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This study aims to investigates the impact of thermal effects on quantum correlations and entanglement in a two-qubit bipartite system modeled by the Heisenberg XXZ spin chain with Dzyaloshinskii-Moriya (DM) interaction along the Y-axis ( D y ). Using both analytical and numerical methods, our study examines how temperature, the DM interaction parameter D y , and the coupling coefficient J (which represents isotropy of the spin-spin interaction along the X and Y axes) influence entanglement, measured by concurrence, and general quantum correlations, quantified by quantum discord. The results reveal that entanglement and quantum discord are affected by temperature, although quantum discord persists beyond the critical temperature T c where entanglement vanishes. Furthermore, the DM interaction mitigates thermal decoherence, with higher values of D y slowing the decay of quantum correlations as temperature increases. These results are significant for developing thermally resilient quantum systems and advancing quantum information processing. [ABSTRACT FROM AUTHOR]

Quantum discord, a key indicator of nonclassical correlations in bipartite systems, has been recently extended to multipartite scenarios [Phys. Rev. Lett. 2020, 124:110401]. We present exact analytic formulas for the quantum discord of special families of N-qubit states, including generalized class of GHZ states. Our formulations span 2, 3, 4n, 4 n + 1 , 4 n + 2 , and 4 n + 3 -qubit configurations where n ∈ 1 , 2 , ... , which refine the assessment of quantum correlations and provide an analytical tool in quantum computation. Moreover, we uncover a "discord freezing" in even-qubit systems under phase flip decoherence which provides a means for preserving quantum coherence in environmental perturbations. [ABSTRACT FROM AUTHOR]

In this paper, we study the dynamical processes of quantum coherence and correlations for two central qubits system coupled with a transverse Ising spin chain. Suppose the initial state of quantum system is the Werner state and the initial state of environment is the ground state of spin chain, and the corresponding time evolution operator, decoherence factor and reduced density matrix are given. We deduce the analytical expressions of evolution of quantum coherence, entanglement and quantum discord. We find that when the spin chain undergoes quantum phase transition (QPT), the entanglement vanished at time t = t c , and the coherence and discord vanished when the decoherence factor decayed to zero. Besides, we also find that the environmental scale N, coupling strength g and parameter P of Werner state do not change the evolution rules of quantum correlation, but accelerate their decay rate with these parameters' increase. [ABSTRACT FROM AUTHOR]

Correlations may arise in quantum systems through various means, of which the most remarkable one is quantum entanglement. Additionally, there are systems that exhibit nonclassical correlations even in the absence of entanglement. Quantum dissonance refers to how Quantum Discord (QD) — the difference between the total correlation and the classical correlation in a given quantum state — appears as a nonclassical correlation in a system without entanglement. It could be said that QD has the potential to provide a more inclusive viewpoint for discerning the nonclassical correlations. In this paper, we address the problem of manipulating the QD between two subsystems through local operations. We propose two explicit procedures for obtaining separable Werner states, a type of mixed state with nonzero QD. Both approaches involve performing local operations on classically correlated states and offers a step-by-step method for obtaining separable Werner states with nonzero discord, providing an alternative (explicit and user-friendly) to existing methods. [ABSTRACT FROM AUTHOR]

Quasi-equilibrium states that can be prepared in solids through Nuclear Magnetic Resonance (NMR) techniques are out-of-equilibrium states that slowly relax towards thermodynamic equilibrium with the lattice. In this work, we use the quantum discord dynamics as a witness of the quantum correlation in this kind of state. The studied system is a set of dipole interacting spin pairs whose initial state is prepared with the NMR Jeener–Broekaert pulse sequence, starting from equilibrium at high temperature and high external magnetic field. It then evolves as an open quantum system within two consecutive dynamic scenarios: adiabatic decoherence driven by the coupling of the pairs to a common phonon field, described within a non-Markovian approach, and spin–lattice relaxation represented by the high-temperature limit of the Born–Markov master equation, and driven by thermal fluctuations. In this way, the studied model is endowed with the dynamics of a realistic solid sample. The quantum discord rapidly increases during the preparation of the initial state, escalating several orders of magnitude compared with thermal equilibrium at room temperature. During decoherence—despite the decay of coherences—the quantum discord oscillates upon this high value, holding the same value as the initial state. Finally, the quantum discord dissipates within a time scale shorter than but comparable to spin–lattice relaxation. [ABSTRACT FROM AUTHOR]

Quantum discord goes beyond entanglement and exists in a wide range of quantum states that may be separable, playing a crucial role in quantum information tasks. In this paper, we firstly proposed a zero-discord criterion for two-qubit system based on the partial transposition of density matrix, and then extended it to the qubit–qudit system. By detailed examples we demonstrate the effectiveness of these criteria in detecting discord. Moreover, we provide an analytical lower bound of geometric quantum discord(GQD) using eigenvalue vectors of density matrix. Finally, we presented a one-way work deficit lower bound based on our lower bound of GQD.

