Effect of the size of environment on the steady-state entanglement and coherence via collision model

The steady-state properties of an open quantum system are investigated via the collision model method of system–reservoir interaction. In our collision model, the system of interest consists of two coupled qubits, each of which interacts with its own independent thermal reservoir. Each thermal reservoir is modeled as a set of clusters of ancillas (qubits or linear harmonic oscillators). We show that collective interaction between the system and the ancillas in the clusters is beneficial to the generation and enhancement of the steady-state entanglement. For the low-temperature thermal reservoir, increasing the size of the clusters is more conducive to the entanglement improvement. Remarkably, it turns out that pronounced entanglement enhancement can be achieved through choosing a suitable size of the clusters of thermal reservoirs. We also study the effect of the size of the cluster on the steady-state coherence. The numerical results show that for the qubit clusters, whether the steady-state coherence of the system can be enhanced depends on the coupling strength between the two system qubits and that between the system and the thermal reservoirs. While for the case of the harmonic oscillator clusters, in addition to the coupling strengths, the enhancement of steady-state coherence also depends on the temperature of thermal reservoirs.