We report a new type of vulnerability in practical implementations of quantum key distribution systems. We show that it is possible to increase the pulse energy of a source laser diode not only by injection-locking it by external light near its emission wavelength of 1550 nm, but also by optically pumping it at a much shorter wavelength. We demonstrate 10% increase in pulse energy when exposing the laser diode to 1310-nm, 1.6-mW cw laser light via its fiber pigtail. This may allow an eavesdropper to steal the secret key. A possible countermeasure is to install broadband optical filters and isolators at the source's output and characterise them during the security certification.
@article{arxiv.2503.11239,
title = {Optical-pumping attack on a quantum key distribution laser source},
author = {Maxim Fadeev and Anastasiya Ponosova and Qingquan Peng and Anqi Huang and Roman Shakhovoy and Vadim Makarov},
journal= {arXiv preprint arXiv:2503.11239},
year = {2025}
}