Vladimir Egorov

We analyze the performance of a quantum repeater scheme that uses multimode Schrödinger cat states. In this scheme, the entanglement generation and swapping protocols utilize entangled cat states produced using electro-optic modulation of single-mode optical cats. In our analysis, we adapt the approach of doubling the number of links that reduces complexity of constructing the final network and simplifies analytical treatment. In order to evaluate the mean waiting time, we employ both the mean-only approximation and exact results and determine the values of the success probabilities of entanglement generation and swapping at which the approximate results are close to the exact ones. Distance dependence of the repeater rate is calculated depending on the number of elementary links. The results are used to perform a comparative analysis of quantum repeater schemes using the modulated cat states and the photon-pair states generated via spontaneous parametric down-conversion. It is shown that the use of the cat states provides several advantages over the photon pairs. Entanglement purification protocols for optical cats are discussed.

The security of quantum key distribution (QKD) systems is constantly being tested, and attacks that utilize probing pulses are have been widely developed recently. One of the most challenging methods is the Trojan-horse attack, which has been successfully demonstrated at 1924 nm. Moreover, the induced-photorefraction attack (IPA) represent attacks that will do better in the visible spectral range. We experimentally investigate the spectral properties of several conventional QKD components. We show that transmission of various fiber optical elements beyond telecommunication range should be taken into account during QKD system design and development due to the possibility for attacks realization.

In the age of measurement-device-independent quantum key distribution (MDI QKD) and twin- field QKD (TF QKD), the source units of these QKD schemes may become a new ``Achilles' heel" of the whole system. An adversary, Eve, can conduct various attacks on the sources by injecting lasers, whose power is limited by the laser-induced damage threshold of the quantum channel. Such an amount of power may modify the characteristics of components in a source. In this work, we study possible components to protect the source from the light-injected attacks, i.e., Trojan-horse attack, the laser-seeding attack, and the laser-damage attack. Experimental testing shows that fiber-optics isolators and circulators are good passive countermeasures because they sacrifice themselves' isolation under a high-power laser to protect other components behind them. Moreover, we find that illuminated by the high-power laser, integrated photonics QKD chips only lose the transmission of the coupler before any other change happens for the other components in the chips.