Coherence and indistinguishability of highly pure single photons from non-resonantly and resonantly excited telecom C-band quantum dots

ABSTRACT The role of resonant pumping schemes in improving the photon coherence is investigated on InAs/InGaAs/GaAs quantum dots (QDs) emitting in the telecom C-band. The linewidths of transitions of multiple exemplary quantum dots are determined under above-band pumping and resonance fluorescence (RF) via Fourier-transform spectroscopy and resonance scans, respectively. The average linewidth is reduced from (9.74 ± 3.3) GHz in the above-band excitation to (3.50 ± 0.39) GHz under RF underlining its superior coherence properties. Furthermore, the feasibility of coherent state preparation with a fidelity of (49.2 ± 5.8) % is demonstrated, constituting a first step toward on-demand generation of coherent, single, telecom C-band photons directly emitted by QDs. Finally, two-photon excitation of the biexciton is investigated as a resonant pumping scheme. A deconvoluted single-photon purity value of 𝑔(2)HBT(0)=0.072 ± 0.104 and a postselected degree of indistinguishability of 𝑉HOM=0.894 ± 0.109 are determined for the biexciton transition. This represents another step in demonstrating the necessary quantum optical properties for prospective applications.