The Transmon Qubit
The transmon trades a controlled loss of anharmonicity for strong immunity to charge noise, and dominates superconducting quantum computing today.
Design idea
The transmon (transmission-line shunted plasma oscillation qubit) is a Josephson junction shunted by a large capacitor. It is characterized by two energies: the Josephson energy E_J and the charging energy E_C. Charge noise sensitivity falls exponentially as the ratio E_J/E_C grows, while the anharmonicity falls only as a power law. Operating at E_J/E_C of roughly 50 to 100 buys near-total charge-noise immunity for a modest price in anharmonicity.
Level structure
The transmon is a weakly anharmonic oscillator. Its 0 to 1 transition sits near 4 to 6 GHz. The anharmonicity, the difference between the 0-1 and 1-2 frequencies, is negative and around -200 to -300 MHz. Because that gap is small, control pulses must be shaped in time and frequency to avoid exciting the 2 state, an error called leakage.
DRAG pulses
Derivative Removal by Adiabatic Gate (DRAG) adds a quadrature component proportional to the derivative of the main pulse envelope, cancelling leakage to the 2 level and phase errors. It is standard for fast single-qubit gates on transmons, enabling gate times near 20 to 40 ns with fidelities above 99.9 percent in good devices.
Tunable and fixed variants
- Fixed-frequency transmons: a single junction, quiet but harder to couple on demand
- Tunable transmons: a SQUID loop of two junctions whose frequency shifts with applied flux, easing two-qubit gates but adding a flux-noise channel
Readout uses a dispersive coupling to a microwave resonator whose frequency shifts depending on qubit state. The transmon's combination of manufacturability, fast gates, and workable coherence is why it anchors most large superconducting roadmaps.