# Single-photon advantage in quantum cryptography beyond QKD

## TL;DR

Researchers demonstrated a quantum advantage in a secure coin-flipping protocol using single photons, which is important for future quantum internet applications.

## Contribution

The study experimentally shows a quantum advantage in coin flipping using single photons instead of probabilistic light sources.

## Key findings

- A quantum strong coin flipping protocol was implemented using single-photon states.
- The protocol demonstrated a quantum advantage over classical and laser-based implementations.
- A deterministic quantum dot light source and polarization encoding achieved low quantum bit error ratio.

## Abstract

Quantum key distribution (QKD) can be used to establish a secret key between trusted parties. Many practical use-cases in communication networks, however, involve parties who do not trust each other. A fundamental cryptographic building block for such distrustful scenarios is quantum coin flipping, which has been investigated only in few experimental studies to date, all of which used probabilistic quantum light sources imposing fundamental limitations. Here, we experimentally implement a quantum strong coin flipping protocol using single-photon states and demonstrate a quantum advantage compared to both classical realizations and implementations using faint laser pulses. We achieve this by employing a state-of-the-art deterministic quantum dot light source in combination with fast, random polarization-state encoding enabling sufficiently low quantum bit error ratio. By demonstrating a single-photon quantum advantage in a cryptographic primitive beyond QKD, our work represents a major advance towards the implementation of complex cryptographic tasks in a future quantum internet.

Quantum coin flipping is an essential cryptographic building block for distrustful scenarios. Here, the authors implement this protocol using polarisation-encoded single-photon states and show a quantum advantage compared to both classical realizations and implementations using faint laser pulses.