Complex sentence: Additionally, one of the mirrors has to be very tiny, fixed on an isolated micromechanical-oscillator allowing to move when the photon is reflected on it, so that it becomes superposed with the photon.
Split into simpler sentences: Additionally, one of the mirrors has to be very tiny and fixed on an isolated micromechanical-oscillator. This allows it to move when the photon is reflected on it, so that it may become superposed with the photon.