A coupled quantum dot laser amplifier: Raman transitions between spin singlet and triplet states

A holy grail of photonics research is the realization of a laser that uses a single quantum emitter as the gain medium. Such a device would exhibit a plethora of new features, including lasing without a well-defined threshold and output intensity fluctuations that remain below the shot-noise limit. While single-atom lasers have been demonstrated, compact devices capable of continuous-wave operation require monolithic structures involving a solid-state quantum emitter. Here, we report the observation of steady-state laser amplification in Raman transitions between the lowest-energy entangled spin states of a quantum-dot molecule. Absorption and resonance fluorescence experiments demonstrate that the singlet and triplet states have electric-dipole coupling to a common optically excited state. Fast spin relaxation ensures population inversion on the triplet transition when the singlet transition is driven resonantly. By embedding the quantum-dot molecule in a cavity of modest quality factor, a solid-state single-emitter laser could be realized.