Jing Wang, Shuqian Shen, Shao-ming Fei · Europhysics Letters (EPL) · 2019
We investigate the norms of the Bloch vectors for any quantum state with subsystems less than or equal to four. Tight upper bounds of the norms are obtained, which can be used to derive tight upper bounds for entanglement measures defined by the norms of Bloch vectors. By using these bounds a trade-off relation of the norms of Bloch vectors is discussed. These upper bounds are then applied on separability. Necessary conditions are presented for different kinds of separable states in four-partite quantum systems. We further present a complete classification of quantum states for four-qudits quantum systems.