@yuka53; @knp86], where $z$ and $t$ are the space and time separations between two constituent particles. Since the mathematics of this physical system is the same as the series given in Eq.(\[seri00\]), the physical concept of entanglement can be transferred to the Lorentz-covariant bound state, as illustrated in Fig. \[resonance\].

![One mathematics for two branches of physics. Let us look at Eq.(\[seri00\]) and Eq.(\[i01\]) applicable to quantum optics and special relativity respectively. They are the same formula from the Lorentz group with different variables as in the case of the LCR circuit and the mechanical oscillator sharing the same second-order differential equation.[]{data-label="resonance"}](resonance11b.eps)

We can approach this problem from the system of two harmonic oscillators. In 1963, Paul A. M. Dirac studied the symmetry of this two-oscillator system and discussed all possible transformations applicable to this oscillator [@dir63]. He concluded that there are ten possible generators of transformations satisfying a closed set of commutation relations. He then noted that this closed set corresponds to the Lie algebra of the $O(3,2)$ de Sitter group which is the Lorentz group applicable to three space-like and two time-like dimensions. This $O(3,2)$ group has two $O(3,1)$ Lorentz groups as its subgroups.

We note that the 