Let's supposed we take the Wigner's friend experiment from the metaphysical arena and try to implement it as an actual physical experiment
Assuming Wigner's lab friend is kept as a coherent superposition of states until the external experimenter tries to open the decoherence-insulated lab door, then the following check to detect coherence should work, even after the enclosed physicists has interacted with the dead/alive cat, but before the sealed lab is allowed to interact with the external experimenter:
- the sealed lab has two lasers (A and B) inside, that mix at a half-mirror, and then the laser goes out of the lab from the half-mirror. This is the only way the state in the inner lab can affect the external universe. If both lasers are on, only one of the external paths has constructive interference, while the other has full destructive interference.
- if the cat is dead, the sealed lab experimenter turns on laser A and keeps B off. Both branches of the outward paths of the half-mirror give half intensities when the single laser A is on while laser B is off
- if the cat is alive, the sealed lab experimenter turns on laser B and keeps A off. Both branches of the outward paths of the half-mirror give half intensities when the single laser B is on while laser A is off
- After some synchronization time, the external experimenter measures photons from each outward path of the half-mirror, and he sees two possible outcomes:
1) one of the paths has 100% intensity, the other has 0% (coherence is kept)
2) both paths have 50% intensity (coherence was lost previously)
Thoughts? What you expect to be the result of this experiment? why?