Gatemon

Figure

Description

The gatemon is a transmon-like superconducting qubit in which the conventional superconductor-insulator-superconductor tunnel junction is replaced by a gate-tunable superconductor-semiconductor-superconductor weak link. Implementations include proximitized InAs nanowires, planar InAs two-dimensional electron gases (2DEGs), and other gate-defined quantum conductors. The gate changes the weak link’s transmission eigenvalues and therefore its Josephson potential, making the qubit frequency voltage-tunable without a SQUID flux-bias loop.

Introduced independently by Larsen et al. and de Lange et al. in 2015, the gatemon operates in the transmon regime ($E_J/E_C\gg 1$), which suppresses ordinary offset-charge dispersion. Voltage control avoids the first-order flux-noise channel of a SQUID-tunable transmon and can use high-impedance, low-static-power wiring. It does not make the device automatically lower-noise: charge traps, gate hysteresis, dielectric loss, and junction-specific dissipation remain important.

Because a semiconductor weak link can contain a few highly transmitting channels, its current-phase relation can be strongly non-sinusoidal. The resulting higher harmonics change the anharmonicity and spectrum relative to the tunnel-junction transmon. The same hybrid junction also provides a circuit-QED probe of Andreev physics and a materials route toward magnetic-field-compatible superconducting circuits; Majorana integration remains a proposed research direction rather than a demonstrated gatemon-computing capability.

The central tradeoff is coherence and stability. A 2025 reliability study achieved repeatable 1 MHz frequency placement across several gigahertz, but a controlled 2026 comparison still found gatemon $T_1$ values in the few-microsecond range while matched tunnel-junction transmons reached tens of microseconds. A 2025 $\beta$-Sn/InAs device reported $T_1=27\mu \text{s}$, showing that the limit is materials- and junction-dependent rather than a fixed architectural constant.

Hamiltonian

The charging term is the same as for a transmon, but the Josephson potential is set by the transmission eigenvalues $T_i(V_g)$ of the semiconductor weak link:

$$H=4E_C(\hat{n}-n_g{)}^2+U_J(\hat{\phi };V_g),U_J(\phi ;V_g)=-\Delta \sum _i\sqrt{1-T_i(V_g){\sin }^2\left(\frac{\phi }{2}\right)}.$$

This short-junction expression assumes the even-parity ground branch at temperatures small compared with the induced gap $\Delta$. In the tunnel or low-transparency limit it reduces, up to an additive constant, to the familiar approximation

$H\simeq 4E_C(\hat{n}-n_g{)}^2-E_J(V_g)\cos \hat{\phi }.$

At higher channel transparency the full $U_J$ must be retained; equivalently, the current-phase relation contains higher harmonics. The gate primarily tunes $T_i$ and $E_J$, while $n_g$ denotes offset charge and is not the same control variable.

Motivation

• Replace local SQUID flux bias with high-impedance electrostatic control of the weak-link transmission and qubit frequency.
• Enable voltage-controlled $Z$ rotations and potentially dense integration with semiconductor processing, while accepting gate-noise and hysteresis engineering challenges.
• Use circuit QED to study few-channel Josephson physics and hybrid materials in a coherent quantum circuit.
• Develop superconducting circuits that may tolerate magnetic fields better than conventional all-aluminum SQUID-tunable devices.

Experimental Status

First demonstrations — Larsen et al. and de Lange et al. (2015):

• Larsen et al. demonstrated coherent control of an epitaxial Al/InAs nanowire gatemon, with first-generation $T_1\sim 0.8\mu \text{s}$ and $T_2\sim 1\mu \text{s}$.
• de Lange et al. independently demonstrated capacitively shunted InAs/NbTiN nanowire Josephson elements. Their two-element loop exhibited a non-sinusoidal current-phase relation and flux-qubit-like behavior near half flux; it was not a two-gatemon entangling-gate demonstration.

Benchmarking and planar scaling — Casparis et al. (2016, 2018):

• A two-nanowire-gatemon device demonstrated randomized-benchmarking single-qubit errors below 0.7% (fidelity above 99.3%) and an estimated 91% controlled-phase-gate fidelity in 2016.
• The 2018 device used a proximitized InAs/Al 2DEG, not Ge/SiGe, and demonstrated voltage-controlled single-qubit rotations, coherent swap operations, and coherence up to about $2\mu \text{s}$.

