Giant photon gain in large-scale quantum circuit-QED systems: Diagrammatic non-equilibrium Green's function approach

Motivated by recent experiments on the generation of coherent light in engineered hybrid quantum systems, we investigate gain in a microwave photonic cavity coupled to quantum dot structures and develop concrete directions for achieving a giant amplification in photon transmission by employing the Keldysh NEGF technique [1]. We propose two architectures for scaling up the electronic gain medium: (i) N double-quantum-dot systems and (ii) M quantum dots arranged in series akin to a quantum cascade laser setup. In both setups, the fermionic reservoirs are voltage biased, and the quantum dots are coupled to a single-mode cavity. Optical amplification is explained based on a sum rule for the transmission function, and it is determined by an intricate competition between two different processes: charge-density response in the gain medium and cavity losses to input and output ports [2]. The same design principle is also responsible for the corresponding giant amplification in other photonic observables, mean photon number, and emission spectrum, thereby realizing a quantum device that behaves as a giant microwave amplifier. \\ [1] Agarwalla et al, Phys. Rev. B 94, 035434 (2016) \\ [2] Agarwalla et al, Phys. Rev. B 94, 121305(R) (2016).