In this paper, we investigate the quantum discord for a family of three-qubit extended X-states and give analytic formulas of the quantum discord for three-qubit states in several cases, including two zero quantum discord cases and the Werner-GHZ state case. Moreover, we inspect the dynamic behavior of the quantum discord for the three-qubit states under the phase damping channel. It is shown that the frozen phenomenon of quantum discord exists for some states under phase damping channel, while it does not exist for some other states. In particular, the quantum discord of the Werner-GHZ state monotonically decreases to zero under the phase damping channel. [ABSTRACT FROM AUTHOR]

We study the dynamics behavior of a two atomic qubits (two two-level atoms) exposed to a dipole–dipole interaction (DDI) under rotating wave approximation (RWA). Each atom resonantly interacts with the vacuum cavity field via one-photon hopping. We assume that the two atoms and field are initially prepared in their excited state and vacuum state, respectively. An exact solution of Milburn equation is given, and then we study a non-classical statistical properties in the full range of decoherence parameters. In the absence and presence of intrinsic decoherence, the influence of dipole–dipole interaction (DDI) on the degree of entanglement between atom-field and atom-atom are evaluated through different entanglement measures, namely, von Neumann reduced entropy and concurrence, respectively. The influence of DDI on geometrical quantum discord (GQD) are discussed. Furthermore, we estimate the sensitive of the atomic system under variation of DDI parameter via the quantum Fisher information. [ABSTRACT FROM AUTHOR]

We study the dynamics of Wigner function, quantum discord, entanglement and quantum dissonance for bipartite Bell-diagonal states independently interacting with dephasing reservoirs. We analyze comparatively their behavior under the decoherence effect. Our main goal is to test the ability of Wigner function negativity to measure all kinds of quantum correlations in such system. Particularly, we find that this negativity can only capture entanglement and discord. Of course it succeeded in predicting the behavior of quantum dissonance, but nevertheless, it failed to detect its presence. As a matter of fact, we conclude that Wigner function negativity cannot be used as measure of quantum dissonance in the two qubit open system. [ABSTRACT FROM AUTHOR]

We investigate the performance of Quantum Fisher Information (QFI) and Quantum Discord (QD) in a two-qubit spin system with Dzyaloshinskii-Moriya (DM) interaction in the presence of an external magnetic field. The current dynamical maps of QFI and QD are investigated by characterizing these functions of the DM interaction, magnetic field strength, and temperature. Parameter optimization can achieve maximal and prolonged QFI and QD under DM interaction in a two-qubit spin chain. The QFI and QD decay monotonically, with no signs of revival, implying they are permanently lost. Although the effects of magnetic field and temperature are both degrading, we find that their consequences can be controlled by increasing the number of DM interactions available, resulting in longer QD and QFI preservation. [ABSTRACT FROM AUTHOR]

In quantum mechanics, quantum discord is a valuable quantum resource. As a generalization of the initial quantum discord, the multipartite quantum discord (MQD) exhibits excellent properties in the complete monogamy relation. In this paper, the dynamics of MQD are investigated for the Greenberger–Horne–Zeilinger (GHZ) state in Markovian environments. More specifically, it indicates that MQD of the GHZ state drops significantly over time in the depolarizing noise channel. Interestingly, quantum decoherence has no influence on MQD of the GHZ state in the Pauli channel σ z . Subsequently, the analytical formulas of MQD are derived with six parameters in the Pauli channels σ x and σ y , respectively. This is conducive to more efficient use of the GHZ state for various quantum tasks. [ABSTRACT FROM AUTHOR]

Entanglement is a uniquely quantum resource giving rise to many quantum technologies. It is therefore important to detect and characterize entangled states, but this is known to be a challenging task, especially for multipartite mixed states. The correlation minor norm (CMN) was recently suggested as a bi-partite entanglement detector employing bounds on the quantum correlation matrix. In this paper, we explore generalizations of the CMN to multipartite systems based on matricizations of the correlation tensor. It is shown that the CMN is able to detect and differentiate classes of multipartite entangled states. We further analyze the correlations within the reduced density matrices and show their significance for entanglement detection. Finally, we employ matricizations of the correlation tensor for introducing a measure of global quantum discord. [ABSTRACT FROM AUTHOR]

