Abstract Quantum teleportation fundamentally depends on shared quantum correlations between spatially separated parties. Although entanglement is widely recognized as the primary resource enabling teleportation, quantum discord represents a more general form of nonclassical correlations that can persist even when entanglement is absent. We provide a thorough analysis combining analytical and numerical evaluations of quantum discord, negativity, and concurrence across varying noise parameters. The study includes simulations under both ide-alized and realistic noisy conditions, incorporating depolarizing noise models alongside experimentally relevant constraints. To improve quantum fidelity and accurately characterize quantum correlations in noisy environments, advanced error mitigation techniques—such as measurement error correction and zero-noise extrapolation—are applied. In this study, we focus on teleportation protocols utilizing two-qubit Werner states, which constitute a significant class of noisy mixed states modeling imperfect entanglement. We systematically examine how the teleportation fidelity varies as a function of the Werner mixing parameter λ and investigate its relationship with the corresponding quantum discord. Our findings indicate that quantum discord remains strictly greater than zero even in parameter regions where entanglement vanishes, emphasizing the role of discord in sustaining quantum advantages in teleportation schemes exposed to noise and imperfections. This highlights the importance of considering non-entanglement quantum correlations for a comprehensive understanding of teleportation resources in realistic settings.
Abstract Quantum teleportation fundamentally depends on shared quantum correlations between spatially separated parties. Although entanglement is widely recognized as the primary resource enabling teleportation, quantum discord represents a more general form of nonclassical correlations that can persist even when entanglement is absent. We provide a thorough analysis combining analytical and numerical evaluations of quantum discord, negativity, and concurrence across varying noise parameters. The study includes simulations under both ide-alized and realistic noisy conditions, incorporating depolarizing noise models alongside experimentally relevant constraints. To improve quantum fidelity and accurately characterize quantum correlations in noisy environments, advanced error mitigation techniques—such as measurement error correction and zero-noise extrapolation—are applied. In this study, we focus on teleportation protocols utilizing two-qubit Werner states, which constitute a significant class of noisy mixed states modeling imperfect entanglement. We systematically examine how the teleportation fidelity varies as a function of the Werner mixing parameter λ and investigate its relationship with the corresponding quantum discord. Our findings indicate that quantum discord remains strictly greater than zero even in parameter regions where entanglement vanishes, emphasizing the role of discord in sustaining quantum advantages in teleportation schemes exposed to noise and imperfections. This highlights the importance of considering non-entanglement quantum correlations for a comprehensive understanding of teleportation resources in realistic settings.
Quantum discord is a prominent measure of quantum correlations, playing an important role in expanding its horizon beyond entanglement. Here we provide an operational meaning of (geometric) discord, which quantifies the amount of nonclassical correlations of an arbitrary quantum system based on its minimal distance from the set of classical states, in terms of teleportation fidelity for general two-qubit and ($d\ensuremath{\bigotimes}d$)-dimensional isotropic and Werner states. A critical value of the discord is found beyond which the two-qubit state must violate Bell's inequality. This is illustrated by an open-system model of a dissipative two-qubit state. For the ($d\ensuremath{\bigotimes}d$)-dimensional states the lower bound of discord is shown to be obtainable from an experimentally measurable witness operator.
