[eliminating the need to carry propellant]...violate conservation of momentum by invoking some sort of vaguely defined quantum woo...
What are we calling "propellant"? It requires power to generate the radiation. How is this diff than say an ion engine? It needs SOME energy source, just not necessarily traditional sources such as flammable chemicals.
And as the radiation bounces around inside the Gumby-head-shaped chamber, it does lose some energy on each bounce such that the radiation generator has keep doing its job.
What's unknown is if we get "bonus" energy beyond what say an ion engine can do with the same amount of electricity.
The speculation is kind of like cold-fusion for space: direct matter-to-energy conversion without the messy side-effects such as high temperatures and dangerous radiation found in traditional nuclear reactors.