Time evolution of the concurrence for different initial states of the qubits for (a) R = 10 and (b) R = 0.1. We have set β = 2 × 10⁻⁹, ω₀/λ = 1.5 × 10⁹ and θ1=π2 , ϕ₁ = 0, θ2=π4 , ϕ₂ = 0 (solid blue line), θ1=π2 , ϕ₁ = 0, θ₂ = 0, ϕ₂ = 0 (dashed red line) and θ1=π2 , ϕ₁ = π, θ2=π4 , ϕ₂ = 0 (dot-dashed green line).