Evgueni Karpov - Academia.edu (original) (raw)

Papers by Evgueni Karpov

Phys Rev a, 2007

We report on the implementation of a reverse-reconciliated coherent-state continuous-variable qua... more We report on the implementation of a reverse-reconciliated coherent-state continuous-variable quantum key distribution system, with which we generated secret keys at a rate of more than 2 kb/s over 25 km of optical fiber. Time multiplexing is used to transmit both the signal and phase reference in the same optical fiber. Our system includes all experimental aspects required for a field implementation of a quantum key distribution setup. Real-time reverse reconciliation is achieved by using fast and efficient LDPC error correcting codes.

We study the short-time behaviour of the survival probability in the framework of the N -level Fr... more We study the short-time behaviour of the survival probability in the framework of the N -level Friedrichs model. We show that depending on initial conditions the decay can be considerably slowed down or even stopped. By choosing proper parameters of the system, the Zeno time can also be considerably extended.

A time superoperator T canonically conjugate to the Liouville superoperator is constructed for a ... more A time superoperator T canonically conjugate to the Liouville superoperator is constructed for a model of unstable particles in the Liouville-von Neumann space of density matrices. While there is no time operator conjugate to the Hamiltonian in the wave function space due to positivity of energy, T may exist in the density matrix space as the spectrum of the Liouvillian covers all the real axIs. This is the first example of an observable that can only be formulated in the Liouville-von Neumann space of density matrices. In our example, the expectation value of T gives the lifetime of the unstable particle. On the basis of T , an entropy superoperator is obtained that manifests the irreversible behavior of unstable systems at the microscopic level.

We study the short-time and medium-time behavior of the survival probability of decaying states i... more We study the short-time and medium-time behavior of the survival probability of decaying states in the framework of the NNN-level Friedrichs model. The degenerated and nearly degenerated systems are discussed in detail. We show that in these systems decay can be considerably slowed down or even stopped by appropriate choice of initial conditions. We analyze the behaviour of the multilevel system within the so-called Zeno era. We examine and compare two different definitions of the Zeno time. We demonstrate that the Zeno era can be considerably enlarged by proper choice of the system parameters.

Dynamical Systems and Irreversibility, 2002

Acta Polymerica

A mathematical model is proposed to describe the processes of cooling and relaxation of an orient... more A mathematical model is proposed to describe the processes of cooling and relaxation of an oriented polymer melt on a tempering surface. On the basis of this model an algorithm is established that makes it possible to calculate the temperature distribution in the melt from the given external parameters and characteristics of the material. With the aid of of this algorithm the rate of melt cooling at tempering is evaluated. The relationship between the tempering time and the degree of relaxation of macromolecules under predetermined conditions was established.Ein mathematisches Modell wird vorgeschlagen, das die Prozesse der Abkühl- und Relaxationsprozesse einer orientierten Polymerschmelze an einer Temperierflche beschreibt. Auf Basis dieses Modells wird ein Algorithmus abgeleitet, der es ermöglicht, die Temperaturverteilung in der Schmelze anhand vorgegebener Parameter und der Materialeigen-schaften zu berechnen. Mit Hilfe dieses Algorithmus wird die Abkühlgeschwindigkeit beim Temp...

Quantum Communications Realized, 2007

We report on both theoretical and experimental aspects of a fully implemented quantum key distrib... more We report on both theoretical and experimental aspects of a fully implemented quantum key distribution device with coherent states. This system features a final key rate of more than 2 kb/s over 25 km of optical fiber. It comprises all required elements for field operation: a compact optical setup, a fast secret bit extraction using efficient LDPC codes, privacy amplification

Polymer Engineering and Science, 1993

ABSTRACT This paper reports on the theoretical and experimental studies of structure formation an... more ABSTRACT This paper reports on the theoretical and experimental studies of structure formation and strengthening (stiffening) of flexible-chain polymers. Two techniques of strengthening relying on the melt extrusion, i.e., orientational crystallization (crystallization initiated by melt extension) and drawing (uniaxial stretching of a crystallized polymer) are analysed by theory. The experiments involved preparation and study of melt extruded films and film fibers of linear polyethylene formed by the two techniques mentioned above. The effect of the degree of orientation and other parameters of the formation processes on the mechanical characteristics and the factors limiting the ultimate values of these characteristics are discussed. It is shown that multistage drawing succeeds in achieving a higher tensile strength and elastic modulus (1.2 and 35 GPa, respectively) than the orientational crystallization, which gives 0.8 and 15 GPa. The strengthening by drawing is accompanied by microcrack formation. In contrast, no discontinuities are observed in orientationally crystallized samples up to their ultimate extension.

Physical Review Letters, 2013

We prove the equivalence of an arbitrary single-mode Gaussian quantum channel and a newly defined... more We prove the equivalence of an arbitrary single-mode Gaussian quantum channel and a newly defined fiducial channel preceded by a phase shift and followed by a Gaussian unitary operation. This equivalence implies that the energy-constrained classical capacity of any single-mode Gaussian channel can be calculated based on this fiducial channel, which is furthermore simply realizable with a beam splitter, two identical single-mode squeezers, and a two-mode squeezer. In a large domain of parameters, we also provide an analytical expression for the Gaussian classical capacity, exploiting its additivity, and prove that the classical capacity cannot exceed it by more than 1/ ln 2 bits.

