Threshold Dynamics of a Semiconductor Single Atom Maser*

Photon emission from single emitters provides fundamental insight into the detailed interaction between light and matter. Here we demonstrate a semiconductor single atom maser (SeSAM) that consists of a single InAs double quantum dot (DQD) that is coupled to a high quality factor microwave cavity. A finite bias results in population inversion in the DQD, enabling sizable cavity gain and stimulated emission. We develop a pulsed-gate approach that allows the SeSAM to be tuned across the masing threshold. The cavity output power as a function of DQD current is in good agreement with single atom maser theory once a small correction for lead emission is included. Photon statistics measurements show that the second-order correlation function of intra-cavity photon number, $n_{\mathrm{c}}$, crosses over from $\langle n_{\mathrm{c}}^{\mathrm{2}}\rangle $/$\langle n_{\mathrm{c}}\rangle^{\mathrm{2}}=$ 2.1 below threshold to $\langle n_{\mathrm{c}}^{\mathrm{2}}\rangle $/$\langle n_{\mathrm{c}}\rangle^{\mathrm{2}}=$ 1.2 above threshold. Large fluctuations are observed at threshold.