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Optics and Spectroscopy

Two-pulse correlations in noisy quantum channels

Alexei Vladimirovich Chizhov, Eduard Schmidt, L. Knöll, D.‐G. Welsch

Published Sep 1, 2001• 1 citation
Publisher

Abstract

The modification of initially entangled light pulses passing through noisy quantum channels, modeled by dispersive and absorbing four-port devices, is studied with the help of recently obtained results on quantum state transformations. The fidelity and indices of quantum correlations based on the von Neumann entropy are calculated. Their dependence on both the pulse shape and the four-port device parameters is studied. It is shown that due to dispersion and absorption, the quantum correlations are reduced, and they decrease with increasing initial entanglement.

DOI
10.1134/1.1405220

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The modification of initially entangled light pulses passing through noisy quantum channels, modeled by dispersive and absorbing four-port devices, is studied with the help of recently obtained results on quantum state transformations. The fidelity and indices of quantum correlations based on the von Neumann entropy are calculated. Their dependence on both the pulse shape and the four-port device parameters is studied. It is shown that due to dispersion and absorption, the quantum correlations are reduced, and they decrease with increasing initial entanglement.
The modification of initially entangled light pulses passing through dispersive and absorbing four-port devices is studied, using recently obtained results on quantum state transformations [Phys. Rev. A 59, 4716 (1999)]. The fidelity and indices of quantum correlations based on the von Neumann entropy are calculated. Their dependence on both the pulse shape and the four-port device parameters is studied. It is shown, that due to dispersion and absorption the quantum correlations are reduced substantially for large initial entanglement.
The problem of decorrelation of entangled (squeezed-vacuum-type) light pulses of arbitrary shape passing through dispersive and absorbing four-port devices of arbitrary frequency response is studied, applying recently obtained results on quantum-state transformation (L Knöll et al 1999 Phys. Rev. A 59 4716). The fidelity and indices of (quantum) correlation based on the von Neumann entropy are calculated, with special emphasis on the dependence on the mean photon number, the pulse shape and the frequency response of the devices. In particular, it is shown that the quantum correlations can decay very rapidly due to dispersion and absorption, and the degree of degradation intensifies with increasing mean photon number.
Dielectric four-port devices play an important role in optical quantum information processing. Since for causality reasons the permittivity is a complex function of frequency, dielectrics are typical examples of noisy quantum channels, which cannot preserve quantum coherence. To study the effects of quantum decoherence, we start from the quantized electromagnetic field in an arbitrary Kramers-Kronig dielectric of given complex permittivity and construct the transformation relating the output quantum state to the input quantum state, without placing restrictions on the frequency. We apply the formalism to some typical examples in quantum communication. In particular we show that for Fock-entangled qubits the Bell-basis states $|{\ensuremath{\Psi}}^{\ifmmode\pm\else\textpm\fi{}}〉$ are more robust against decoherence than the states $|{\ensuremath{\Phi}}^{\ifmmode\pm\else\textpm\fi{}}〉.$
Using the information-theoretic inequality proposed by Braunstein and Caves, we study the evolution of quantum correlation in the presence of different noisy channels. First, we experimentally prepare the high-fidelity two-bit entangled state and simulated the bit-flip and phase-shift noise using an all-optical experimental setup. Then, we study the quantum correlation evolution of a two-bit entangled system using the information difference of entropy under different noisy channels and noisy environments. It is found that the quantum correlation degradation depends on the type of noise and noisy channels. Our work provides a new essential tool for quantum information processing and measurement, and offers new insight into the evolution of quantum correlation in open systems.
Abstract We investigate how the correlations between consecutive actions of the two-qubit noisy channels affect quantum teleportation, entanglement, and Bell nonlocality. For the initial Bell and Werner states used for teleportation, we established connections among these three quantities, and obtained the minimum correlation strengths for achieving the nonclassical teleportation fidelity, nonvanishing entanglement, and Bell-nonlocal states. It was found that these three quantities can be noticeably enhanced due to the existence of the channel correlations. By considering a colored dephasing channel, we further investigated the combined effects of the channel correlation and the (non-)Markovianity of the channel on the teleportation fidelity, entanglement, and Bell nonlocality. The result shows that while they decrease monotonically with time in the Markovian regime, their strengths can be further enhanced due to the non-Markovian memory effect in a wide time region.