Interference of single photons emitted from solid state sources Solid state emitters such as quantum dots are thought of as being amongst the best single photon sources for quantum technology.
There have been impressive demonstrations of >10 photon experiments with a quantum dot (most recently arXiv:1910.09930) which shows >90% indistinguishability of photons emitted from the same dot at different times. This allowed the authors to have good interference when demultiplexing the source into different spatial modes.
If we want to go to higher and higher photon number eventually demultiplexing one source will be less favourable compared to using multiple emitters in terms of absolute detection rate.
When searching for two photon interference experiments between different emitters however the quoted interference visibility is pretty poor, in the 40% region which is less than the classical limit of 50%.
I am not an expert in solid state photonics and would like to understand why the indistinguishability is so bad between distinct sources and if this is likely to improve in the future?
If this is not likely to improve then surely this is a huge problem if we want to use solid state emitters as a scalable quantum technology?
 A: In short, the main reason is that it is extremely difficult to fabricate solid-state emitters like quantum dots of exactly the same dimensions. 
To conceptually understand this, you can think of a quantum dot as a quantum particle-in-a-box. If the box dimensions are slightly different for different quantum dots, then the energy level separations are slightly different leading to the emitted photons having slightly different frequencies. As a result, the photons lose their indistinguishability. 
On a more fundamental note, I believe that this physical concept of indistinguishability is rather poorly understood. The primary method to quantify the indistinguishability of two photons is to perform a Hong-Ou-Mandel (HOM) interference experiment. The common understanding is that the dip in the coincidence count rate in the HOM experiment is a measure of the indistinguishability of the photons. 
However, HOM interference is NOT an interference between two photons; it is a two-photon interference effect. In other words, it arises due to the interference of two-photon probability amplitudes, NOT one-photon probability amplitudes. For instance, look at this paper , where the two photons are produced from the same source, but don't arrive at the beam-splitter at the same time. Yet, there is almost perfect HOM interference. This is because although the individual photons are distinguishable at the beam-splitter, the two-photon paths from source to detectors are indistinguishable. 
