Alexei Kiselev

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.

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.