Transmon

The transmon is a specific superconducting qubit design, short for transmission-line shunted plasma oscillation qubit, and the dominant design across the industry today. It takes a Josephson junction, the nonlinear element that creates a usable two-level system, and shunts it with a large capacitor. The large capacitance flattens the qubit's sensitivity to charge noise, the random movement of stray charges near the junction that was the main coherence killer in earlier charge-qubit designs, at the price of slightly weaker nonlinearity. That trade was worth it: transmon coherence times jumped from nanoseconds to tens of microseconds, and the design became the workhorse behind IBM, Google, and Rigetti machines. Fast single-qubit and two-qubit gates, dispersive readout through a coupled resonator, and straightforward fabrication are the reasons the transmon dominates commercial superconducting hardware.