Short answer: The gatemon is a transmon-like superconducting qubit whose Josephson energy is set by a voltage on a gate instead of a magnetic flux. A 2015 Physical Review Letters paper introduced it with a semiconductor nanowire and reported relaxation of about 0.8 microseconds, and a 2018 Nature Nanotechnology paper built gatemons from a wafer-scale two-dimensional electron gas with coherence up to about 2 microseconds. These early devices have short coherence times and are laboratory demonstrations.
What a gatemon is
The 2015 abstract introduces a hybrid qubit based on a semiconductor nanowire with an epitaxially grown superconductor layer. The Josephson energy of the transmon-like device, which the authors name the gatemon, is controlled by an electrostatic gate that depletes carriers in a semiconducting weak-link region. The 2018 abstract adds context: the nonlinear inductance of Josephson junctions sets qubit frequencies, coupling strengths and parametric amplifier gain, and is usually set by junction dimensions or tuned magnetically with flux lines. Superconductor-semiconductor hybrid junctions are described as a tantalizing all-electric alternative.
2015: the first nanowire gatemon
The authors demonstrate strong coupling to an on-chip microwave cavity and coherent qubit control with gate voltage pulses. They report relaxation times of about 0.8 microseconds and dephasing times of about 1 microsecond, which they say exceed gate operation times by two orders of magnitude in these first-generation devices. They list reasons for voltage control: dissipation in resistive control lines is reduced, screening reduces crosstalk, and the absence of flux control allows operation in a magnetic field, which they say is relevant for topological quantum information. These are the authors' stated advantages, not tested outcomes in the abstract.
2018: wafer-scale semiconductor
The Nature Nanotechnology paper goes beyond proof of concept. The authors demonstrate that semiconducting channels etched from a wafer-scale two-dimensional electron gas are a suitable platform for a scalable gatemon-based quantum computer. They show that these gatemons perform voltage-controlled single-qubit rotations and two-qubit swap operations. They measure coherence times up to about 2 microseconds, limited by dielectric loss in the 2DEG substrate. The claim of suitability for a scalable computer is their assessment, supported by the two operations shown.
Reading the progression
Between the two papers the platform moved from a single nanowire device coupled to a cavity to etched channels from a wafer-scale material, and from single-qubit control to a two-qubit swap. The coherence number moved from about 1 microsecond to about 2 microseconds. The authors of the later paper tie the ceiling to a material property of the substrate, which suggests the limit is about materials rather than the gate-voltage control idea itself. That reading is an interpretation of their statement and not a result in the abstracts.
Limits
Coherence times of about 1 to 2 microseconds are short compared with other superconducting qubits, and the 2018 authors name substrate dielectric loss as the limit. The abstracts do not report gate fidelities, so no comparison with other qubit types is made here. A third copy of the 2015 paper from a university repository was listed as a source and is the same work.
FAQ
How is a gatemon controlled?
By a voltage on a gate that changes the Josephson energy, instead of a magnetic flux.
How long did early gatemons last?
About 0.8 microseconds relaxation in 2015, and coherence up to about 2 microseconds in 2018.
Why use voltage control?
The 2015 authors cite reduced dissipation, less crosstalk and operation in a magnetic field.
What limited the 2018 devices?
Dielectric loss in the 2DEG substrate, per the authors.
Is it a competitor to the transmon?
The abstracts do not make that comparison. They report early demonstrations.
All field notes