Actually, I was thinking about a kind of "triple threat" propulsion system, something that would combine the Bussard ramjet, a zero-point energy laser (ZPE), and matter conversion.
In reverse order: matter-to-energy conversion begins with setting up conditions where atoms disintegrate into protons and neutrons, and the neutrons undergo beta decay into protons, thus producing hydrogen of various flavors and some energy. This releases the binding energy of the nucleus and you end up with very hot, highly ionized hydrogen gas.
Now, you can let this gas exit the back of your spaceship's engine and get a good deal of thrust from it. But what if you first superheat it with a ZPE? Thanks to that, you now have hydrogen ions leaving the engine somewhere close to the speed of light, and as a bonus you're going to get some fusion out of it.
Any starship that moves close to the speed of light needs to be protected from radiation and dust. You have a fairly thick shield of ice at the bow of the ship to deal with dust impacts and to act as a radiation shield, but your engines produce enough energy to let you run a big electromagnet, and if you're feeling particularly plucky you can shape it into a ramscoop and funnel what it catches down the core of the ship into the reaction chamber where the ZPE is superheating your hydrogen plasma.
...in a pinch you can probably use boost from the ramjet alone, but only above certain speeds. Still, it beats walking. The main point is that you're heating your hydrogen even hotter, getting more fusion from it, and thus more energy.
Thought about all this as I was going to sleep last night, and in the sober light of morning I realized the Bussard ramscoop is probably not worth the energy outlay, not when you compare what you get from it to the output of the ZPE and the mass convertor.
But all that is secondary to the thoughts I had on the ZPE itself. As noted in prior posts on ZPE as propulsion, a laser spacedrive is also a pretty powerful cutting torch. (See also Larry Niven's short story The Warriors.) You turn that thing on and point it at Earth, and there's going to be trouble.
You end up doing one of two things: either you collimate the beam down to something absurdly thin, or you diffuse the hell out of it. In theory it does not matter to the overall efficiency of the drive, because you are spewing the same number of photons.
A ZPE propulsion beam the thickness of your little finger, pointed at Earth from geostationary orbit, would not itself kill a large number of people, but holding the beam perfectly still with respect to the surface of the Earth would be impossible. It would wander, and it would slice right through anything it touched. Further, the explosive venting of superheated gas from vaporized rock would be disastrous, and it would leave cracks in the crust which were miles deep.
The same energy diffused--there's a practical limit to how much you can diffuse a laser beam of a set aperture; being generous, I'm assuming an aperture of ten meters and a diffuser which broadens the beam to a hundred kilometers at 25,000 miles.
Problem: you're still baking a spot 100 kilometers across. We're talking about a lot of coherent light, here, and it's probably hot enough to crisp everything in that circle to ash.
...that's why the hypothetical starship for this story has a hybrid system, the matter conversion/fusion coupled to the ZPE. The ZPE acts like an afterburner, something you switch on when you're well away from Earth. It also lets me take a step back from the extreme of having ZPE for the sole propulsion system. Also it gets me away from total matter-to-energy conversion, which is kind of sticky for other reasons.
In truth I would be happy with a reactor that would only get as far as reducing the input mass to hydrogen, because that alone would release a lot of energy and it would run on anything. (I picked ammonia for the ship's preferred fuel because most of its mass comes from nitrogen and it's extremely common.)
...easy for me to say, coming from a civilization that hasn't yet figured out continuous fusion, but a man can dream.