Nitrogen-Vacancy (NV) centers in diamonds provide a room-temperature platform for various emerging quantum technologies, e.g., the long nuclear spin coherence times as potential quantum memory registers. We demonstrate a freezing protocol for an NV center to isolate its intrinsic nuclear spin from a noisy electromagnetic environment. Any initial state of the nuclear spin can be frozen when the hyperfine-coupled electron and nuclear spins are simultaneously driven with unequal Rabi frequencies. Through numerical simulations, we show that our protocol can effectively shield the nuclear spin from strong drive or noise fields. We also observe a clear suppression of quantum correlations in the frozen nuclear spin regime by measuring the quantum discord of the electron–nuclear spin system. These features can be instrumental in extending the storage times of NV nuclear spin-based quantum memories in hybrid quantum systems. [ABSTRACT FROM AUTHOR]

Spin qubits are at the heart of technological advances in quantum processors and offer an excellent framework for quantum information processing. This work characterizes the time evolution of coherence and nonclassical correlations in a two-spin XXZ Heisenberg model, from which a two-qubit system is realized. We study the effects of intrinsic decoherence on coherence (correlated coherence) and nonclassical correlations (quantum discord), taking into consideration the combined impact of an external magnetic field, Dzyaloshinsky–Moriya (DM) and Kaplan–Shekhtman–Entin–Wohlman–Aharony (KSEA) interactions. To fully understand the effects of intrinsic decoherence, two extended Werner-like (EWL) states were considered in this work. The findings indicate that intrinsic decoherence leads to a decay in the quantum coherence and quantum correlations and that their behavior depends strongly on the initial EWL state parameters. Likewise, we found that the robustness of correlated coherence and quantum discord can be controlled through an appropriate choice of the initial state. These findings give us insights into engineering a quantum system to achieve quantum advantages. [ABSTRACT FROM AUTHOR]

Based on Hubbard model with the Hartree-Fock approximation, we study the properties of quantum discord (QD) between the nearest-neighbor sites A and B in zigzag graphene nanoribbons thermalized with a reservoir at temperature T. Several influences of the site position, on-site Coulomb repulsion U , temperature, and ribbon width on QD are discussed in detail. The results show that QD is robust against thermal fluctuations, and QD for the leg pairs along the zigzag chain near ribbon edges is always larger than that for the rung pairs linking two adjacent zigzag chains. QD for the rung pairs increases and then approaches to saturation as the ribbon width increases, where the velocity of saturation is strongly correlated to U. Moreover, for rung pairs the values of U at the QD peaks perform the scaling behaviors with increasing ribbon width. • Analytical solution for electronic and magnetic structures of zigzag graphene nanoribbons (ZGNRs). • The influences of the A-B pair position, on-site Coulomb repulsion, temperature, and ribbon width on quantum discord. • Scaling behaviors of the on-site Coulomb repulsion at the peak of quantum discord with increasing width of ZGNR. [ABSTRACT FROM AUTHOR]

Bipolar spin systems are expected to provide a reliable and scalable platform for advances in quantum computing and nanotechnology. Thus, the survey of the dynamics of the quantum properties of such systems is of paramount importance. This work explores the dynamics of bipartite entanglement and nonclassical correlations in a dipole-dipole two spin system with Dzyaloshinsky-Moriya (DM) interaction and under the influence of the intrinsic decoherence effects. We employ logarithmic negativity to quantify quantum entanglement. Local quantum uncertainty and quantum discord are employed to capture non-classical correlations beyond entanglement in the considered system. For this purpose, we consider that the system is initially prepared in a Werner state and we explore the effect of intrinsic decoherence rate, dipolar coupling parameters between the spins, strength of the Dzyaloshinsky-Moriya interaction and the intensities of the homogeneous magnetic fields on the dynamics of the three quantifiers of correlations within the considered system. The findings reveal that intrinsic decoherence deteriorates quantum correlations, while the dipolar coupling constant diminishes the oscillatory behavior observed but enhances the robustness of bipartite entanglement and nonclassical correlations. The negative effects of the intrinsic decoherence on the quantum correlations can be mitigated by adjusting the values of the system’s parameters as well as the Dzyaloshinsky-Moriya coupling parameter. We also show that the three studied measures behave quasi-similarly. Finally, we depict that the amounts of entanglement and nonclassical correlations within the system are closely tied to the system’s degree of purity. [ABSTRACT FROM AUTHOR]

