Improved protocols of secure quantum communication using w states

Abstract

Two effective stable quantum communication protocols using 3-qubit and 4-qubit symmetric W states have recently been suggested. These two protocols based on dense coding are extended and their efficiencies are greatly increased. Simplebounds are obtained on the qubit efficiency of determinist safe quantum communication (DSQC) and quantum secure direct communication (QSDC) protocols, and it is shown that dense coding is not sufficient for the design of DSQC and QSDC protocols that are maximally effective. This fact is used to develop DSQC and QSDC protocols that are maximally effective using 3-qubit and 4-qubit W states.