Here is a report on slowing down c in measurement, slowing down c
The report is from Jan-15 at phys.org site. However, this does not change the velocity of c as measured using the Galilean moon eclipses and transits or the velocity of c as it passes by the Sun during a total solar eclipse where stars are observed displaced according to Einstein near the Sun as their starlight passes by the Sun, measured in May 1919 total solar eclipse, other total solar eclipses since then, and also in August 2017, total solar eclipse across the USA. The quantum world is not altering the velocity of c as astronomers measure in the vacuum of space. We also have the fine structure constant and the electromagnetic force too, spectrums of remote starlight shows this as well as here on Earth.