In order to avoid unwanted effects where the signal of the two-qubit is lost (decoherence), it is considered a two qubits system exposed to a common reservoir at very low temperatures. It is derived as an expression for Mutual Quantum Information. Such a quantity has a classical component and also a quantum component. It is found to be a strictly quantum limit where classical correlations vanish. By invoking the First Law of Thermodynamics, it is shown that the concept of conservation behind the generation (loss) of Mutual Quantum Information will be proportional to the energy conservation of the system at constant temperature. In case of generation (loss) of Quantum Information the system will absorb (transfer) internal energy from (towards) the environment. [ABSTRACT FROM AUTHOR]

We apply the local optimal universal asymmetric cloning machine on an initially pure entangled state of two qubits. As output, we obtain two final states which present quantum correlations. We analyze three types of quantum correlations among the final states, namely, concurrence, quantum discord, and consonance. A detailed comparison between concurrence, quantum discord, and consonance is made, and we find that consonance is greater than quantum discord, which is in turn greater than concurrence. [ABSTRACT FROM AUTHOR]

We discover an environment-assisted quantum discord mechanism (ENAQD), similar to the phenomenon termed environment-assisted quantum transport (ENAQT), in the chromophores network of the Fenna–Matthews–Olson (FMO) pigment–protein complex from green sulfur bacteria Chlorobium tepidum. We use the Local Quantum Uncertainty (LQU), a discord-like quantifier, to identify the subsets of chromophores whose quantum correlations simultaneously increase with transport efficiency across the FMO complex. The subsets of chromophores that exhibit the ENAQD dynamics correspond to the final part of the energy transport pathways across the FMO complex and their LQU might be working as a resource for the energy transfer task. [ABSTRACT FROM AUTHOR]

Local available quantum correlations (LAQC), as defined by Mundarain et al., are analyzed for nonsymmetric 2-qubit X states, that is, X states that are not invariant under the exchange of subsystems and therefore have local Bloch vectors whose norms are different. A simple analytic expression for their LAQC quantifier is obtained. As an example, we analyze the local application of the amplitude damping channel for Werner states and general X states. Although this local quantumchannel can create quantum discord in some cases, no such outcome is possible for LAQC, which hints toward their monotonicity under LOCC operations. This work, along with our previous result for the so-called symmetric and anti-symmetric X states, completes the pursuit of exact analytical expressions for the LAQC quantifier for 2-qubit X states. [ABSTRACT FROM AUTHOR]

We here investigate the ground state phase transitions of the one-dimensional J 1 - J 2 Heisenberg model. The phase transition is characterized by analyzing the quantum discord, global quantum discord (GQD), bipartite nonlocality, and multipartite nonlocality of the ground state. Our findings reveal that these correlations serve as effective indicators of the phase transition point at J 2 / J 1 = 0.5. Specifically, quantum discord and bipartite nonlocality display more pronounced critical behavior in small-sized systems. Conversely, for large-scale systems, GQD and multipartite nonlocality offer a more practical and reliable method for detecting many-body driven phase transitions. Additionally, several linear relationships among these correlations are identified. • Quantum discord and bipartite nonlocality effectively indicate phase transitions in small-sized systems at J2/J1=0.5. • GQD and multipartite nonlocality are more reliable for detecting phase transitions in large-scale systems. • Linear relationships among different quantum correlations are identified in the study. [ABSTRACT FROM AUTHOR]

Quantum discord goes beyond entanglement and exists in a wide range of quantum states that may be separable, playing a crucial role in quantum information tasks. In this paper, we firstly proposed a zero-discord criterion for two-qubit system based on the partial transposition of density matrix, and then extended it to the qubit–qudit system. By detailed examples we demonstrate the effectiveness of these criteria in detecting discord. Moreover, we provide an analytical lower bound of geometric quantum discord(GQD) using eigenvalue vectors of density matrix. Finally, we presented a one-way work deficit lower bound based on our lower bound of GQD. • Zero discord criterion for two-qubit quantum states based on the partial transposition of the density matrix. • Analytical lower bound of geometric quantum discord for arbitrary bipartite quantum states. • Lower bound of one-way work deficit. [ABSTRACT FROM AUTHOR]

