Possibly a bit on the futuristic side - though here is some physical reasoning for a relativistic thrust rocket:

https://www.sciencedirect.com/science/artic...3179?via%3Dihub

This concept seems to be based on an exceedingly unusual state of matter that appears to be investigated by only a very small number of investigators since a decade, at least. I wonder why I didn't hear anything of that exotic matter, although it would be highly interesting, if it would really exist. So, I'm inclined to consider that the paper, and the cluster of related papers, may not be about actual science.
Rydberg atoms exist, but they are much larger than ordinary atoms. And I don't see how extremely dense Bose-Einstein condensates or anything similar should form from such Rydberg atoms, especially none stable up to temperatures of 1 MK. To the contrary, these atoms are so sensitive that they react on the presence of single nearby photons.
Beyond this, protons won't decay or annihilate into kaons or pions by low-energy laser pulses, not even if they would be as close to each other like suggested for the supposed exotic state of matter.

The source article was published in a relevant peer-reviewed journal, so that's good. However, it does seem to have some questionable physics as Gerald pointed out, so in accordance with rule 1.9 re SF engineering this topic is closed.