Schematics of the nonequilbrium V-type three-level system in (a) the original framework and (b) the polaron framework. The three horizontal solid black lines represent the central three-level model (|u〉, u = l,r,0), and the double-arrowed solid brown line shows the coherent tunneling between two excited states |l〉 and |r〉; the rectangular left red, top-middle purple, and right blue boxes describe three thermal baths, which are characterized by temperatures T_l, T_m, and T_r, respectively; the double-arrowed solid red, purple, and blue curves describe interactions between the system and thermal baths, and the double-arrowed dashed black lines describe transitions between different states assisted by phonons in the corresponding thermal bath.