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arXiv:2609.01988

Contrasting Effects of Control on Fidelity and Fidelity Deviation in Controlled Teleportation

Jeonghyeon Shin, Minjin Choi

quant-phSubmitted Sep 2, 2026• Announced Sep 3, 2026• 0 citations
arXivHTMLPDF

Abstract

Teleportation performance is commonly characterized by the average teleportation fidelity, while its variation over input states provides additional information captured by the fidelity deviation. In controlled teleportation, the controller's measurement introduces an additional source of fidelity variation through its measurement outcomes. We investigate the fidelity deviation in controlled teleportation with three-qubit pure states. We derive a lower bound on the fidelity deviation and show that it is attainable together with the maximal average teleportation fidelity. Among the measurements attaining the maximal average fidelity, however, the fidelity deviation can vary depending on the controller's measurement. We further examine the effect of controller assistance by comparing controlled teleportation with direct teleportation using the reduced state. Unlike the maximal average fidelity, which cannot decrease with controller assistance, the minimal fidelity deviation is not necessarily reduced by controller assistance. In particular, for any W-class pure state, controller assistance cannot reduce the fidelity deviation. These results reveal contrasting effects of control on teleportation fidelity and its deviation in controlled teleportation.

Comments
7 pages, 2 figures

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Teleportation performance is commonly characterized by the average teleportation fidelity, while its variation over input states provides additional information captured by the fidelity deviation. In controlled teleportation, the controller's measurement introduces an additional source of fidelity variation through its measurement outcomes. We investigate the fidelity deviation in controlled teleportation with three-qubit pure states. We derive a lower bound on the fidelity deviation and show that it is attainable together with the maximal average teleportation fidelity. Among the measurements attaining the maximal average fidelity, however, the fidelity deviation can vary depending on the controller's measurement. We further examine the effect of controller assistance by comparing controlled teleportation with direct teleportation using the reduced state. Unlike the maximal average fidelity, which cannot decrease with controller assistance, the minimal fidelity deviation is not necessarily reduced by controller assistance. In particular, for any W-class pure state, controller assistance cannot reduce the fidelity deviation. These results reveal contrasting effects of control on teleportation fidelity and its deviation in controlled teleportation.
We consider the controlled quantum teleportation performed on a single copy of entangled three-qubit pure states and show how the fidelity of teleportation and the control power are related to the tripartite entanglement measures, namely, three-tangle and genuine concurrence. We characterize the states with extreme properties, and use them to derive tight lower and upper bounds on both the teleportation fidelity and control power for a given amount of entanglement. Furthermore, we discuss the usefulness of these two quantities for experimental detection and/or quantification of tripartite entanglement. In particular, we present the results which imply that the control power cannot be considered as a candidate for a degree of three-qubit entanglement complementary to the three-tangle as conjectured recently [K. Jeong, J. Kim, and S. Lee, Phys. Rev. A 93, 032328 (2016)]. Furthermore, we prove a mutual relation between teleportation fidelity and control power which provides attainable limits of the controller's authority for existing controlled teleportation schemes. Although those relations are derived for pure states, their extensions for mixed states are also examined.
In this work, we investigate controlled quantum teleportation in the presence of noisy channels acting on the three-qubit resource state. We employ a series of generalized noisy channels that bridge the dephasing channels and amplitude damping channels while encompassing extensive intermediate scenarios. We provide an in-depth analysis of the degradation of the maximal average fidelity and the optimal average fidelity in controlled quantum teleportation induced by such noisy channels by deriving the analytical expression and examining several special cases. The analytical expression shows that attaining the optimal average fidelity requires Charlie's cooperation in performing a measurement at suitably chosen angles, and is also related to the initial state and the channel parameters. Our analysis reveals that the optimal average fidelity does not always decrease monotonically with the evolution parameter, instead, it first decreases and then increases. This non-monotonic behavior depends on the entanglement of the initial resource state, as well as on the parameters of the channel traversed by the first qubit.
Performance of quantum teleportation is typically measured by the average fidelity, an overlap between the input and output states. Along with the first moment, we introduce the second moment of fidelity in continuous-variable (CV) teleportation, i.e., the fidelity deviation as the figure of merit to assess the protocol's efficiency. We show that CV states, both Gaussian and non-Gaussian, can be better characterized by considering both average fidelity and fidelity deviation, which is not possible with only average fidelity. Moreover, we shed light on the performance of the teleportation protocol in two different input scenarios: One is when input states are sampled from constrained uniform distribution while the other one is Gaussian suppression of the input states which again leads to a different classification of CV states according to their performance. The entire analysis is carried out in noiseless and noisy scenarios with noise being incorporated in the measurement and the shared channels. We also report that one type of noise can make the protocol robust against the other one, which leads to a ``constructive effect,'' and identify the noise models which are responsible for decreasing average fidelity and the increment in fidelity deviation.
Abstract Quantum teleportation with an arbitrary two-qubit state can be appropriately characterized in terms of maximal fidelity and fidelity deviation. The former quantifies optimality of the process and is defined as the maximal average fidelity achievable within the standard protocol and local unitary strategies, whereas the latter, defined as the standard deviation of fidelity over all input states, is a measure of fidelity fluctuations. The maximal fidelity for a two-qubit state is known and is given by a simple formula that can be exactly computed, but no such formula is known for the fidelity deviation. In this paper, we derive an exact computable formula for the fidelity deviation in optimal quantum teleportation with an arbitrary state of two qubits. From this formula, we obtain the dispersion-free condition, also known as the universality condition: the condition that all input states are teleported equally well and provide a necessary and sufficient condition for a state to be both useful (maximal fidelity larger than the classical bound) and universal (zero fidelity deviation). We also show that for any given maximal fidelity, larger than the classical bound, there always exist dispersion-free or universal states and argue that such states are the most desirable ones within the set of useful states. We illustrate these results with well-known families of two-qubit states: pure entangled states, Bell-diagonal states, and subsets of X states.
Abstract We analyze the performance of quantum teleportation in terms of average fidelity and fidelity deviation. The average fidelity is defined as the average value of the fidelities over all possible input states and the fidelity deviation is their standard deviation, which is referred to as a concept of fluctuation or universality. In the analysis, we find the condition to optimize both measures under a noisy quantum channel—we here consider the so-called Werner channel. To characterize our results, we introduce a 2D space defined by the aforementioned measures, in which the performance of the teleportation is represented as a point with the channel noise parameter. Through further analysis, we specify some regions drawn for different channel conditions, establishing the connection to the dissimilar contributions of the entanglement to the teleportation and the Bell inequality violation.
We investigate the influence of amplitude damping at every stage of the unitary operation of controlled teleportation of a qubit by a tripartite W state in the Bloch sphere representation. We use the average fidelity to describe how much information is transferred from the initial state to the teleported state. It is shown that when a depolarized three-particle W state is used as the quantum channel, the average fidelity of teleportation is a function of the decoherence rate p and the depolarizing rate p '. The larger the depolarizing rate is, the smaller the amplitude damping effect on the average fidelity is. Moreover, the average fidelity of teleportation with values larger than 2/3 can be obtained when the values of p and p ' are chosen properly.