Fig. 4: Microscopic picture of the QCL dynamics.

a Temporal evolution of the QCL’s density matrix, shown in a reduced Bloch sphere with a radius of 0.2 for a delay time of τ = 0 ps and a bias current of I_B = 780 mA. b Close-up view of the trajectory of the Bloch vector shown in a. c As b, but for I_B = 1080 mA. d Simulated band diagram of the QCL under an electric field of F = 1.1 kV cm⁻¹. The blue and orange lines represent the squared modulus of the wave function of the upper and lower laser level (|Ψ₂|² and |Ψ₁|², respectively). Additionally, the highest miniband state |Ψ_mb1|² as well as the other miniband states |Ψ_mb|² are shown in black and gray. e As in d but for a bias voltage of F = 1.7 kV cm⁻¹. Inset: Calculated dependence of T₂ on the bias current I_B