Representation of Quantum Entanglement using Thermo Field Dynamics

Several states are entangled essentially through non-separable quantum fluctuations appearing in quantum systems such as the singlet state. These behaviors of quantum states are called “quantum entanglements”. Recently, we have introduced an interesting method to understand the quantum entanglement using the thermo field dynamics. Especially, we focus on the definition of the extended density matrix in the double Hilbert space and thereby we study such typical behaviors of finite-size quantum systems as the classical-quantum crossover-oscillation.