Gatemon

Figure

Description

The gatemon is a superconducting transmon-style qubit where the conventional tunnel-barrier Josephson junction is replaced by a semiconductor nanowire (typically InAs or InAs/Al) with a gate-tunable weak link. The supercurrent through the nanowire depends on the gate voltage applied to the semiconductor channel, making $E_J$ — and therefore the qubit frequency — voltage-tunable rather than flux-tunable.

Introduced independently by Larsen et al. and de Lange et al. in 2015, the gatemon inherits the transmon’s charge-noise insensitivity ($E_J/E_C\gg 1$) while gaining two advantages:

1. Voltage tunability: no magnetic flux required, eliminating flux-noise sensitivity and simplifying wiring.
2. Semiconductor integration: the nanowire channel can host exotic physics (Andreev bound states, Majorana modes) while simultaneously serving as the qubit element.

The current-phase relation of the semiconductor weak link can deviate from sinusoidal, introducing higher harmonics ($I(\phi )=I_c\sin \phi +I_2\sin 2\phi +\cdots$) that modify the anharmonicity and energy-level structure compared to a standard transmon.

The tradeoff is that semiconductor junctions currently have higher loss and lower reproducibility than aluminum oxide tunnel junctions, resulting in shorter coherence times than state-of-the-art transmons.

Hamiltonian

Same form as the transmon, but with gate-voltage-dependent Josephson energy:

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

The current-phase relation of the semiconductor weak link can deviate from sinusoidal, introducing higher harmonics:

$I(\phi )=I_c\sin \phi +I_2\sin 2\phi +\cdots$

which modifies the anharmonicity and energy-level structure compared to a standard transmon.

Motivation

• Flux-tunable transmons require magnetic flux bias lines that add wiring complexity and introduce flux noise.
• The gatemon replaces this with a DC gate voltage — simpler, lower noise, and compatible with semiconductor qubit co-integration on the same chip.
• Provides a platform for exploring novel semiconductor-superconductor physics (Andreev states, Majorana modes) within a circuit-QED-compatible architecture.

Experimental Status

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

• Larsen et al. demonstrated the first gatemon using an InAs nanowire Josephson junction, showing gate-voltage tunability of the qubit frequency from 4–8 GHz.
• de Lange et al. independently demonstrated a gatemon with two-qubit coupling.
• Coherence times of $T_1\sim 1$–$10\mu$s, limited by semiconductor junction losses.

Ge/SiGe gatemon — Casparis et al. (2018):

• Extended the gatemon concept to a proximitized two-dimensional electron gas (2DEG) platform.
• Improved junction reproducibility compared to nanowire devices.

Ongoing development (2020s):

• Continued materials and fabrication improvements targeting higher coherence.
• Integration with Majorana-based topological qubit proposals.

Key Metrics

Metric	Value	Notes	Fidelity reference
$T_1$	1–10 μs	Limited by semiconductor junction loss	Larsen et al. 2015
$T_2$	1–5 μs	Dominated by charge and junction noise	—
Frequency tunability	4–8 GHz	Via gate voltage	—
1Q gate fidelity	98–99.5%	Improving with materials	Larsen et al. 2015
Operating temperature	10–20 mK	Dilution refrigerator	—

References

Original demonstrations

• T. W. Larsen et al., “Semiconductor-Nanowire-Based Superconducting Qubit,” Phys. Rev. Lett. 115, 127001 (2015)

2DEG gatemon

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

Linked Papers

• larsen-2015-gatemon
• casparis-2018-ge-gatemon

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
• ferbo-qubit — uses same nanowire weak link platform in high-impedance regime for dual noise protection