Entangled spin coherent states are a type of quantum states that involve two or more spin systems that are correlated in a nonclassical way. These states can improve metrology and information processing, as they can surpass the standard quantum limit, which is the ultimate bound for precision measurements using coherent states. However, finding entangled coherent states in physical systems is challenging because they require precise control and manipulation of the interactions between the modes. In this work we show that entangled unique coherent states can be found in the ground state of the spin-1/2 XX chain model with three-spin interaction, which is an exactly solvable model in quantum magnetism. We use the spin squeezing parameter, the l_{1}-norm of coherence, and the entanglement entropy as tools to detect and characterize these unique coherent states. We find that these unique coherent states exist in a gapless spin liquid phase, where they form a line that separates two regions with different degrees of squeezing. We call this line the entangled unique coherent line, as it corresponds to the almost maximum entanglement between two halves of the system. We also study the critical scaling of the spin squeezing parameter and the entanglement entropy versus the system size.
Entangled spin coherent states are a type of quantum states that involve two or more spin systems that are correlated in a nonclassical way. These states can improve metrology and information processing, as they can surpass the standard quantum limit, which is the ultimate bound for precision measurements using coherent states. However, finding entangled coherent states in physical systems is challenging because they require precise control and manipulation of the interactions between the modes. In this work we show that entangled unique coherent states can be found in the ground state of the spin-1/2 XX chain model with three-spin interaction, which is an exactly solvable model in quantum magnetism. We use the spin squeezing parameter, the l_{1}-norm of coherence, and the entanglement entropy as tools to detect and characterize these unique coherent states. We find that these unique coherent states exist in a gapless spin liquid phase, where they form a line that separates two regions with different degrees of squeezing. We call this line the entangled unique coherent line, as it corresponds to the almost maximum entanglement between two halves of the system. We also study the critical scaling of the spin squeezing parameter and the entanglement entropy versus the system size.
In this paper, we investigate the ground state properties of the spin-1/2 XX chain model with anisotropic three-spin interaction using the fermionization technique. By exactly diagonalizing the Hamiltonian, we analyze the dispersion relation, ground state energy, and order parameters. Our findings identify two gapless phases, distinguished by a Lifshitz transition line: one exhibiting long-range chiral correlations and the other characterized by chiral-nematic correlations. These correlations do not correspond to conventional symmetry-breaking local order parameters typically associated with gapped one-dimensional phases but instead signify emergent quantum coherence in the gapless regime. Further, we study the ground state phase diagram through the concurrence and quantum discord between nearest-neighbor spins, finding that these measures are maximized at the critical line, with an additional entangled region observed. Finally, we examine the spin squeezing parameter and entanglement entropy, demonstrating that the ground state is squeezed throughout and becomes extremely squeezed at the critical line. Notably, in the gapless chiral-nematic phase, the Heisenberg limit is achieved. By dividing the system into two equal parts, we observe significant entanglement in the gapless chiral phase. The central charge calculation confirms the critical nature of the gapless chiral-nematic phase, while the entanglement entropy follows volume-law scaling in the gapless chiral phase.
We investigate the one-dimensional spin-1/2 XX chain with anisotropic three-spin interactions, which hosts four distinct topological phases separated by critical lines. Using exact fermionization, we analyze the spin squeezing parameter, entanglement entropy, and bulk entanglement spectrum to characterize the ground state. Our results show that the system remains unsqueezed across the entire phase diagram, with maximal unsqueezing at criticality. The entanglement entropy exhibits cusp-like singularities at the critical lines, while the extracted central charges reveal distinct critical behaviors. Furthermore, the lowest level of the bulk entanglement spectrum is four-fold degenerate in two phases and two-fold in the others, reflecting the topological nature of each phase. These findings offer deeper insight into the interplay between topology, criticality, and quantum correlations in low-dimensional spin systems.