We study quantum discord dynamic of two atomic qubits moving inside a leaky cavity, where the two-qubit system is driven by a classical field. We suppose that the qubits are coupled to each other through dipole-dipole interaction and interacting asymmetrically with the cavity field via a two-photon relaxation. Using the time-dependent Schrödinger equation, we obtain the quantum state of the two-qubit system and study the dynamical behavior of the corresponding quantum discord. We discuss in detail the effects of the dipole-dipole interaction, atomic motion, atom-cavity coupling strength, classical driving and detuning between the qubit and the classical field (laser detuning) on the protection of initial qubit-qubit correlation in both the Markovian and non-Markovian regimes. We show that the dipole-dipole interaction and laser detuning destroy the initial correlation between the qubits in the stationary limit, however, the classical driving can effectively eliminate their destructive effects on the maintaining the initial correlation. [ABSTRACT FROM AUTHOR]

We study the influence of a two-mode Gaussian state under the Gaussian channel that describes the curvature effect in de Sitter space. Interestingly, the classical correlation may be unaffected by the curvature effect, while the quantum discord generally vanishes in the limit of infinite curvature. Under the action of curvature effect, the symmetry of both the classical correlation and quantum discord is destroyed. We obtain a useful monogamy relation for correlations between causally disconnected open charts, which means that quantum discord and classical correlation can convert each other but the total correlation is conserved in de Sitter space. [ABSTRACT FROM AUTHOR]

Local available quantum correlations (LAQC), as defined by Mundarain et al., are analyzed for two-qubit X states with local Bloch vectors of equal magnitude. Symmetric X-states are invariant under the exchange of subsystems, hence having the same Bloch vector. On the other hand, anti-symmetric X states have Bloch vectors with an equal magnitude but opposite direction. In both cases, we obtain exact analytical expressions for their LAQC quantifier. We present some examples and compare this quantum correlation to concurrence and quantum discord. We have also included Markovian decoherence, with Werner states under amplitude damping decoherence. As is the case for depolarization and phase damping, no sudden death behavior occurs for the LAQC of these states with this quantum channel. [ABSTRACT FROM AUTHOR]

A quantum illumination radar uses quantum entanglement to enhance photodetection sensitivity. The entanglement is quickly destroyed by the decoherence in an environment, although the sensitivity enhancement could survive thanks to quantum correlations beyond the entanglement. These quantum correlations are quantified by the quantum discord. Here, we use a toy model with an amplitude damping channel and Lloyd's binary decision strategy to highlight the possible role of these correlations from the perspective of a quantum radar. [ABSTRACT FROM AUTHOR]

Radhakrishnan et al. (Phys. Rev. Lett. 124:110401, 2020) proposed quantum discord to multipartite systems and derived explicit formulae for any states. These results are significant in capturing quantum correlations for multiqubit systems. In this paper, we evaluate the geometric measure of multipartite quantum discord and obtain the results for a large family of multiqubit states. Furthermore, we investigated the dynamic behavior of geometric discord for the family of two-, three- and four-qubit states under phase noise acting on the first qubit. And we discover that sudden change of multipartite geometric discord can appear when phase noise act only on one part of the two-, three- and four-qubit states. [ABSTRACT FROM AUTHOR]

We discuss the dynamics of quantum dissonance, quantum discord and entanglement for bipartite Bell-diagonal states independently interacting with dephasing reservoirs. Our aim is to reveal mainly the behavior of dissonance comparatively to that of the two other kinds of quantum correlations. Particularly, we show that the sensitivity of these quantum correlations against the environment effect increases upon the increase in the independent random variable e. Also, we show that the dissonance is proved to be more robust to the decoherence effect than the rest of quantum correlations; it survives along the time of evolution. This indicates its potential to be fully responsible for the quantum characteristics of the system evolution under the environment effect and ensures that the system never loses its non-classicality in such conditions. Moreover, we show that its role in capturing the quantum correlations without entanglement still preserved under the influence of the system–environment interaction. Further, we find that it is very useful for predicting the quantum correlations behavior in an open bipartite system; also it can be an efficient tool for the discrimination of its state. [ABSTRACT FROM AUTHOR]

Transport phenomena in photosynthetic systems have attracted a great deal of attention due to their potential role in devising novel photovoltaic materials. In particular, energy transport in light-harvesting complexes is considered quite efficient due to the balance between coherent quantum evolution and decoherence, a phenomenon coined Environment-Assisted Quantum Transport (ENAQT). Although this effect has been extensively studied, its behavior is typically described in terms of the decoherence’s strength, namely weak, moderate or strong. Here, we study the ENAQT in terms of quantum correlations that go beyond entanglement. Using a subsystem of the Fenna–Matthews–Olson complex, we find that discord-like correlations maximize when the subsystem’s transport efficiency increases, while the entanglement between sites vanishes. Our results suggest that quantum discord is a manifestation of the ENAQT and highlight the importance of beyond-entanglement correlations in photosynthetic energy transport processes.

