# Preparation-Attack-Immune Quantum Secure Direct Communication Using Two-Fold Photon Degree of Freedom

## TL;DR

This paper introduces a new quantum secure direct communication protocol that remains secure even if an eavesdropper observes all device settings, leveraging two degrees of freedom in a single photon to achieve preparation-attack immunity.

## Contribution

The authors propose a novel preparation-attack-immune QSDC protocol using two degrees of freedom in single photons, enhancing security without extra resources.

## Key findings

- Protocol maintains security even under full device setting observation.
- Utilizes polarization and spatial modes of single photons for security.
- Can be generalized to other single-photon quantum protocols.

## Abstract

Quite recently, enhancing security against device-attack vulnerability has been theoretically challenging but also practically important in quantum cryptographic communication. For dealing with this issue in a general and strict scenario, we design a seemingly-new kind of quantum secure direct communication (QSDC) in a linear-optical regime, which we call "preparation-attack-immune QSDC." We address that in our "naive" analysis, it is quite formidable to extract even a piece of information of the secret message, and any malicious eavesdropping attempts will be unsuccessful. The most remarkable feature is that even in the case where a powerful eavesdropper can peep at all preparation device settings, our protocol still keeps a high level of security without invoking any additional resources and physical properties. This novel advantage that we call "preparation-attack immunity" comes from the simultaneous use of the two degrees of freedom involved in a single-photon (polarization and spatial modes), which enables one to faithfully deal with the single-photon quantum superposition nature. Our idea can be generalized to other single-photon based protocols.