Recently, Hwang et al. [Eur. Phys. J. D. 61, 785 (2011)] and Yuan et al. [Int. J. Theo. Phys. 50, 2403 (2011)] have proposed two efficient protocols of secure quantum communication using 3-qubit and 4-qubit symmetric W state respectively. These two d
ense coding based protocols are generalized and their efficiencies are considerably improved. Simple bounds on the qubit efficiency of deterministic secure quantum communication (DSQC) and quantum secure direct communication (QSDC) protocols are obtained and it is shown that dense coding is not essential for designing of maximally efficient DSQC and QSDC protocols. This fact is used to design maximally efficient protocols of DSQC and QSDC using 3-qubit and 4-qubit W states.
Recently Yang et al. [Int. J. Theo. Phys. 48 (2009) 516] have shown that an unknown qubit can be teleported by using a particular GHZ-like state as quantum channel. However, there are several errors in the calculation which lead to incorrect conclusi
ons. The errors have been indicated and corrected. It is also noted that their scheme and the independently proposed teleportation scheme of Zhang et al. [Int. J. Theo. Phys. 48 (2009) 3331] uses quantum channel from the same family and any state of that family may be used for teleportation.