The quantum discord of bipartite systems is one of the best-known measures of non-classical correlations and an important quantum resource. In the recent work appeared in [Phys. Rev. Lett 2020, 124:110401], the quantum discord has been generalized to multipartite systems. In this paper, we give analytic solutions of the quantum discord for tripartite states with fourteen parameters. [ABSTRACT FROM AUTHOR]

It is well known that environmental decoherence is a crucial barrier in realizing various quantum information processing tasks; on the other hand, it plays a pivotal role in explaining how a quantum system's fragile state leads to a robust classical state. Zurek (Nat Phys 5(3):181–188, 2009) was the first to develop the theory which successfully describes the emergence of classical objectivity of quantum systems using decoherence, introduced by the environment. Here, we consider an n-qubit generalized quantum circuit for the quantum system–environment interaction model, where the first qubit represents the quantum system, and the rest are for the environmental fragments. This quantum circuit is implemented on ibmq_athens and ibmq_16_melbourne for n = 2 , 3 , 4 , 5 , 6 . Its accuracy is checked using quantum state tomography and enhanced using the quantum error mitigation procedure. The reconstructed density matrices are used to investigate quantum-classical correlation and the mutual information between the quantum system and the environment. The investigation proves the quantum Darwinism principle when the quantum circuits are executed on the noise-less simulator; however, it shows the unaccountable behavior when implemented on the real quantum devices. The results via the noise-less simulator successfully prove that the environmental fragment size and the interaction strength play a crucial role in the emergence of classicality. [ABSTRACT FROM AUTHOR]

We study the decoherence process induced by a spin chain environment on a central spin consisting of R spins and we apply it on the dynamics of quantum correlations (QCs) of three interacting qubits. To see the impact of the initially prepared state of the spin chain environment on the decoherence process, we assume the spin chain environment prepared in two main ways, namely, either the ground state or the vacuum state in the momentum space. We develop a general heuristic analysis when the spin chain environment is prepared in these states, to understand the decoherence process against the physical parameters. We show that the decoherence process is mainly determined by the choice of the initially prepared state, the number of spins of the chain, the coupling strength, the anisotropy parameter and the position from the quantum critical point. In fact, in the strong coupling regime, the decoherence process does not appear for the environment prepared in the vacuum state and it behaves oscillatory in the case of evolution from the ground state. On the other hand, in the weak coupling regime and far from the quantum critical point, decoherence induced by the ground state is weaker than that of the vacuum state. Finally, we show that QCs are completely shielded from decoherence in the case of evolution from the W state and obey the same dynamics as the decoherence factors for the GHZ state. [ABSTRACT FROM AUTHOR]

Quantum discord is a measure of quantumness of correlations in bipartite states, which characterizes the difference between quantum mutual information and classical correlations. Based on the generators of Lie algebra s u (d) , we study the quantum discord for two families of two-qudit states with maximally mixed marginals, and derive some upper bounds of quantum discord for such states by evaluating classical correlations. Furthermore, we make a comparison among total correlations, classical correlations and quantum discord for a class of special states in high-dimensional two-partite systems. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the dynamics of quantum correlations for the continuous-variable bipartite optomechanical system in the presence of homodyne mediated quantum feedback. The system is constructed using the Fabry–Perot cavity and a mechanical oscillator. In detail, we have studied the quantum entanglement via logarithmic negativity, quantum discord, and quantum steering between the optical and mechanical modes. The results show that the maximum entanglement between the optical and mechanical modes occurred out of the resonance frequency at the steady-state region due to the presence of the optical feedback. We also employ quantum steering to see the steerability between the modes. It is observed that the quantum entanglement increases when the effective detuning frequency increases, however, the quantum discord and steering decrease when the effective detuning frequency increases for a given quantum feedback strength with other parameters of the system are to be fixed. Moreover, the presence of quantum feedback has an enhancement effect on the quantum correlations.

