Steering coherence in quantum dots by carriers injection via tunneling

Coherent control is a key experimental technique for quantum optics and quantum information processing. We demonstrate a new degree of freedom in coherent control of semiconductor quantum dot (QD) ensembles operating at room temperature using the tunneling injection (TI) processes in which charge carriers tunnel directly from a quantum well reservoir to QD confined states. The TI scheme was originally proposed and implemented to improve QD lasers and optical amplifiers, by providing a direct injection path of cold carriers thereby eliminating the hot carrier injection problem which enhances gain nonlinearity. The impact of the TI processes on the coherent time of the QDs was never considered, however. We show here that since the cold carriers that tunnel to the oscillating QD state are incoherent, the rate of injection determines the coherent time of the QDs thereby controlling coherent light-matter interactions. Coherent interactions by means of Rabi oscillations were demonstrated in absorption and for weak excitation pulses in the gain regime. However, Rabi oscillations are totally diminished under strong excitation pulses which increase the rate of stimulated emission, causing the tunneling processes to dominate what shortens the coherence time significantly. Since the tunneling rate, and hence, the coherence time, were controlled by the optical excitation and electrical bias, our finding paves the way for TI-based coherence switching on a sub-picosecond time scale in room-temperature semiconductor nanometric structures.