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Acta Physica Sinica

Teleportation of a two-mode Gaussian state through double two-mode-squeezed-state quantum channels

Qian Zhang, Li Fu-Li, Hongrong Li, (1)西安交通大学应用物理系,西安 710049; (2)西安交通大学应用物理系,西安 710049;西安邮电学院,西安 710061

Published Jan 1, 2006• 11 citations
Publisher

Abstract

Teleportation of a general two-mode Gaussian state through double two-mode squeezed state quantum channels is studied. It is found that the quantum channels must be enough entangled for the output state to be inseparable. The required minimum entanglement for the quantum channels to maintain entanglement in the output state depends on the entanglement of the input state when the input state is pure. The minimum entanglement depends not only the on entanglement but also on the global purity of the input state when the input state is mixed.

DOI
10.7498/aps.55.2275

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Teleportation of a general two-mode Gaussian state through double two-mode squeezed state quantum channels is studied. It is found that the quantum channels must be enough entangled for the output state to be inseparable. The required minimum entanglement for the quantum channels to maintain entanglement in the output state depends on the entanglement of the input state when the input state is pure. The minimum entanglement depends not only the on entanglement but also on the global purity of the input state when the input state is mixed.
Entanglement swapping of two-mode Gaussian states is investigated by use of quantum teleportation protocol of continuous variables. In the entanglement swapping protocol, we take the two-mode squeezed vacuum which is exposed in a thermal environment as a mixed-quantum-channel state and a two-mode standard form Gaussian state as an input state to be teleported. When the input state is pure, we find that the swapped state is inseparable if and only if both the input state and the quantum-channel state are entangled. When the input state is mixed and entangled, we find that the swapped state is inseparable only if the quantum-channel state is sufficiently much entangled. We also notice that the required minimum entanglement in the mixed quantum channel to maintain entanglement in the swapped state strongly depends on the global purity of the input state.
Abstract Quantum state teleportation is an important protocol that plays a pivotal role in various quantum information tasks. Here we theoretically investigate quantum state teleportation by exploiting a general two-mode Gaussian entangled state produced by a parametric converter when two single-mode Gaussian states (SMGSs) in terms of the non-classicality and purity are employed as inputs to the parametric converter. In particular, the time-dependent teleportation fidelity is analyzed with respect to the squeezing parameter and phase-space quadratures of the teleported squeezed coherent state. We show that the teleportation fidelity is maximal when ratio of the expectation values of photon number in the two modes of the evolved Gaussian entangled state is equal to 1. Quantum state teleportation in terms of the purity and non-classicality of the general two SMGSs seems to be a good choice for experimental realization of quantum communication and information processing.
The inseparability of photon-added Gaussian states which are generated from two-mode Gaussian states by adding photons is investigated. According to the established inseparability conditions [New J. Phys. 7, 211 (2005); Phys. Rev. Lett. 96, 050503 (2006)], we find that even if a two-mode Gaussian state is separable, the photon-added Gaussian state becomes entangled when the purity of the Gaussian state is larger than a certain value. The lower bound of entanglement of symmetric photon-added Gaussian states is derived. The result shows that entanglement of the photon-added Gaussian states is involved with high-order moment correlations. We find that fidelity of teleporting coherent states cannot be raised by employing the photon-added Gaussian states as a quantum channel of teleportation.
We propose a reliable entanglement measure for a two-mode squeezed thermal state of the quantum electromagnetic field in terms of its Bures distance to the set of all separable states of the same kind. The requisite Uhlmann fidelity of a pair of two-mode squeezed thermal states is evaluated as the maximal transition probability between two four-mode purifications. By applying the Peres-Simon criterion of separability we find the closest separable state. This enables us to derive an insightful expression of the amount of entanglement. Then we apply this measure of entanglement to the study of the Braunstein-Kimble protocol of teleportation. We use as input state in teleportation a mixed one-mode Gaussian state. The entangled state shared by the sender (Alice) and the receiver (Bob) is taken to be a two-mode squeezed thermal state. We find that the properties of the teleported state depend on both the input state and the entanglement of the two-mode resource state. As a measure of the quality of the teleportation process, we employ the Uhlmann fidelity between the input and output mixed one-mode Gaussian states.
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.
Non-Gaussian entangled states of light have been found to improve the success of quantum telepor- tation. Earlier works in the literature focussed mainly on two-mode non-Gaussian states generated by de-Gaussification of two-mode squeezed vacuum states. In the current work, we study quan- tum teleportation with a class of non-Gaussian entangled resource states that are generated at the output of a passive beam splitter (BS) with different input single mode non-Gaussian states. In particular, we consider input states that are generated under successive application of squeezing and photon addition/subtraction operations in various orders. We focus on identifying what attributes of the resource states are necessary or sufficient for quantum teleportation (QT). To this end we first evaluate two attributes considered in the literature, viz. squeezed vacuum affinity (SVA) and EPR correlation. While SVA is not non-zero for all two-mode resource states, EPR correlation is neither necessary nor sufficient of QT. We consider yet another attribute, viz. two-mode quadrature squeezing as defined by Simon et. al. [Phys. Rev. A 49, 1567 (1994)]. Our numerical results on the de-Gaussified two-mode squeezed vacuum state as well as the BS generated non-Gaussian states lead us to the conclusion that two-mode quadrature squeezing is a necessary condition for QT, in general. We further demonstrate the plausibility of this conclusion by giving an analytical proof that two-mode quadrature squeezing is a necessary condition for QT in the case of symmetric two-mode Gaussian resource states.