Vladimir Tsvetkov

In this work, we demonstrate a new kind of attack on laser sources in quantum key distribution systems - the optical-pumping attack. We investigated its influence on a single distributed feedback laser diode and an optically injection-locked source configuration. The spectral dependency of this attack was also examined. We managed to increase the energy of emitted pulses using attackers light at several wavelengths. The developed optical-pumping attack should be considered as a possible threat to the security of QKD systems because the increase of pulse energy leads to overestimation of secret key rate between Alice and Bob, giving more information about secret key to Eve.

Quantum key distribution (QKD) technology allows sharing secret keys between two parties over an insecure channel. But there are vulnerabilities in the technical implementation of systems. Laser seeding attack is one of the examples of imperfections in QKD systems. Recent works have demonstrated that Eve can manipulate output power of Alice's laser. This leads to an increase of the average photon number, emitted by Alice. But this attack can be prevented by using passive fiber-optic elements such as isolators or DWDM-filters. In this work, we demonstrate a new kind of attack namely, the optical pumping attack. This attack utilises imperfections in passive optic elements that are used in QKD systems to prevent other types of attack. Eve can use source at different wavelength to seed Alice laser, 1064 nm for example. This radiation would be absorbed by crystal within laser and create additional population inversion to inversion created by bias current, that drives Alice laser. This pumping would change average photon number at the output of Alice, leading to wrong estimation of lower bound on the secret key rate. This creates a side-channel for Eve for obtaining key information. In this work we performed this kind of attack for several wavelengths: 1064 nm, 1310 nm, 1480 nm and 2000 nm, measured changing of pulse energy, average output power and pulse shape under attacks at different wavelengths. Finally, we provide theoretical estimation of required isolation at the tested wavelengths to protect the source against the optical-pumping attack.