Investigating the Qualitative Behaviors of Entanglement and Coherence in an Identical Two-Qubit Coupled System by a Large Capacitor: Application in Quantum Circuits

In this paper, we model an entangled system of two identical superconducting qubits in which Josephson junctions are coupled by a fixed capacitor. This coupling of the capacitor with Josephson junctions is added due to increase the coherence and neutralize the effects of decoherence in the system. To better understand the state of the system, using the theory of quantum computing, the qualitative behaviors of entanglement, coherence, and comparison between them are numerically investigated. It was observed that there is a limit for increasing the mutual coupling energy between two qubits, and beyond that, it will weaken the system performance. It was also seen that the behaviors of the entanglement and the coherence are almost similar and they can be maintained at high temperatures under some conditions. A remarkable point was is that, as the Josephson energy of the qubits increases, the coherence and entanglement increase even at high temperatures. Also, by increasing the mutual coupling energy between qubits, coherence and entanglement are maintained at high temperatures, which can be considered in the construction of entanglement sources. In addition, the difference between an identical and a dissimilar two-qubit system is also stated.