Standard quantum theory allows for superpositions of macroscopically different states of macroscopic objects. Such states, called Schrödinger's Cats, collapse under standard quantum measurements. Massive Schrödinger Cats are relevant and problematic in quantum gravity, and also remain problematic in the Newtonian non-relativistic limit. The speaker [1] and Penrose [2] have introduced a tiny universal decoherence - proportional to Newton's constant G - to modify the standard unitary qauntum dynamics. Accordingly, any Schrödinger Cat collapses spontaneously, without any measurement. After 30 years, dedicated tests using quantum nanomechanics are under construction or planning in earth- (e.g. [3]) as well as in space-based labs (e.g. [4]), results are expected in the coming years.

Indirect tests were already reported (e.g. [5] based on Lisa Pathfinder's data) to constrain the short-length cutoff parameter of the theory. A most recent indirect test in the Gran Sasso underground lab [6], co-authored by the speaker, has led to headlines, raising the question whether collapse of Schrödinger Cats has anything to do with gravity.

 

 

[1] Diosi, PLA 120,377 (1987)

[2] Penrose, GRG 28, 581 (1996)

[3] Pontin et al, PRR 2, 023349 (2020)

[4] Kaltenbaek et al, EPJ Quant.Tech. 3:5 (2016)

[5] Helou et al, PRD 95, 084054 (2017)

[6] Donadi et al, Nat.Phys. https://doi.org/10.1038/s41567-020-1008-4 (2020)