Anton Kozubov

We have studied the spectral characteristics of fiber-optical components in the ranges of 400-800 nm and 1500-2100 nm: different types of attenuators, isolators, circulators and wavelength-division multiplexing (WDM) filters were considered for both ranges. Our work has demonstrated that the spectral characteristics of isolators under test significantly depend on temperature, and their changes may result in vulnerabilities.

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.

We investigate multi-pixel quantum random number generators as a Gaussian entropy source. The procedure of randomness extraction implies maximization of the conditional probability as a step of conditional min-entropy estimation, implying an eavesdropper may influence the signal. Usually, it requires to artificially limit the dynamic range of the variable that an eavesdropper may control. We propose the approach that may expel a necessity of this assumption by utilizing Pearson’s goodness-of-fit criterion, such that the histogram of measured outcomes fits well into the dynamical range and well approximated by a Gaussian. The latter greatly simplifies the min-entropy estimations and may be incorporated in security criterion estimation.

We theoretically study dynamical regimes of the observables that govern the operating conditions of continuous variable (CV) quantum key distribution (QKD) systems, depending on quantum-channel-induced intermode interactions. In contrast to the widely used approach, where losses and thermal broadening are introduced through a beamsplitter transformation, our analysis uses the exactly solvable quantum channel model describing the Lindblad dynamics of multimode bosonic systems interacting with a heat bath and additionally takes into account imperfections of the homodyne detection scheme. The analytical results for the photon count difference and the quadrature probability distributions are used to derive the expression for the mutual information between legitimate parties, which explicitly links the information properties of CV QKD and the parameters of the channel. For the important special case of a two-mode photonic system propagating in a fiber channel, the latter can be conveniently parameterized using the frequency and the relaxation rate vectors that characterize the coherent (dynamical) intermode couplings and the incoherent (environment mediated) interaction between the bosonic modes, respectively. It turned out that these vectors determine four qualitatively different dynamical regimes of the mutual information and the phase difference between the signal and the local oscillator that may significantly affect the operation of CV QKD.