It seems complicated and it is, yet this is one of the most interesting an counter intuitive physical quantum behaviors. Without it, lots of good SF books wouldn't have been written.

All of the steps – from photon detection to mirror movement – take about 7.1 microseconds, which is significantly less than the 60 microseconds it would take a photon to cover the 18 km between interferometers. So measurements made simultaneously at each of the interferometers could not be been influenced by anything traveling at – or even a few times more than – the speed of light.