A quantum state is said to be objective if multiple observers are able to recover information about the state and agree among themselves. This is in turn possible only if said information was encoded multiple times into the surrounding environment. In this paper, we show how it is not always possible for the observers to extract all of the relevant information that was initially encoded into the environment. We do this by introducing two quantities with a rigorous definition and a clear operative interpretation: “redundancy”, which quantifies how many times the information was written into the environment, and “consensus”, which is the maximum number of observers able to extract said information.

Article reference: 

D.A. Chisholm, L. Innocenti, G.M. Palma, Quantum 7, 1074 (2023)

Link to the article: “https://quantum-journal.org/papers/q-2023-08-03-1074/”

We show that any dynamics collapsing the wave function in space implies diffusion in momentum. This is relevant since it means that, in order to test the validity of thiesse models, and in general of the quantum superposition principle, one can perform experiments, known as “non-interferometric”, which look for this diffusion effect. The main advantage is that these kind of experiments do not require the creation of a large and stable superposition in space, which is typically very hard to do. This approach was successfully employed in the last decade to constrain the parameters of models of spontaneous wave function collapse and it ruled out the simplest version of the Diósi-Penrose model. Until now, one might have argued that the diffusive effects were just a feature of the specific models considered. In this article, we show that this is not the case: ny space-translation covariant dynamics that complies with the no-signaling constraint, if collapsing the wave function in space, must change the average momentum of the system and/or its spread.

Article reference: 

S. Donadi, L. Ferialdi and A. Bassi, Collapse Dynamics Are Diffuse, Phys. Rev. Lett. 130, 230202 (2023). 

Link to the article: “https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.230202”