Josephson Junction
Two superconductors separated by a whisper-thin insulating barrier: the nonlinear, non-dissipative circuit element at the heart of every superconducting qubit.
A Josephson junction is two superconductors separated by a whisper-thin insulating barrier, thin enough that Cooper pairs of electrons tunnel through without resistance. Predicted by Brian Josephson in 1962, it's the only nonlinear, non-dissipative circuit element physicists have, and that combination is why it sits at the heart of every superconducting qubit.
The nonlinearity matters because a plain superconducting LC circuit has evenly spaced energy levels, so you can't single out two of them as your 0 and 1. Add a Josephson junction and the level spacing becomes uneven, so the bottom two states can be addressed without accidentally exciting the rest. That's what turns a circuit into a qubit.