Classical communication capacity of a channel can be enhanced either through a device called a 'quantum switch' or by putting the channel in a quantum superposition. The gains in the two cases, although different, have their origin in the use of a quantum resource, but is it the same resource? Here this question is explored through simulating large sets of random channels. We find that quantum superposition always provides an advantage, while the quantum switch does not: it can either increase or decrease communication capacity. The origin of this discrepancy can be attributed to a subtle combination of superposition and non-commutativity.

We use the galaxy rotation curves in the SPARC database to compare nine different dark matter (DM) and modified gravity models on an equal footing, paying special attention to the stellar mass-to-light ratios. We compare three noninteracting DM models: a self-interacting DM model, two hadronically interacting DM models, and three modified Newtonian dynamics type models, modified Newtonian dynamics (MOND), a radial acceleration relation, and a maximal-disk model. The models with DM–gas interactions generate a disky component in the DM, which significantly improves the fits to the rotation curves compared to all other models except an Einasto halo; the MOND-type models give significantly worse fits.

A rotation curve inequality that holds for spherically symmetric mass distributions is derived, and tested against the SPARC galaxy rotation curves dataset. We identify several Galaxies, e.g. NGC7793 and UGC11557, which are candidates for hosting non-spherical dark matter structures that could be detected by more precise measurements.