Lets just say I had practically inifinte energy. How do I go about turning this into a stream of protons? Dont hold back on the quantum field theory. Smashing existing particles together & filtering out what we want(protons) is not a good enough answer.
The first practical complication is that you cannot (as far as we know) create matter without also creating an equal amount of anti matter. Of course the fact that the observable universe is mostly regular matter indicates that there is some lopsidedness to this summitry and so it may be possible to find conditions that at least create slightly more matter then antimatter. Still, this is a problem that you would need to be overcome to get your pure stream of regular matter protons.
Another problem is the fact that to create particles we simply amass a very large amount of energy in a very small space and see what pops out. We have no way to command that only certain particles be created. For example, even if I amass enough energy to allow for the spontaneous creation of a pair of protons (the proton and its anti matter partner) i have no way to know if the protons are what is going to be created, or other particles whose combined mass and energy add up to the mass of the proton pair. We can only predict the frequency that certain particles will be created.
Finally, although the transformation of energy into matter and matter into energy is a common occurrence in nature, and an entire industry (the nuclear power industry) has been made possible by our understanding of the transition we still aren’t anywhere close to having a mass-energy conversion machine.
I can try and explain the conversion machine and our current methods of conversion if you want, but I think its off the topic of your question, and it looks like I’ve mad an ugly wall of text already.
If any of you reading this see something that I’ve got wrong, or want to explain in more detail please do! This is a topic I’ve been curious about for years.
Edit: I saw your post in r/physics. No, we can’t do better then smash particles together and see what comes out. Think of it this way. We don’t create matter, we simply create the conditions that allow matter to be created. The conditions that are needed are a very high concentration of energy, and the only way we have to achieve those conditions are particle collides. Unfortunately if we have enough energy to allow for the creation of a proton, then we have also allowed for the creation of many smaller particles that will need to be filtered out. So I’m sorry if smashing existing particles together & filtering out what we want is not a good enough answer, because right now its the only answer.