Physical Review A, 2001

We analyze the short-time behavior of the survival probability in the frame of the Friedrichs mod... more We analyze the short-time behavior of the survival probability in the frame of the Friedrichs model for different formfactors. We have shown that this probability is not necessary analytic at the time origin. The time when the quantum Zeno effect could be observed is found to be much smaller than usually estimated. We have also studied the anti-Zeno era and have estimated its duration.

Physical Review A, 2009

We address the classical capacity of a quantum bosonic memory channel with additive noise, subjec... more We address the classical capacity of a quantum bosonic memory channel with additive noise, subject to an input energy constraint. The memory is modeled by correlated noise emerging from a Gauss-Markov process. Under reasonable assumptions, we show that the optimal modulation results from a "quantum water-filling" solution above a certain input energy threshold, similar to the optimal modulation for parallel classical Gaussian channels. We also derive analytically the optimal multimode input state above this threshold, which enables us to compute the capacity of this memory channel in the limit of an infinite number of modes. The method can also be applied to a more general noise environment which is constructed by a stationary Gauss process. The extension of our results to the case of broadband bosonic channels with colored Gaussian noise should also be straightforward.

Physical Review A, 2002

ABSTRACT

Physical Review A, 2000

In a series of interesting papers G. C. Hegerfeldt has shown that quantum systems with positive e... more In a series of interesting papers G. C. Hegerfeldt has shown that quantum systems with positive energy initially localized in a finite region, immediately develop infinite tails. In our paper Hegerfeldt's theorem is analysed using quantum and classical wave packets. We show that Hegerfeldt's conclusion remains valid in classical physics. No violation of Einstein's causality is ever involved. Using only positive frequencies, complex wave packets are constructed which at t = 0 are real and finitely localized and which, furthemore, are superpositions of two nonlocal wave packets. The nonlocality is initially cancelled by destructive interference. However this cancellation becomes incomplete at arbitrary times immediately afterwards. In agreement with relativity the two nonlocal wave packets move with the velocity of light, in opposite directions.

Physical Review A, 2012

A non trace-preserving map describing a probabilistic but heralded noiseless linear amplifier has... more A non trace-preserving map describing a probabilistic but heralded noiseless linear amplifier has recently been proposed and experimentally demonstrated. Here, we exhibit another remarkable feature of this peculiar transformation, namely its ability to serve as a universal single-mode squeezer regardless of the quadrature that is initially squeezed. Hence, it acts as an heralded phase-insensitive optical squeezer, conserving the signal-to-noise ratio just as a phase-sensitive optical amplifier but for all quadratures at the same time, which may offer new perspectives in quantum optical communications. Although this ability to squeeze all quadratures seemingly opens a way to instantaneous signaling by circumventing the quantum no-cloning theorem, we explain the subtle mechanism by which the probability for such a causality violation vanishes, even on an heralded basis.

Physical Review A, 2005

We consider two three-dimensional isotropic harmonic oscillators interacting with the quantum ele... more We consider two three-dimensional isotropic harmonic oscillators interacting with the quantum electromagnetic field in the Coulomb gauge and within dipole approximation. Using a Bogoliubovlike transformation, we can obtain transformed operators such that the Hamiltonian of the system, when expressed in terms of these operators, assumes a diagonal form. We are also able to obtain an expression for the energy shift of the ground state, which is valid at all orders in the coupling constant. From this energy shift the nonperturbative Casimir-Polder potential energy between the two oscillators can be obtained. When approximated to the fourth order in the electric charge, the well-known expression of the Casimir-Polder potential in terms of the polarizabilities of the oscillators is recovered.

Physical Review A, 2006

We study the capacity of d-dimensional quantum channels with memory modeled by correlated noise. ... more We study the capacity of d-dimensional quantum channels with memory modeled by correlated noise. We show that, in agreement with previous results on Pauli qubit channels, there are situations where maximally entangled input states achieve higher values of mutual information than product states. Moreover, a strong dependence of this effect on the nature of the noise correlations as well as on the parity of the space dimension is found. We conjecture that when entanglement gives an advantage in terms of mutual information, maximally entangled states saturate the channel capacity.

Physical Review A, 2007

Open Systems & Information Dynamics, 2006

We consider explicitly two examples of d-dimensional quantum channels with correlated noise and s... more We consider explicitly two examples of d-dimensional quantum channels with correlated noise and show that, in agreement with previous results on Pauli qubit channels, there are situations where maximally entangled input states achieve higher values of the output mutual information than product states. We obtain a strong dependence of this effect on the nature of the noise correlations as well as on the parity of the space dimension, and conjecture that when entanglement gives an advantage in terms of mutual information, maximally entangled states achieve the channel capacity.