Free-orbit experiment with laser interferometry X-rays

The Free-orbit Experiment with Laser Interferometry X-Rays (FELIX) belongs to a category of experiments exploring whether macroscopic systems can be in superposition states. It was originally proposed by the physicist Roger Penrose in his 2004 book The Road to Reality specifically to prove whether unconventional decoherence processes such as gravitationally induced decoherence or spontaneous wave-function collapse of a quantum system occur.

Later revised to take place as a tabletop experiment, 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).