We explore the monogamy of multipartite quantum discord. In the article [Quantum Science and Technology 6, 4, 045028], Guo et al. show that quantum discord for multiqubit systems is monogamous provided that it does not increase under discard of subsystems. we illustrate that the above-mentioned preconditions are valid for a family of multiqubit states. Based on the analytical expressions of quantum discord and geometric discord of the family of states obtained in the articles [Phys. Rev. A 104, 012428] and [arXiv:], we investigate the dynamic behavior of these under the local decoherence channel, and show that the quantum discord of even partite systems have frozen phenomenon while the odd partite systems does not exist. For geometric discord, this family of states has the frozen phenomenon under local decoherence conditions. The results show that compound noises are not necessary for sudden changes in quantum correlation, and one qubit of the quantum noise is sufficient. The research of these non-loss conditions is of great significance for understanding the evolution of quantum systems in the environment. [ABSTRACT FROM AUTHOR]

Information is encoded in a qubit in the form of its Bloch vector. In this paper, we propose protocols for remote transfers of information in a known and an unknown qubit to qudits using SU(2)-invariant 2 × N -level separable discordant states as quantum channels. These states have been identified as separable equivalents of the two-qubit entangled Werner states in Bharath and Ravishankar (Phys Rev A 89:062110, 2014). Due to S U (2) × S U (2) invariance of these states, the remote qudit can be changed by performing appropriate measurements on the qubit. We also propose a protocol for transferring information of a family of unknown qudits to remote qudits using 2 × N -level states as channels. Finally, we propose a protocol for swapping of quantum discord from 2 × N -level systems to N × N -level systems. All the protocols proposed in this paper involve separable states as quantum channels. [ABSTRACT FROM AUTHOR]

Optimal measurement is required to obtain the quantum and classical correlations of a quantum state, and the crucial difficulty is how to acquire the maximal information about one system by measuring the other part; in other words, getting the maximum information corresponds to preparing the best measurement operators. Within a general setup, we designed a variational hybrid quantum–classical algorithm to achieve classical and quantum correlations for system states under the Noisy-Intermediate Scale Quantum technology. To employ, first, we map the density matrix to the vector representation, which displays it in a doubled Hilbert space, and it is converted to a pure state. Then, we apply the measurement operators to a part of the subsystem and use variational principle and a classical optimization for the determination of the amount of correlation. We numerically test the performance of our algorithm at finding a correlation of some density matrices, and the output of our algorithm is compatible with the exact calculation. [ABSTRACT FROM AUTHOR]

In a pair of correlated quantum systems, a measurement in one corresponds to a change in the state of the other. Measurement along which set of projectors would accompany minimum loss in information content is the optimization problem of quantum discord and is an important aspect of a classical to quantum transition because it asks us to look for the most classical states. This optimization problem is known to be NP-complete and is important because discord is defined through it making it a major obstacle on every computation. The standard zero discord condition helps us move to a stronger measure for correlated observables; in such a context, we show that discord minimizes at the diagonal basis of the reduced density matrices and derive an analytical expression. [ABSTRACT FROM AUTHOR]

The analytic form of quantum discord by Radhakrishnan et al. (2020) is derived for three qubits under a non-Markovian dephasing channel, where tripartite entanglement is explicitly calculated for reference. The necessary and sufficient conditions for the frozen discord are presented. It is shown that tripartite discord can exhibit multiple intervals of freezing under such conditions, while the entanglement displays multiply sudden deaths and revivals. • The analytic form of tripartite discord is presented for initial states in three qubits under a non-Markovian channel. • The conditions for the frozen discord are given for such channel. • Tripartite discord can exhibit multiple intervals of freezing under such conditions. • The tripartite entanglement is explicitly presented, which displays multiply sudden deaths and revivals. [ABSTRACT FROM AUTHOR]

In this study, we consider a composed system consisting of two identical non-interacting subsystems. Each sub-system is made of a moving qubit into a leaky cavity. The study of the dynamic of the composed system revealed that compared with the stationary qubits, entanglement, quantum discord and coherence between two moving qubits remained close to their initial values as time went by. In particular, we show that the entanglement, quantum discord and the initial coherence between the two qubits could be strongly protected from decay by increasing the velocity of the qubits. Furthermore, based on such results, we have drawn a comparison between the three quantum resources (entanglement, quantum discord and coherence), demonstrating that the coherence of a bipartite system could be more robust than the two other quantum resources.