In distributed quantum computing, quantum networks provide a potential infrastructure for long-range quantum information processing. Controlled quantum teleportation (CQT) is a key practically relevant protocol that can connect two parties on a network and enables a third party, the controller to allow or abort the secure teleportation of an arbitrary single-qubit state between the two parties. We investigate the robustness of a three-qubit GHZ resource under bit-flip, phase-flip, depolarizing, and amplitude-damping noise. Using the Kraus-operator formalism, we characterize the noisy resource through multipartite quantum discord, genuine multipartite concurrence (GMC), localizable entanglement (LE), and CQT fidelity. We also outline a proof-of-concept photonic implementation for preparing the three-qubit GHZ resource and realizing the CQT protocol, which could enable experimental investigation of the effects of noise. Our results show that bit-flip and amplitude-damping noise generally preserve useful mutipartite quantum correlations over broader noise parameter ranges. Interestingly, multipartite discord exhibits freezing under bit-flip noise over a finite parameter regime. Moreover, discord can persist beyond the sudden death of genuine multipartite entanglement, revealing the distinct robustness of nonclassical correlations. We further demonstrate that CQT fidelity which characterizes the performance of CQT can exceed the classical threshold even after GMC vanishes, while LE more closely reflects the operational usefulness of the noisy resource. These findings highlight the complementary roles of quantum correlations and entanglement in assessing the resilience of GHZ-based protocols.
Abstract Quantum key distribution (QKD) facilitates secure communication by harnessing non-classical correlations within quantum systems. Although entanglement is fundamental to many QKD protocols, it is highly susceptible to noise and decoherence effects. In this work, we analyze the performance of Werner and Bell-diagonal states within commonly used QKD protocols such as BB84, Six-state, and their device-independent variants, considering realistic noise environments and eavesdropping strategies. Employing a Qiskit-based simulation toolkit , we assess critical performance metrics including quantum bit error rate (QBER), fidelity, concurrence, quantum discord, violations of the CHSH inequality, and Devetak, Winter key rates. Our findings emphasize the resilience of quantum discord as a resource in scenarios where entanglement is diminished, and reveal that Bell-diagonal states may exhibit superior performance compared to Werner states under certain noise conditions. We further discuss practical consequences for secure key generation and propose directions for extending this work toward physical device implementation and machine learning–enhanced detection of eavesdropping attempts. These results shed light on the robustness of QKD protocols that utilize mixed quantum states and the significant role quantum discord can play in secure quantum communication.
We study nonclassical correlations beyond entanglement in a family of two-mode non-Gaussian states which represent the continuous-variable counterpart of two-qubit Werner states. We evaluate quantum discord and other quantumness measures obtaining exact analytical results in special instances, and upper and lower bounds in the general case. Non-Gaussian measurements such as photon counting are in general necessary to solve the optimization in the definition of quantum discord, whereas Gaussian measurements are strictly suboptimal for the considered states. The gap between Gaussian and optimal non-Gaussian conditional entropy is found to be proportional to a measure of non-Gaussianity in the regime of low squeezing, for a subclass of continuous-variable Werner states. We further study an example of a non-Gaussian state which is positive under partial transposition, and whose nonclassical correlations stay finite and small even for infinite squeezing. Our results pave the way to a systematic exploration of the interplay between nonclassicality and non-Gaussianity in continuous-variable systems, in order to gain a deeper understanding of---and to draw a bigger advantage from---these two important resources for quantum technology.
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.
Abstract The significance of photon addition in engineering the single‐ and two‐mode (bipartite correlations) nonclassical properties of a quantum state is investigated. Specifically, the behavior of the Wigner function of two quasi‐Werner states constructed by superposing two normalized bipartite m‐photon added coherent states are analyzed. This allows the authors' to quantify the nonclassicality present in the quantum states using Wigner logarithmic negativity (WLN), while quantum correlations are measured using concurrence, entanglement of formation, and quantum discord. The WLN for a two‐mode state corresponds to the sum of the single‐mode nonclassicality and quantum correlations, and both of these are observed to enhance with photon addition manifesting the efficacy of photon addition in the entanglement distillation. Usefulness of photon addition is further established by showing that the performance of the quasi‐Werner states as quantum channel for the teleportation of single‐mode coherent and squeezed states, as quantified via teleportation fidelity, improves with the photon addition. The non‐Gaussian operation is shown effective in stalling the detrimental effects of transmission through noisy channel and/or inefficient detection under realistic scenario. Further, in contrast to a set of existing results, it is established that the negative values of two‐mode Wigner function is not a witness of quantum correlation.