Materials, stability, and present limits (2024–2026):

• Strickland et al. (2024) measured $T_1=100\text{ns}$ across a 1.5 GHz tunable band in one InAs-2DEG process, illustrating that wafer-scale hybrid materials can dominate loss.
• Feldstein-Bofill et al. (2025) achieved reliable 1 MHz frequency placement over several gigahertz and found grounded shunt capacitors more stable than floating designs.
• Purkayastha et al. (2025, preprint) reported a $\beta$-Sn/InAs nanowire gatemon with $T_1=27\mu \text{s}$ and echo $T_2=1.8\mu \text{s}$.
• Sun et al. (2026, preprint) co-fabricated nominally matched gatemons and tunnel-junction transmons. Representative devices gave $T_1=4.5\mu \text{s}$ versus $25.8\mu \text{s}$, and the measured gatemon loss exceeded their Purcell, control-line, and dielectric-loss budget, pointing to additional junction-intrinsic dissipation.

Key Metrics

Metric	Value	Notes	Fidelity reference
$T_1$	27 μs	High-coherence $\beta$-Sn/InAs nanowire gatemon; preprint result	Purkayastha et al. 2025
Echo $T_2$	1.8 μs	Same $\beta$-Sn/InAs device; maximum occurred at a different bias from maximum $T_1$	Purkayastha et al. 2025
Matched-device $T_1$	4.5 μs	Representative Al/InAs gatemon; matched tunnel-junction transmon: 25.8 μs	Sun et al. 2026
Frequency placement	1 MHz precision over several GHz	Grounded gatemon design	Feldstein-Bofill et al. 2025
1Q gate fidelity	>99.3%	Randomized benchmarking; error below 0.7%	Casparis et al. 2016
2Q controlled-phase fidelity	91% (estimated)	Two-nanowire-gatemon device	Casparis et al. 2016

References

Original demonstrations

• T. W. Larsen et al., “Semiconductor-Nanowire-Based Superconducting Qubit,” Phys. Rev. Lett. 115, 127001 (2015); arXiv:1503.08339.
• G. de Lange et al., “Realization of Microwave Quantum Circuits Using Hybrid Superconducting-Semiconducting Nanowire Josephson Elements,” Phys. Rev. Lett. 115, 127002 (2015); arXiv:1503.08483.

Gate benchmarking

• L. Casparis et al., “Gatemon Benchmarking and Two-Qubit Operations,” Phys. Rev. Lett. 116, 150505 (2016); arXiv:1512.09195.

2DEG gatemon

• L. Casparis et al., “Superconducting gatemon qubit based on a proximitized two-dimensional electron gas,” Nature Nanotech. 13, 915–919 (2018); arXiv:1711.07665.

2024–2026 updates

• W. M. Strickland et al., “Characterizing losses in InAs two-dimensional electron gas-based gatemon qubits,” Phys. Rev. Research 6, 023094 (2024); arXiv:2309.17273.
• D. Feldstein-Bofill et al., “Gatemon qubit revisited for improved reliability and stability,” Phys. Rev. Applied 24, 044099 (2025); arXiv:2412.11611.
• A. Purkayastha et al., “Transmon qubit using Sn as a junction superconductor,” arXiv:2508.04007 (2025 preprint).
• Z. Sun et al., “Junction-Intrinsic Dissipation in Hybrid Superconductor-Semiconductor Gatemon Qubits,” arXiv:2603.29498 (2026 preprint).

Linked Papers

• larsen-2015-gatemon
• de-lange-2015-hybrid-nanowire-josephson-elements
• casparis-2016-gatemon-benchmarking
• casparis-2018-ge-gatemon
• strickland-2024-gatemon-losses
• feldstein-bofill-2025-gatemon-reliability
• purkayastha-2025-sn-inas-gatemon
• sun-2026-junction-intrinsic-dissipation-in-hybrid
• hays-2021-andreev-spin-qubit
• shim-2014-bottom-up-sc
• shim-2016-semiconductor-inspired
• strickland-2024-gatemonium

Evergreen context

• josephson-junction-as-nonlinear-element — the gatemon keeps transmon physics but replaces the oxide tunnel barrier with a semiconductor weak link whose $E_J$ is tuned electrostatically.
• dispersive-readout-mechanism — in practice the measurement story is still the standard circuit-QED one: gate tuning changes the qubit, but the state is read out from a small resonator shift rather than from a new measurement primitive.
• charge-noise-in-superconducting-qubits — even with a semiconductor weak link, the underlying CPB/transmon Hamiltonian still inherits the same offset-charge noise channel that large $E_J/E_C$ is meant to suppress.
• charge-noise-sweet-spot — its protection story is still the transmon one: keep charge dispersion flat enough that the added gate knob does not drag the device back toward charge-qubit fragility.
• quantum-hardware — this is the cleanest bridge between mainstream superconducting control stacks and semiconductor-process ambitions.

Related Entries

• transmon — parent qubit architecture; gatemon replaces the tunnel junction
• gatemonium — related semiconductor-superconductor hybrid qubit
• andreev-spin-qubit — same material platform, spin degree of freedom
• quantum-transduction — hybrid superconducting-semiconductor circuits are also studied as interfaces and field-compatible circuit elements