Qubit ZooDescription
The Gmon is a superconducting qubit architecture developed by Google that adds a tunable coupler between neighboring Xmon-style qubits. Introduced by Chen et al. (2014), the “g” in Gmon refers to the tunable coupling strength between qubits.
The tunable coupler is itself a frequency-tunable transmon-like element placed between two computational qubits. By adjusting its frequency (via a flux line), the effective qubit-qubit coupling can be tuned from a finite positive value through zero to a finite negative value. This enables:
- Fast two-qubit gates: bringing qubits into resonance with strong coupling for SWAP-like gates in ~10–20 ns.
- Idle isolation: parking the coupler to cancel residual coupling, suppressing always-on errors.
- CZ gates: diabatic flux pulses that accumulate a conditional phase.
The Gmon/tunable-coupler architecture was used in Google’s Sycamore (2019) and Willow (2024) processors and has become the dominant paradigm for frequency-tunable superconducting qubit arrays.
Figure
Hamiltonian
The three-body system (qubit 1 — coupler — qubit 2):
The effective qubit-qubit coupling after adiabatically eliminating the coupler:
where is the coupler detuning. Setting achieves zero effective coupling (idle point).
Motivation
Fixed-coupling architectures suffer from always-on interaction, causing idle errors and frequency-crowding constraints. The tunable coupler solves both: it allows fast gates when coupling is “on” and near-perfect isolation when “off,” dramatically improving circuit fidelity for multi-qubit algorithms.
Key Metrics
| Metric | Value | Notes | Fidelity reference |
|---|---|---|---|
| 2Q gate fidelity | 99.5–99.9% | CZ or SWAP | Chen 2014 |
| 2Q gate time | 10–30 ns | Fast parametric gates | — |
| Residual (off) | <10 kHz | At idle point | — |
| 1Q gate fidelity | 99.9%+ | Same as Xmon | Chen et al. 2014 |
| Operating temperature | 10–20 mK | Dilution refrigerator | — |