  In this article we present a new prepare and measure quantum key distribution
protocol that uses an experimentally accessible measure of single qubit
contextuality to warranty the security of the quantum channel. The definition
of contextuality used is that due to Spekkens in which any underlying hidden
variable model of the physical system is noncontextual if the probability
distribution of the model's hidden variables is independent of the preparation
and measurement context. Under this more general definition of noncontextuality
the measurement outcomes of a single qubit can be shown to require a contextual
model in order to reproduce the results of some non-projective, positive
operator valued measurements (POVMs). The proposed protocol utilizes a
particular set of POVMs to exhibit the degree of contextuality of the qubits
exiting the QKD system's quantum channel. The contextuality measure is defined
in terms of the weak values of a set of projective measurements that are
implemented as POVMs using weak measurements. The resulting QKD protocol should
have a key rate that is equal to or greater than BB84 since none of the sifted
key is used to test for the presence of an eavesdropper. Additionally, the new
protocol is shown to be immune to detector based attacks.
