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International Conference on Telecommunications and Signal Processing (TSP)

Compensating the Cross-Talk in Two-Mode Continuous-Variable Quantum Communication

Vladyslav C. Usenko, Olena Kovalenko, Radim Filip

Published Jul 1, 2018• 1 citation
Publisher

Abstract

We address distribution of two-mode continuous-variable entanglement in the presence of linear cross-talk and channel imperfections. We show how cross-talk degrades entanglement and leads to necessity of optimization of the initial entangled resource. We suggest a method for compensating the cross-talk by a phase-flipping beamsplitter, which can optimally remove the negative effect of cross-talk once the modes undergo almost the same channel attenuation.

DOI
10.1109/tsp.2018.8441510

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We address distribution of two-mode continuous-variable entanglement in the presence of linear cross-talk and channel imperfections. We show how cross-talk degrades entanglement and leads to necessity of optimization of the initial entangled resource. We suggest a method for compensating the cross-talk by a phase-flipping beamsplitter, which can optimally remove the negative effect of cross-talk once the modes undergo almost the same channel attenuation.
Two-mode squeezed states are scalable and robust entanglement resources for continuous-variable and hybrid quantum information protocols that are realized at a distance. We consider the effect of a linear cross talk in the multimode distribution of two-mode squeezed states propagating through parallel similar channels. First, to reduce degradation of the distributed Gaussian entanglement, we show that the initial two-mode squeezing entering the channel should be optimized already in the presence of a small cross talk. Second, we suggest simultaneous optimization of relative phase between the modes and their linear coupling on a receiver side prior to the use of entanglement, which can fully compensate the cross talk once the channel transmittance is the same for all the modes. For the realistic channels with similar transmittance values for either of the modes, the cross talk can be still largely compensated. This method relying on the mode interference overcomes an alternative method of entanglement localization in one pair of modes using measurement on another pair and feed-forward control. Our theoretical results pave the way to more efficient use of multimode continuous-variable photonic entanglement in scalable quantum networks with cross talk.
We address the role of linear inter-mode cross talk in continuous-variable quantum key distribution in the most typical prepare-and-measure scenario. Considering coherent and well as squeezed signal states and either homodyne or heterodyne detection in the receiver station, we show how the cross talk deteriorates the secret key rate and robustness of the protocols to the channel noise. We show how optimization of the signal modulation can improve the performance of the protocols in the presence of cross talk and point out possibilities for further cross talk compensation.
Preserving entanglement during the teleportation of multipartite quantum states is a central challenge in continuous-variable quantum communication, particularly when experimentally feasible finite-dimensional entangled resources are employed. In this work, we investigate whether the entanglement of a two-mode continuous-variable state can be preserved when only one of its modes is teleported through a multimode interferometry-based protocol using a three-dimensional discrete entangled channel. Two representative entangled states, namely the two-mode squeezed vacuum and discrete-modulation states, are considered under both ideal conditions and four standard quantum noise channels: bit-flip, phase-flip, depolarizing, and amplitude-damping. The protocol is evaluated in terms of teleportation fidelity, success probability, logarithmic negativity, Wigner-function evolution, and state purity to provide a comprehensive assessment of both teleportation quality and entanglement preservation. The results show that, in the absence of noise, the protocol preserves most of the initial entanglement while maintaining high teleportation fidelity. Under noisy conditions, the residual entanglement remains remarkably robust against amplitude-damping and phase-flip noise, whereas bit-flip noise causes the most pronounced degradation. The phase-space structure and purity of the teleported states are likewise largely retained over a broad range of noise strengths. These findings demonstrate that multimode interferometry with finite-dimensional entangled channels provides an effective and noise-resilient approach for preserving continuous-variable entanglement during quantum teleportation, offering a practical route toward realistic quantum communication networks.
Preserving entanglement during the teleportation of multipartite quantum states is a central challenge in continuous-variable quantum communication, particularly when experimentally feasible finite-dimensional entangled resources are employed. In this work, we investigate whether the entanglement of a two-mode continuous-variable state can be preserved when only one of its modes is teleported through a multimode interferometry-based protocol using a three-dimensional discrete entangled channel. Two representative entangled states, namely the two-mode squeezed vacuum and discrete-modulation states, are considered under both ideal conditions and four standard quantum noise channels: bit-flip, phase-flip, depolarizing, and amplitude-damping. The protocol is evaluated in terms of teleportation fidelity, success probability, logarithmic negativity, Wigner-function evolution, and state purity to provide a comprehensive assessment of both teleportation quality and entanglement preservation. The results show that, in the absence of noise, the protocol preserves most of the initial entanglement while maintaining high teleportation fidelity. Under noisy conditions, the residual entanglement remains remarkably robust against amplitude-damping and phase-flip noise, whereas bit-flip noise causes the most pronounced degradation. The phase-space structure and purity of the teleported states are likewise largely retained over a broad range of noise strengths. These findings demonstrate that multimode interferometry with finite-dimensional entangled channels provides an effective and noise-resilient approach for preserving continuous-variable entanglement during quantum teleportation, offering a practical route toward realistic quantum communication networks.
Given a certain amount of entanglement available as a resource, what is the most efficient way to accomplish a quantum task? We address this question in the relevant case of continuous variable quantum teleportation protocols implemented using two-mode Gaussian states with a limited degree of entanglement and energy. We first characterize the class of single-mode phase-insensitive Gaussian channels that can be simulated via a Braunstein-Kimble protocol with nonunit gain and minimum shared entanglement, showing that infinite energy is not necessary apart from the special case of the quantum limited attenuator. We also find that apart from the identity, all phase-insensitive Gaussian channels can be simulated through a two-mode squeezed state with finite energy, albeit with a larger entanglement. We then consider the problem of teleporting single-mode coherent states with Gaussian-distributed displacement in phase space. Performing a geometrical optimization over phase-insensitive Gaussian channels, we determine the maximum average teleportation fidelity achievable with any finite entanglement and for any realistically finite variance of the input distribution.