# Indistinguishable Encoding for Bidirectional Quantum Key Distribution: Theory to Experiment

## TL;DR

This paper introduces a novel bidirectional quantum key distribution protocol using indistinguishable nonorthogonal unitary transformations, enhancing security and demonstrating practical feasibility through experimental implementation with polarized photons.

## Contribution

It presents the first protocol employing indistinguishable nonorthogonal unitary transformations for quantum key distribution, with theoretical security analysis and a proof-of-concept experiment.

## Key findings

- Protocol outperforms previous methods in security by limiting eavesdropper information.
- Experimental demonstration using polarized photons confirms practical feasibility.
- Indistinguishability of transformations enhances security against eavesdropping.

## Abstract

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.