  We present for the first time, a bidirectional Quantum Key Distribution
protocol with minimal encoding operations derived from the use of two
`nonorthogonal' unitary transformations selected from two mutually unbiased
unitary bases; which are indistinguishable in principle for a single use. Along
with its decoding procedure, it is a stark contrast to its `orthogonal
encoding' predecessors. Defining a more relevant notion of security threshold
for such protocols, the current protocol outperforms its predecessor in terms
of security as the maximal amount of information an eavesdropper can glean is
essentially limited by the indistinguishability of the transformations. We
further propose adaptations for a practical scenario and report on a proof of
concept experimental scheme based on polarised photons from an attenuated
pulsed laser for qubits, demonstrating the feasibility of such a protocol.
