A Pseudo-Hermitian Hybrid Model at Finite Temperature: The Role of the Exceptional Points

Abstract

We study a hybrid system formed by an ensemble of colour nitrogen-vacancy centres in diamond interacting with a superconducting flux-qubit at finite temperature. The presence of impurities in the system is modelled through pseudo-hermitian Hamiltonian, by introducing an asymmetry parameter in the interaction between the superconducting flux qubit and the ensemble of colour nitrogen-vacancy centres in diamond. We construct the exact grand partition function of the system, and from it we derive the thermodynamic quantities, e.g. entropy, internal energy, and Helmholtz free energy. In the broken symmetry phase, we observe the existence of zeros in the partition function. These zeros are related to the existence of complex-pair-conjugate eigenvalues with real parts lying among the low levels of energy. In line with the Yang-Lee framework, these zeros in the complex plane signal phase transitions, and the proposed hybrid model exhibits transitions of first-order. To account for metastable regions in parameter space, we perform a Maxwell construction and a spinodal-decomposition analysis. We determine the critical temperature at which the first zero of the partition-function appears, as a function of the asymmetry parameter and the coupling constant of the interaction between the ensemble of colour nitrogen-vacancy centres in diamond and the superconducting flux-qubit. We also design a Carnot cycle that traverses Exceptional Points in the broken symmetry phase for temperatures above the critical value, achieving the same efficiency as the classical Carnot cycle. Furthermore, we implement a Stirling cycle whose efficiency surpasses its classical counterpart, particularly when operating near Exceptional Points. Finally, we outline how the model can be scaled to larger Hilbert-space dimensions.