  The first two party Quantum Key Agreement (QKA) protocol, based on quantum
teleportation, was proposed by Zhou et al. (Electronics Letters 40.18 (2004):
1149-1150). In this protocol, to obtain the key bit string, one of the parties
use a device to obtain inner product of two quantum states, one being unknown,
and the other one performs Bell measurement. However, in this article, we show
that it is not possible to obtain a device that would output the inner product
of two qubits even when only one of the qubit is unknown. This is so because
existence of such device would imply perfectly distinguishing among four
different states in a two-dimensional vector space. This is not permissible in
quantum mechanics. Furthermore, we argue that existence of such a device would
also imply violation of the "No Signalling Theorem" as well. Finally, we also
comment that this protocol is not a valid key agreement protocol at all.
