It's a superconducting switch, not a semiconductor per se. Lookup "Josephson junction". IBM spent a fortune on a huge R&D program to make computers from this, but eventually abandoned it. I think they got some circuitry working but eventually decided it wasn't practical enough to commercialize.
Also, some of today's work in quantum computers uses superconducting qubits. Maybe that's in the same research stage now. No idea if it will ever become practical.
