Schrödinger cat and Werner state disentanglement simulated by trapped ion systems

Influences of two collective noises, i.e., the dephasing noise and the rotating noise, on quantum correlations in Werner states are considered in this note. It is found that the collective noises do not alter the correlations as far as quantum discord and geometry discord as quantifiers are concerned. Alternatively, quantum correlations in Werner states are robust against the noises.

In this work, we mainly analyze the dynamics of geometric quantum discord under a common dissipating environment. Our results indicate that geometric quantum discord is generated when the initial state is a product state. The geometric quantum discord increases from zero to a stable value with the increasing time, and the variations of stable values depend on the system size. For different initial product states, geometric quantum discord has some different behaviors in contrast with entanglement. For initial maximally entangled state, it is shown that geometric quantum discord decays with the increasing dissipated time. It is found that for EPR state, entanglement is more robust than geometric quantum discord, which is a sharp contrast to the existing result that quantum discord is more robust than entanglement in noisy environments. However, for GHZ state and W state, geometric quantum discord is more stable than entanglement. By the comparison of quantum discord and entanglement, we find that a common dissipating environment brings complicated effects on quantum correlation, which may deepen our understanding of physical impacts of decohering environment on quantum correlation. In the end, we analyze the effects of collective dephasing noise and rotating noise to a class of two-qubit X states, and we find that quantum correlation is not altered by the collective noises.

We consider the effect of a thermal bath on quantum correlations induced by the gravitational interaction in the weak field limit between two massive cat states, called gravitational cat (gravcat) states. The main goal of this paper is to provide a good understanding of the effects of temperature and several parameters in the entanglement (measured by the concurrence) and quantum coherence (measured by the l1-norm that is defined from the minimal distance between the quantum state and the set of incoherent states) which are derived from the thermal quantum density operator. Our results show that the thermal concurrence and l1-norm can be significantly optimized by increasing the masses or decreasing the distance between them. We investigate and discuss the behavior of these quantities under temperature variations in different regimes, including some that are expected to be experimentally feasible in the future. In particular, we observe that thermal fluctuations raise non-entangled quantum correlations when entanglement suddenly drops.

Werner derivatives are a special kind of mixing states transformed from Werner states by unitary operations (Hiroshima and Ishizaka in Phys Rev A 62:044302, 2000). In this paper, the inherent quantum correlations in Werner derivatives are quantified by two different quantifiers, i.e., quantum discord and geometric discord. Different analytic expressions of the two discords in Werner derivatives are derived out. Some distinct features of the discords and their underlying physics are exposed via discussions and analyses. Moreover, it is found that the amount of quantum correlations quantified by either quantifier in each derivative cannot exceed that in the original Werner state. In other words, no unitary operation can increase quantum correlation in a Werner state as far as the two quantifiers are concerned.

We investigate quantum discord of the "q-deformed" Werner state via Yang–Baxterization approach. There are two parameters q and u in this "q-deformed" Werner state. The parameter u, which plays an important role in some typical models, is related to the probability of the "q-deformed" two-qubit spin singlet state in this study. The "q-deformed" parameter q is related to the single loop through d = q + q-1. When topological parameter d approaches 2 (i.e. q → 1), the "q-deformed" Werner state degenerates into the well-known Werner state. The results show that topological parameter d has great influence on quantum correlations of the "q-deformed" Werner state. When we fix the parameter u, the quantum correlations decrease with increasing the single loop d. When d approaches +∞ (i.e. q → 0+or +∞), quantum discord, geometric measure of quantum discord and entanglement all tend to 0. While d approaches 2 (i.e. q → 1), all of them just have the same results with the Werner state.

Measuring complex properties in quantum systems, such as measures of quantum entanglement and Bell nonlocality, is inherently challenging. Traditional methods, like quantum state tomography (QST), require a full reconstruction of the density matrix for a given system and demand resources that scale exponentially with system size. We propose an alternative strategy that reduces the required information by combining multicopy measurements with artificial neural networks (ANNs), resulting in a 67% reduction in measurement requirements compared to QST. We have successfully measured two-qubit quantum correlations of Bell states subjected to a depolarizing channel (resulting in Werner states) and an amplitude-damping channel (leading to Horodecki states) using the multicopy approach on real quantum hardware. Our experiments, conducted with transmon qubits on IBMQ quantum processors, quantified the violation of Bell's inequality and the negativity of two-qubit entangled states. We compared these results with those obtained from the standard QST approach and applied a maximum likelihood method to mitigate noise. We trained ANNs to estimate degrees of entanglement and nonlocality measures using optimized sets of projections identified through Shapley's (SHAP) analysis for the Werner and Horodecki states. The ANN output, based on this reduced set of projections, aligns well with expected values and enhances noise robustness. This approach simplifies and improves the error robustness of multicopy measurements, eliminating the need for complex nonlinear equation analysis. It represents a significant advancement in AI-assisted quantum measurements, making the practical implementation on current quantum hardware more feasible. The experimental results demonstrate improved noise robustness on the current noisy intermediate-scale quantum (NISQ) hardware, representing a practical advance in resource-efficient characterization of quantum correlations.

