The quantum flux parametron is a really fascinating design and I always wondered why no one talks about it. You can get to GHz range easily and computing will be adiabatic. Its based on Josephson-junctions so you need to provide very low temperatures. I always thought it was a more promising next gen compute technology than the current quantum computers. Especially when you get to write your own SQUIDs in a SEM. Its like two long rectangles with pads at the end+thin insulator on top+two wires for the contacts for a SQUID sandwich and suddenly you can do complex quantum circuits.