Quantum decoherence leads to environment-induced superselection of preferred states. Some information in the measurement apparatus is lost during communication. Even if the measurement apparatus is not entangled with the system of interest, the loss of information would occur. In this paper, we propose a feasible quantum network coding scheme reducing decoherence effect to transmit the mutual information between source node and target node. With the help of entanglement distribution by separable states, the quantum network coding scheme initially achieves quantum entanglement distribution of two crossing source-target pairs in a butterfly network. Furthermore, by means of transmission of correlations, the maximal mutual information resulting from local operations and classical communication will be transmitted to a distant receiver. Compared with the representative quantum network coding schemes, the proposed scheme transmits correlations rather than quantum state over the butterfly network. The upper bound to the concentrated information is also quantified. Analysis indicates that the proposed scheme can effectively defend against active attacks with fewer resource consumption and good region rate. [ABSTRACT FROM AUTHOR]

Two-qubit X-state is a large class of quantum states which plays an important role in the quantification and dynamical study of quantum correlations. However, the corresponding quantification of quantum discord is still missing for bona fide discord measures, like original quantum discord, Bures distance of discord, and relative entropy of discord. In this paper, we consider the calculation of Bures distance of discord, which is a kind of correlations satisfying all criteria of a discord measure, for two-qubit X-states. Firstly, we derive explicit expression for Bures distance of discord for a kind of five-parameters family of states. Moreover, for general two-qubit X-states, we not only calculate the Bures distance of discord for a subset of two-qubit X-states by classifying and analyzing the optimal local measurements and the optimal projection operators, but also provide an analytic upper bound for entirety. [ABSTRACT FROM AUTHOR]

We investigate the nonlocal advantage of quantum coherence (NAQC) and quantum discord (QD) for both the thermal equilibrium state and all eigenstates of the Heisenberg XXX chain. It is shown that both the NAQC and QD of two neighboring spins are completely determined by a thermodynamic potential, i.e., the internal energy for the thermal equilibrium state and the kth-level eigenenergy for the kth-level eigenstate. From the dependence of the NAQC and QD on the internal energy, we further show that they approach to their thermodynamic limits very quickly with an increase in the number of spins in the chain. We also investigate the NAQC and QD versus energy in the anisotropic Heisenberg XXZ model. [ABSTRACT FROM AUTHOR]

We study the effect of unentangled states with nonzero discord on Quantum Stackelberg duopoly game (QSD). We show that the initial state parameter and quantum discord will cause remarkable change in the Nash equilibrium of the QSD game and the profits of the two players. We also find that under certain conditions there is a 'specific point' of the initial state parameter and quantum discord. The two players have the same moves and payoffs at the 'specific point'. The QSD game can switch between the first-mover advantage game and the follower-mover advantage game when the initial state parameter and quantum discord varies from the left-hand-side regime of the 'specific point' to the right-hand-side regime. [ABSTRACT FROM AUTHOR]

We study dynamics of local quantum uncertainty (LQU) for a system of two cavities and two reservoirs. In the start, the cavities are treated as two qubits which are quantum correlated with each other, whereas reservoirs (also qubits) are neither correlated with each other nor with cavities. We answer two main questions in this work. First, how local quantum uncertainty decays from two quantum correlated cavities and grows among reservoirs. The second question is the examination of LQU developed among four qubits and also shed some light on its dynamics. We observe that LQU develops among reservoirs as kind of mirror image to its decay from cavities. For four qubits, we propose how to compute LQU such that the method is intuitive and analytically computable. We find that among four qubits, LQU starts growing from zero to some maximum value and then decays again to zero as the asymptotic state of cavities is completely transferred to reservoirs. We suggest the experimental setup to implement our results. [ABSTRACT FROM AUTHOR]

In an optomechanical system consisting of two Fabry–Pérot cavities fed by squeezed light and coupled via Coulomb interaction, we respectively use the logarithmic negativity, Gaussian discord and Gaussian coherence to analyze the behavior of three different indicators of nonclassicality, namely the entanglement, quantum discord and quantum coherence. We perform the rotating wave approximation and work in the resolved sideband regime. In two bi-mode states (optical and mechanical), the coherence is generally found to be greater than entanglement and discord. More interestingly, we show that the Coulomb interaction can be used either to degrade or enhance the nonclassical properties of the optical subsystem. In addition, compared with the discord and coherence, the mechanical entanglement is found strongly sensitive to both thermal and Coulomb effects, and it requires a minimum value of cooperativity to be generated. Remarkably, this minimum increases when increasing the Coulomb coupling strength. Finally, we notice that an optimal transfer of quantum correlations between the optical and mechanical subsystems is achieved in the absence of the Coulomb interaction. [ABSTRACT FROM AUTHOR]