Entanglement of formation and quantum discord in multipartite j-spin coherent states

We study the entanglement of formation and the quantum discord contained in even and odd multipartite [Formula: see text]-spin coherent states. The key element of this investigation is the fact that a single [Formula: see text]-spin coherent state is viewed as comprising [Formula: see text] qubit states. We compute the quantum correlations present in the n even and odd [Formula: see text]-spin coherent states by considering all possible bipartite splits of the multipartite system. We discuss the different bi-partition schemes of quantum systems and we examine in detail the conservation rules governing the distribution of quantum correlations between the different qubits of the multipartite system. Finally, we derive the explicit expressions of quantum correlations present in even and odd spin coherent states decomposed in four spin sub-systems. We also analyze the properties of monogamy and we show in particular that the entanglement of the formation and the quantum discord obey the relation of monogamy only for even multipartite [Formula: see text]-spin coherent states.

Maximum violation of Wigner inequality for two-spin entangled states with parallel and antiparallel polarizations

The experimental test of Bell’s inequality is mainly focused on Clauser–Horne–Shimony–Holt (CHSH) form, which provides a quantitative bound, while little attention has been paid to the violation of Wigner inequality (WI). Based on the spin coherent state quantum probability statistics, we in the present paper extend the WI and its violation to arbitrary two-spin entangled states with antiparallel and parallel spin-polarizations. The local part of density operator gives rise to the WI while the violation is a direct result of nonlocal interference between two components of the entangled states. The Wigner measuring outcome correlation denoted by [Formula: see text] is always less than or at most equal to zero for the local realist model ([Formula: see text]) regardless of the specific initial state. On the other hand, the violation of WI is characterized by any positive value of [Formula: see text], which possesses a maximum violation bound [Formula: see text] [Formula: see text]. We conclude that the WI is equally convenient for the experimental test of violation by the quantum entanglement.

Collapse and revival of entanglement between qubits coupled to a spin coherent state

We extend the study of the Jayne–Cummings (JC) model involving a pair of identical two-level atoms (or qubits) interacting with a single mode quantized field. We investigate the effects of replacing the radiation field mode with a composite spin, comprising [Formula: see text] qubits, or spin-1/2 particles. This model is relevant for physical implementations in superconducting circuit QED, ion trap and molecular systems. For the case of the composite spin prepared in a spin coherent state, we demonstrate the similarities of this set-up to the qubits-field model in terms of the time evolution, attractor states and in particular the collapse and revival of the entanglement between the two qubits. We extend our analysis by taking into account an effect due to qubit imperfections. We consider a difference (or “mismatch”) in the dipole interaction strengths of the two qubits, for both the field mode and composite spin cases. To address decoherence due to this mismatch, we then average over this coupling strength difference with distributions of varying width. We demonstrate in both the field mode and the composite spin scenarios that increasing the width of the “error” distribution increases suppression of the coherent dynamics of the coupled system, including the collapse and revival of the entanglement between the qubits.