The entanglement in quantum XY spin chains of arbitrary length is investigated via the geometric measure of entanglement. The emergence of entanglement is explained intuitively from the perspective of perturbations. The model is solved exactly and the energy spectrum is determined and analyzed in particular for the lowest two levels for both finite and infinite systems. The overlaps for these two levels are calculated analytically for arbitrary number of spins. The entanglement is hence obtained by maximizing over a single parameter. The corresponding ground-state entanglement surface is then determined over the entire phase diagram, and its behavior can be used to delineate the boundaries in the phase diagram. For example, the field-derivative of the entanglement becomes singular along the critical line. The form of the divergence is derived analytically and it turns out to be dictated by the universality class controlling the quantum phase transition. The behavior of the entanglement near criticality can be understood via a scaling hypothesis, analogous to that for free energies. The entanglement density vanishes along the so-called disorder line in the phase diagram, the ground space is doubly degenerate and spanned by two product states. The entanglement for the superposition of the lowest two states is also calculated. The exact value of the entanglement depends on the specific form of superposition. However, in the thermodynamic limit the entanglement density turns out to be independent of the superposition. This proves that the entanglement density is insensitive to whether the ground state is chosen to be the spontaneously Z2 symmetry broken one or not. The finite-size scaling of entanglement at critical points is also investigated from two different view points. First, the maximum in the field-derivative of the entanglement density is computed and fitted to a logarithmic dependence of the system size, thereby deducing the correlation length exponent for the Ising class using only the behavior of entanglement. Second, the entanglement density itself is shown to possess a correction term inversely proportional to the system size, with the coefficient being universal (but with different values for the ground state and the first excited state, respectively).
We consider the spin-1/2 XX chain model with uniform Gamma interaction, exactly solvable using the fermionization approach. The ground state phase diagram reveals two regions: a composite gapless phase and a single gapped phase, separated by a second-order critical line. Within the composite phase, a special line is identified where one coexisting phase vanishes, leaving a single gapless phase. By examining the concurrence and quantum discord (QD) between nearest-neighbor spin pairs, we probe the ground state phase diagram. Our results highlight a discontinuity along the quantum critical line, indicating the phase transition. The special line in the gapless region is detected through concurrence and QD results, showing the sensitivity of these quantum correlations to system parameter changes. We also explore the scaling behavior of concurrence and QD near the quantum critical line, providing insights into the system's critical properties. Furthermore, we identify a line within the gapped phase where both concurrence and QD vanish. Investigating the density matrix along this line, we assess whether the zero QD condition adheres to a novel criterion for arbitrary bipartite states. Our findings indicate that without Heisenberg interaction, the criterion is not met, but with both Heisenberg interaction and Gamma interaction, the criterion for zero QD is satisfied.
In this study, the detection method of entanglement of 3-mode spin coherence states is investigated. For this purpose, by properly defining the computational codes, these states are mapped to a three qubit quantum state, and then, by using the Mermin- Kalyshko inequality, the entanglement of these states is studied and an analytical relationship to determine the entanglement region is presented. The results of numerical analysis of the extracted inequality show that the entanglement region of these states depends on the coherence parameters, the amount of spin as well as the phase of the states. For example, in the case where the values of the coherence parameters are equal but with opposite signs, by adjusting the coherence phase, the degree of entanglement can be controlled such that the maximum entanglement occurs . More important, as the value of spin increases, the allowable range of the coherence parameter for entanglement detection increases. These results are consistent with the data reported in the study of the degree of entanglement of 2-mode superposition of spin coherent states using other measures and criteria of entanglement. The findings of this study can be used in the study of non-classical and quantum systems and quantum correlations in quantum information science.
Abstract We investigate the ground-state quantum correlations in the spin-1/2 XX chain with anisotropic four-spin interaction (AFSI), employing an exact analytical approach via fermionization. The phase diagram of the system hosts three distinct quantum critical lines that separate non-trivial topological phases characterized by winding numbers ν = ±1 and ν = ±3. To probe the system’s critical behavior and topological nature, we analyze two key quantum correlation measures: concurrence and quantum discord between nearest-neighbor spin pairs. Our results reveal that while concurrence is limited in detecting all critical lines particularly in phases with higher winding numbers, quantum discord successfully identifies all quantum phase transitions, either through cusps or discontinuities in its first derivative. Furthermore, quantum discord assumes larger values in topological phases with lower winding numbers, suggesting that the structure and localization of quantum correlations are influenced by topological order. These findings underscore the effectiveness of quantum discord as a robust diagnostic tool for both criticality and topology in many-body quantum systems. Our results offer valuable insight for advancing correlation-based techniques in quantum information science and highlight the potential role of topological spin chains in future quantum technologies.