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Journal of Physics B Atomic Molecular and Optical Physics

Asymmetry and decoherence of a quantum channel in the quantum communication system of continuous variables

Masashi Ban

Published Feb 13, 2006• 5 citations
Publisher

Abstract

Influences of an asymmetric noisy quantum channel on continuous variable entanglement and quantum teleportation are investigated. It is shown that when the resource of entanglement is a two-mode squeezed-vacuum state, the amount of the shared entanglement and the performance of the quantum teleportation are greater (smaller) in the symmetric quantum channel than in the asymmetric one if , where and are the average photon numbers of the two-mode squeezed-vacuum state and the thermal noise. When the quantum teleportation is carried out in the asymmetric quantum channel, introducing an additional loss makes better the performance of the quantum teleportation under certain conditions. The introduction of an additional loss makes smaller not only the shared entanglement but also the degree of the channel asymmetry.

DOI
10.1088/0953-4075/39/5/011

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Influences of an asymmetric noisy quantum channel on continuous variable entanglement and quantum teleportation are investigated. It is shown that when the resource of entanglement is a two-mode squeezed-vacuum state, the amount of the shared entanglement and the performance of the quantum teleportation are greater (smaller) in the symmetric quantum channel than in the asymmetric one if , where and are the average photon numbers of the two-mode squeezed-vacuum state and the thermal noise. When the quantum teleportation is carried out in the asymmetric quantum channel, introducing an additional loss makes better the performance of the quantum teleportation under certain conditions. The introduction of an additional loss makes smaller not only the shared entanglement but also the degree of the channel asymmetry.
Symmetric and asymmetric quantum channels which act on bipartite bosonic states are considered. The linear dissipative channel and the quantum teleportation channel are applied. The influences of the symmetric and asymmetric quantum channels on bipartite Gaussian states are investigated by means of the inseparability condition. Furthermore, quantum teleportation and quantum dense coding of continuous variables performed by means of two-mode squeezed-vacuum states under the influence of the noisy quantum channels are discussed.
A quantum channel consisting of continuous variable quantum teleportation based on the standard protocol is investigated. It is shown that a quantum channel for continuous variable teleportation by an aribitrary entangled state is equivalent to a generalized thermalizing channel. We also study the transmission of nonclassical states by a teleportation channel with a two-mode squeezed-vacuum state in a noisy environment. The transmisson performance is evaluated by using the Glauber-Sudarshan P function and the nonclassical depth. Finally, we compare the results obtained to those of a noisy direct transmission channel. It is shown that the teleportation channel performs better than the direct transmission channel in a certain region.
Quantum teleportation of a continuous-variable state is studied for the quantum channel of a two-mode squeezed vacuum influenced by a thermal environment. Each mode of the squeezed vacuum is assumed to undergo the same thermal influence. It is found that when the mixed two-mode squeezed vacuum for the quantum channel is separable, any nonclassical features, which may be imposed in an original unknown state, cannot be transferred to a receiving station. A two-mode Gaussian state, one of which is a mixed two-mode squeezed vacuum, is separable if and only if a positive well-defined P function can be assigned to it. The fidelity of teleportation is considered in terms of the noise factor given by the imperfect channel. It is found that quantum teleportation may give more noise than direct transmission of a field under the thermal environment, which is due to the fragile nature of quantum entanglement of the quantum channel.
Noisy teleportation of nonclassical quantum states via a two-mode squeezed-vacuum state is studied with the completely positive map and the Glauber-Sudarshan P -function. Using the nonclassical depth as a measure of transmission performance, we compare the teleportation scheme with the direct transmission through a noisy channel. The noise model is based on the coupling to the vacuum field. It is shown that the teleportation channel has better transmission performance than the direct transmission channel in a certain region. The bounds for such a region and for obtaining the nonvanished nonclassicality of the teleported quantum states are also discussed. Our model shows a reasonable agreement with the teleportation fidelity observed in the experiment performed by Furusawa et al (1998 Science 282 706). We finally mention the required conditions for transmitting nonclassical features in real experiments.
Quantum teleportation enables transferring quantum states by leveraging entanglement. The distribution of entangled states is challenged by decoherence, which degrades the teleportation performance. Continuous-variable teleportation with coherent-state qubits has gained particular interest because it can provide better performance than discrete-variable teleportation with polarization-encoded qubits and is more resilient to decoherence. However, most existing schemes employ a specific configuration of coherent-state qubits, namely Schrödinger cat states, and assume symmetric photon-loss channels. This work explores the teleportation of qubits based on coherent states through asymmetric quantum channels impaired by photon loss. First, we characterize the teleportation protocol for arbitrary coherent-state qubit configurations considering asymmetric and photon-loss channels. Then, we present case studies under both ideal entanglement distribution and lossy conditions. Results reveal a trade-off between state distinguishability and teleportation fidelity, providing insight into the optimal design of coherent-state qubits in practical teleportation protocols.
Quantum communication networks can be built on quantum teleportation, which is the transmission of an unknown quantum state from a sending station to a remote receiving station supported by entangled states and classical communication. We use a continuous variable two-mode squeezed vacuum state as a resource state for the quantum teleportation. This state is shared by Alice and Bob, and their system comes into contact with a squeezed thermal environment. The conditions for a secure quantum teleportation require a teleportation fidelity larger than 2/3 and two-way steering of the resource state. We investigate the time evolution of the steering and the fidelity of teleportation in order to determine the values of the parameters required for a successful secure quantum teleportation of a coherent Gaussian state. We show that the temperature, dissipation rate and squeezing parameter of the squeezed thermal reservoir limit the feasible duration for secure quantum teleportation, while by increasing the squeezing parameter of the initial state one can effectively expand the temporal range for a successful secure quantum teleportation.