The gatemon is a superconducting transmon qubit whose Josephson junction is a gate-voltage-tunable semiconductor weak link, promising all-electric qubit-frequency control, CMOS-compatible group-IV materials, and protected-qubit architectures. Every demonstrated gatemon — in InAs, Ge, carbon nanotubes, and graphene — nonetheless relaxes orders of magnitude faster than fixed-junction (aluminium-oxide-barrier) transmons: group-IV devices reach 75 ns to 1.27 microseconds, and even co-fabricated InAs gatemons saturate at a few microseconds while otherwise-identical fixed-junction transmons on the same chip reach tens of microseconds, the excess being attributed to temperature-independent, junction-intrinsic dissipation of unidentified microscopic origin. The open problem is whether voltage-tunable junction qubits can reach the coherence, stability, and reproducibility of fixed-junction transmons, and whether the junction-intrinsic dissipation and charge-noise-driven instabilities that currently cap them can be identified and eliminated.

Background

A superconducting transmon qubit encodes quantum information in the two lowest energy levels of a weakly anharmonic superconducting oscillator whose nonlinearity is provided by a Josephson junction — a weak link between two superconductors that transmits Cooper pairs. In conventional hardware the junction is a superconductor–insulator–superconductor (SIS) device, typically Al/AlOx_x/Al, whose oxide tunnel barrier is fixed at fabrication; individual qubit frequencies can then be shifted only magnetically, via flux-bias lines, which add wiring, crosstalk, and noise channels that complicate scaling. State-of-the-art fixed-junction transmons reach energy-relaxation times T1T_1 (the time over which the excited-state population decays) of tens to hundreds of microseconds.

A gatemon replaces the oxide barrier with a superconductor–semiconductor–superconductor (SNS) weak link: a semiconducting channel in which superconductivity is induced by the proximity effect (superconducting correlations leaking from attached superconducting contacts, usually aluminium). Because the channel's carrier density is controlled by an electrostatic gate, the Josephson energy — and hence the qubit frequency — becomes voltage-tunable, removing flux lines and enabling in-situ frequency calibration. Built in CMOS-compatible group-IV materials (Ge, Si, carbon; CMOS = complementary metal-oxide-semiconductor, the standard industrial semiconductor technology), gatemons additionally promise integration with semiconductor processing and with spin qubits, and their highly transparent junctions underpin proposed parity-protected cos⁡(2ϕ)\cos(2\phi) qubits (whose double-well energy potential suppresses charge and flux noise but requires a junction that transmits Cooper pairs in pairs) and Andreev- and spin-based hybrid qubits (which would encode information in discrete subgap states bound within the junction, or in the spin of its carriers).

Gatemons were first demonstrated with InAs nanowire junctions (Larsen et al. 2015), reached coherence up to about 2 μ2\,\mus in InAs two-dimensional-electron-gas devices, limited by substrate dielectric loss (Casparis et al. 2018), and have since been realized in graphene (Wang et al. 2019), in InAs grown selectively on silicon (Hertel et al. 2022), and — the technologically central instance — in group-IV platforms: hole gatemons in Ge/Si core/shell nanowires with T1∼180T_1\sim 180 ns (Zhuo et al. 2023) and a group-IV record T1=1.27 μT_1=1.27\,\mus in a two-channel device (Zheng et al. 2024), planar Ge gatemons with T1T_1 and Ramsey coherence time T2∗T_2^* up to 75 ns (Sagi et al. 2024), Ge quantum-well gatemons with T1T_1 up to 119 ns (Kiyooka et al. 2025), and carbon-nanotube gatemons with coherence up to 200 ns, limited by charge noise (Mergenthaler et al. 2021; Riechert et al. 2025).

Across all platforms the pattern is the same: voltage tunability works, but coherence sits one to three orders of magnitude below fixed-junction transmons, and the semiconductor junction brings excess noise sensitivity. A semiconductor junction carries an effective offset charge (the net gate-induced charge that shifts the junction's energy levels), so moving trapped charges cause charge jumps — abrupt qubit-frequency discontinuities, seen for example in the planar-Ge device, whose anharmonicity (the spacing difference between the 0→10\to1 and 1→21\to2 transition frequencies, which must be resolved for selective qubit control) also fluctuated without explanation. A 2026 co-fabrication study made the comparison controlled: SIS transmons and gatemons built on the same chips with nominally identical layouts, dielectrics, and control lines show transmons at T1T_1 of tens of microseconds while gatemons saturate at a few microseconds; the constructed loss budget (Purcell decay — relaxation by photon emission into the measurement line through the coupled readout resonator — spontaneous emission through the control line, and internal dielectric loss) would permit T1T_1 more than an order of magnitude above every measured gatemon, and the temperature-dependent part of the relaxation follows a common model of thermally activated quasiparticles (unpaired excitations above the superconducting ground state), with similar superconducting gaps for both junction types — leaving a dominant, temperature-independent, junction-intrinsic dissipation whose microscopic origin is not understood (Sun et al. 2026). Separately, gatemon frequency-versus-gate-voltage stability and hysteresis have been systematically characterized and partially improved through shunt-capacitor design (Feldstein-Bofill et al. 2025), and theory has begun to address gate-dependent offset-charge shifts and anharmonicity in weak-tunneling junctions (Güngördü et al. 2026). Whether voltage-tunable junction qubits can reach the coherence, stability, and reproducibility of fixed-junction transmons — and what physics stands in the way — is the open problem.

Problem Statement

Can a superconducting qubit whose Josephson element is a gate-voltage-tuned semiconductor weak link — in the CMOS-compatible group-IV platforms (planar Ge/SiGe, Ge/Si core/shell nanowires, carbon nanotubes, graphene) or any other material system — achieve relaxation and coherence times, gate-voltage stability, and device-to-device reproducibility comparable to those of fixed-junction SIS transmons fabricated in the same circuit environment (as benchmarked, for example, by co-fabricating both junction types on one chip with identical layouts, dielectrics, and control lines)? And can the mechanisms currently responsible for the shortfall — above all the temperature-independent, junction-intrinsic dissipation that caps even co-fabricated gatemons at a few microseconds while otherwise-identical transmons reach tens of microseconds, together with charge-noise-driven offset-charge fluctuations that cause qubit-frequency discontinuities, hysteresis, and dephasing — be identified microscopically and eliminated? The problem is stated in its natural generality: closing the coherence-and-control gap for voltage-tunable junction qubits, not reaching any particular coherence time in any single material or geometry. Resolution requires both parts of the question together: the gap counts as closed only when relaxation and coherence times, gate-voltage stability, and device-to-device reproducibility have all been brought to the level of fixed-junction SIS transmons in the same circuit environment — judged against that co-fabrication benchmark rather than any absolute coherence target — and the junction-limiting mechanisms must at the same time be identified microscopically and eliminated; establishing one part without the other leaves the problem open.

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