A transmon is a superconducting qubit designed to reduce sensitivity to charge noise. It is built from a Josephson junction shunted by a comparatively large capacitance.
Core Idea
The transmon operates in a regime where Josephson energy is much larger than charging energy. This makes the qubit transition frequency less sensitive to small charge fluctuations, improving coherence.
The tradeoff is reduced anharmonicity: the energy levels become more evenly spaced, so control pulses must avoid accidentally driving transitions outside the computational subspace.
Useful Mental Model
Think of a transmon as a weakly anharmonic microwave oscillator. The lowest two energy levels encode |0> and |1>, while higher levels exist and must be managed during gate design.
Control
- Single-qubit gates use shaped microwave pulses.
- Two-qubit gates often use tunable couplers, cross-resonance interactions, or flux modulation.
- Readout usually couples the qubit dispersively to a resonator in circuit QED.