Composing Quantum Key Distribution with Superposition-Based Quantum Image Transmission: Confidentiality Without an Additional Reconstruction-Fidelity Penalty

This paper presents a secure image transmission framework for superposition-based quantum communication systems that integrates quantum key distribution (QKD) with one-time-pad encryption. Existing superposition-based image transmission systems achieve reliable image reconstruction over noisy channels but do not provide confidentiality: an eavesdropper intercepting the transmission can recover the image. To address this limitation, we introduce a key-based security layer and analyse its interaction with the quantum modulation and channel coding components. In the proposed approach, a secret key is generated using the BB84 quantum key distribution protocol and then used to encrypt the image data before Hadamard modulation. The system is evaluated both with and without polar channel coding. Simulation results show that encryption does not degrade image quality at the legitimate receiver: its reconstruction performance is indistinguishable from that of the corresponding unsecured baseline under the evaluated conditions. In contrast, the eavesdropper obtains only noise-like plaintext reconstructions, with an empirical PSNR floor of approximately 8.1 dB, even under more favourable channel conditions. Attempts to intercept the BB84 key-generation process remain detectable through the quantum bit error rate. Polar coding provides approximately 8 dB of performance gain for the legitimate receiver; although it can improve ciphertext recovery, it does not improve the eavesdropper’s plaintext reconstruction under the evaluated conditions. These results demonstrate confidentiality without an additional reconstruction-fidelity penalty, while leaving key-rate and protocol costs as separate practical considerations.