Superconducting Qubit

A superconducting qubit stores quantum information in the current or charge states of a superconducting circuit built around a Josephson junction, most commonly in the transmon design used across the industry today. The circuit is fabricated on a chip using processes similar to conventional semiconductor lithography, then cooled inside a dilution refrigerator to roughly 10-15 millikelvin, close enough to absolute zero that thermal noise cannot flip the qubit's state. Qubits are controlled and read out with microwave pulses delivered through coaxial wiring, and neighboring qubits are coupled directly or through tunable couplers, which keeps gate times fast (tens of nanoseconds) but makes wiring density and crosstalk the main scaling bottlenecks as chips grow past a few hundred qubits. IBM, Google, and Rigetti all build on this modality. Typical coherence times run from tens of microseconds to just over a millisecond on today's best chips, short enough that error correction and fast gates matter more than on slower modalities.