We demonstrate a hierarchy of various classes of quantum correlations on\nexperimentally prepared two-qubit Werner-like states with controllable white\nnoise. Werner states, which are white-noise-affected Bell states, are\nprototypal examples for studying such a hierarchy as a function of the amount\nof white noise. We experimentally generated Werner states and their\ngeneralizations, i.e., partially entangled pure states affected by white noise.\nThese states enabled us to study the hierarchy of the following classes of\ncorrelations: separability, entanglement, steering in three- and\ntwo-measurement scenarios, and Bell nonlocality. We show that the generalized\nWerner states (GWSs) reveal fundamentally different aspects of the hierarchy\ncompared to the Werner states. In particular, we find five different parameter\nregimes of the GWSs, including those steerable in a two-measurement scenario\nbut not violating Bell inequalities. This regime cannot be observed for the\nusual Werner states. Furthermore, we find threshold curves separating different\nregimes of the quantum correlations and find the optimal states which allow for\nthe largest amount of white noise, which does not destroy their specific\nquantum correlations (e.g., unsteerable entanglement). Thus, we could identify\nthe optimal Bell-nondiagonal GWSs, which are, for this specific meaning, more\nrobust against white noise compared to the Bell-diagonal GWSs (i.e., Werner\nstates).\n

We propose a total measure of multi-particle quantum correlation in a system of N two-level atoms (N qubits). We construct a parameter that encompasses all possible quantum correlations among N two-level atoms in arbitrary symmetric pure states and define its numerical value to be the total measure of the net atom-atom correlations. We use that parameter to quantify the total quantum correlations in atomic Schrödinger cat states, which are generated by the dispersive interaction in a cavity. We study the variation of the net amount of quantum correlation as we vary the number of atoms from N=2 to N=100 and obtain some interesting results. We also study the variation of the net correlation, for fixed interaction time, as we increase the number of atoms in the excited state of the initial system, and notice some interesting features. We also observe the behaviour of the net quantum correlation as we continuously increase the interaction time, for the general state of N two-level atoms in a dispersive cavity.

Sudden death of entanglement: Classical noise effects

This study advances economical Quantum Teleportation (QT) schemes for cat states across arbitrary dimensional Hilbert spaces. First, we develop a protocol for teleporting an unknown two-dimensional cat state using a three-dimensional maximally entangled three-qutrit quantum channel. The approach involves constructing a complete orthogonal nonsymmetric measurement basis, where the sender performs joint measurements on three particles. The receiver then applies dimension-specific unitary operations conditioned on measurement outcomes to reconstruct the target state. Subsequently, we extend this protocol by replacing the maximally entangled channel with a nonmaximally entangled three-qutrit state. Through auxiliary qubit introduction and optimized operations, the two-qubit cat state is probabilistically recovered. We derive the success probability for both protocols, demonstrating that the nonmaximally entangled case generalizes the former scheme. Further generalization is achieved through two nonsymmetric basis constructions: (i) Scaling quantum channel dimension from 3 to [Formula: see text] dimensions; (ii) Extending transmitted cat states from 2 to [Formula: see text] dimensions [Formula: see text]. These dual-dimensional generalizations establish distinct economical advantages over existing QT schemes.

We present an effective method of coherent state superposition (cat state) generation using single trapped ion in a Paul trap. The method is experimentally feasible for coherent states with amplitude α ≤ 2 using available technology. It works both in and beyond the Lamb-Dicke regime.

In this paper we study quantum correlation (QC) swapping between two different kinds of quantum-correlated states. To be concrete, one is the famous Werner state while another is a kind of separable state. We employ measurement-induced disturbance (Luo 2008 Phys. Rev. A 77 02230) as the quantifier to characterize and quantify QC. We work out QCs in all the concerned states and concretely analyze their behaviors and properties such monotony features and thresholds. We find that a long-distance shared QC can be generated from two short-distance ones via intermediate measure. Moreover, the amount of the long-distance shared QC cannot reach that in the Werner state but that in the separable state.

In the present work, we consider a scenario where an arbitrary two-qubit pure state is applied for the quantum key generation (QKG). Using the twirling procedure to convert the pure state into a Werner state, the error rate of the key can be reduced by a factor of 2/3. This effect indicates that entanglement is not the sufficient resource of QKG protocol since it is not increased in the twirling procedure. Based on the fact that the geometric discord is increased in the twirling procedure, we argue that the geometric discord should be taken as a necessary resource for the QKG task. Besides the pure state, we also give other two types of mixtures where twirling may increase the discord and reduce the error rate of the generated key.

We investigate the e ect of eld changing on two qubit entanglement. We choose the coherent state, the cat state and the squeezed state as di erent states of interacting eld with atom. The correlation between two qubits can be degraded by environmental noise, this phenomenon has been labelled sudden death of entanglement. The double Jaynes Cummings model as a solvable model is used for describing the dynamic of sudden death of entanglement. We measure two-qubit entanglement using the concurrence and the negativity. The Fisher information is introduced, too, and compares results with previous entanglement measures. Finally we conclude that the Fisher information is not an entanglement measure.

We suggest a measure of classical correlation in a two-qubit state, which is computable and implementable in experiments. As examples, we investigate the classical and quantum correlations in the Werner states and in a group of particular states mixed with three parts: an entangled, a separable and a product one. It is shown that the quantum correlation in a two-qubit state cannot exceed the classical correlation in the same state.