Entanglement and coherence of the wobbling mode in the particle–triaxial-rotor model

The entanglement and coherence of the wobbling mode are studied in the framework of the particle plus triaxial-rotor model for the one-quasiparticle nucleus $^{135}\mathrm{Pr}$ and the two-quasiparticles nucleus $^{130}\mathrm{Ba}$. The focus lies on the coupling between the total and the particle angular momenta. Using the Schmidt decomposing, it is quantified in terms of the von Neumann entropy of the respective subsystems, which measures their mutual entanglement. The entropy and the entanglement increase with spin $I$ and number of wobbling quanta $n$. The coherence of the wobbling mode is studied by means of the eigenstate decomposition of its reduced density matrix. To a good approximation, the probability distributions of the total angular momentum can be interpreted as the incoherent combination of the coherent contributions from the first two pairs of eigenvectors with the largest weight of the reduced density matrix. Decoherence measures are defined, which, in accordance, scatter between 0.1 to 0.2 at low spin and between 0.1 and 0.3 at high spin. Entanglement in the framework of the adiabatic approximation is further analyzed. In general, the coherent eigenstates of the effective collective Hamiltonian approximate the reduced density matrix with the limited accuracy of its pair of eigenstates with the largest weight. As the adiabatic approximation becomes more accurate with decreasing excitation energy, the probability distribution of the angle of the total angular momentum around a principal axis approaches the one of the full reduced density matrix. The $E2$ transition probabilities and spectroscopic quadrupole moments reflect this trend.