Free-orbit experiment with laser interferometry X-rays

The Free-orbit Experiment with Laser Interferometry X-Rays (FELIX)

Later revised to take place as a tabletop experiment,^[2][3] if successful, it is estimated that a mass of roughly 10¹⁴ atoms would have been superposed, approximately nine orders of magnitude more massive than any superposition observed to that date (2003).

The proposed experimental setup is basically a variation of the Michelson interferometer but for a single photon. 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. The purpose is to vary the size of the mirror to investigate the effect of the mass on the time it takes for the quantum system to collapse.

Originally the arms of the interferometer had to stretch into the hundreds of thousands of kilometers to achieve a photon roundtrip-time comparable to the oscillator's period, but that meant that the experiment had to take place in-orbit, reducing its viability. The revised proposal^[2] requires that the mirrors be placed into high-finesse optical cavities that will trap the photons long enough to achieve the desired delay.

There are various technological challenges, but all are within high-end laboratory capabilities. The primary requirement is that the mass of the cavity remains as small as possible. To avoid noise on the interferometer and have a low probability of emitting more than one photon each time, a very low absolute temperature for the experiment is needed, on the order of 60 μK. For similar reasons, and to avoid