transmon
A transmon is the most common kind of superconducting qubit, the type of artificial atom used in most superconducting quantum computers (including the big machines from IBM and Google). The basic idea is to build a tiny electrical circuit out of superconducting metal that, when chilled to near absolute zero, has neatly separated energy levels. You pick the lowest two of those levels and call them |0> and |1>, and that circuit now behaves like a qubit you can control with microwave pulses.
Earlier superconducting qubits were painfully sensitive to charge noise: stray electric charges drifting nearby would jostle the energy levels and scramble the information almost immediately. The transmon's clever fix is to add a relatively large capacitor across the circuit. This flattens out the qubit's dependence on charge so much that charge noise barely matters anymore, which is the main reason transmons hold their state long enough to be useful. The trade-off is that the energy levels become more evenly spaced (less anharmonicity), so |0> and |1> sit closer in spacing to higher, unwanted levels. Engineers manage this with carefully shaped pulses so a gate nudges the qubit between |0> and |1> without accidentally leaking it up to level 2 or beyond.
A transmon stores one qubit in its two lowest energy levels; control pulses are tuned to stay inside this |0>/|1> space and avoid exciting higher levels.
Transmons are a leading platform, not a finished one: they still need millikelvin refrigeration, their coherence times are measured in microseconds, and today's devices are noisy and far from large-scale fault tolerance.