Approaching transform-limited linewidths in telecom-wavelength transitions of ungated quantum dots

Highly coherent quantum emitters operating in the telecommunication C-band (1530 - 1565nm), where ultra-low-loss fibers and photonic circuits are available, are crucial to the development of scalable quantum technologies. In this work, we report on a modified Stranski-Krastanov growth scheme using chemical beam epitaxy to enable the generation of high-quality InAs/InP quantum dots, characterized by near-transform-limited linewidths ($\Gamma_{\mathrm{TL}}$). We demonstrate the growth of highly-symmetric quantum dots with aspect ratios >0.8 and densities ranging from 2 to 22$\,\mu$m$^{-2}$. Optical characterization of these sources reveal fine-structure splittings down to $25\pm4\,\mu$eV and a single-photon purity of $g^{(2)}(0) = 0.012\pm\mathrm{0.007}$, confirming the quality of these dots. Further, using an etalon to measure the linewidth, in combination with rigorous modelling, we find an upper-bound to the mean, low-power linewidths of only $12.1\pm 6.7\,\Gamma_\mathrm{TL}$ and, in the best case, $2.8\pm 1.8\,\Gamma_\mathrm{TL}$. These results represent a significant step in the development of telecom-wavelength quantum light sources which are essential for complex quantum networks and devices.