George Miroshnichenko

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.

In this paper we consider the Liouville equation that describes the quantum non-unitary dynamics of quantum states in optical fiber.We consider particular case of thermalization aiming to applications related to various quantum information protocols; however the model can be generalized in various ways taking into account more features (external pump, nonlinear interactions, continuous spectra, free space propagation, etc.). In order to obtain the appropriate evolution models for states in the channel we use the SU(1,1) algebra formalism in the Liouville representation. Developed model is applied in cases of two different initial states as an example. The first is single- and multi-frequency-mode (e.g. wavelength-division-multiplexed) weak coherent states in quantum notation as rather simple but useful example. This particular example is of interest since weak coherent states are commonly used tool in various fields of optics, e.g. optical communication, quantum key distribution, etc. Results for coherent states are well agreed with classical theories however in order to highlight quantum features of developed theory we also consider the case of non-classical light, in particular Fock states. Considered implementation of model takes into account dichroism, retardance, thermalization, dispersion, decoherence in polarization domain. We derive expressions of evolved states, mean photon number and estimate the Stokes parameters as well as degree of polarization. Considered examples explicitly demonstrates all the features and effects of developed approach. Described approach allows to connect the information properties of quantum channels with its physical ones. In order to illustrate this statement we consider BB84 quantum key distribution protocol and investigate behavior of quantum bit error rate affected by considered physical phenomena in optical fiber.

In this paper we present for the first time two possible techniques for deterministic two-step complete Bell state analyzer of optical (polarization) qubits using semi-demolition or entangled non-demolition measurements. Main difference to a prior studies in the field is that we do not use hyperentanglement or representation of the Bell states as concatenated Greenber–Horne–Zeilinger (C-GHZ) state to provide the discrimination. We demonstrate two different approaches for complete Bell state measurement based on different types of filtration. In entangled non-demolition measurement we allocate two pairs of the states from each other as the filtration process. The approach can be based on the utilization of cubic (Kerr) nonlinearity and auxiliary mode. In semi-demolition measurement two states are unambiguously discriminated and hence destroyed; however two other states passes the filter without modification. The measurement destroys the single photon subspace in every mode and preserves the superposition of zero and two photons. It can be realized with discrete photodetection based on microresonator with atoms. Such filtration can be considered as quadratic nonlinearity just as any measurement. The most significant about this approach is that we do not transform the initial states using any type of filtration based on different nonlinearities.