Creating fully integrated, gate-tunable qubits has proven challenging for advanced materials platforms. Zhenhai Sun and colleagues at University of Copenhagen achieved coherent control of gatemon qubits fabricated directly within a single process; these use indium arsenide antimonide/aluminium Josephson junctions, key components enabling superconductivity. The team tuned qubit transitions over several gigahertz with representative energy relaxation times reaching zero point seven nine microseconds.

The team successfully created functioning qubits, the essential building blocks for quantum computing, using indium arsenide antimonide and aluminium; this new material combination simplifies manufacturing compared with previous approaches. These qubits are controlled by electrical signals allowing precise tuning of their properties over several gigahertz, demonstrated via experiments showing coherent control of the qubit states.

A key component is the Josephson junction, a tiny switch that allows electrons to flow through even when they shouldn’t, like water finding a leak between two connected containers, enabling superconductivity within these devices. Researchers employed cQED architecture, using microwaves akin to sonar mapping an underwater landscape, to carefully examine energy flows in the materials. These junctions are reliable components for quantum circuits but raise questions about optimising performance across different frequencies and chip designs.

Improved qubit coherence via in-situ fabricated indium arsenide antimonide Josephson junctions

Energy relaxation times, a key metric for quantum information processing, improved from approximately 0.2 microseconds to 1 microsecond. This improvement surpasses previously achievable coherence within in-situ fabricated junctions, historically damaged during definition by etching techniques. At Niels Bohr Institute and University of Copenhagen, researchers successfully created gate-tunable superconducting qubits using indium arsenide antimonide/aluminium Josephson junctions grown directly during material deposition, avoiding these prior limitations.

Rabi oscillations confirmed controlled quantum behaviour alongside Ramsey interference with representative values of T1 equaling 0.79 microseconds and T2* reaching 0.91 microseconds. Relaxation times varied between approximately 0.2 to 1 microsecond across five ‘gatemons’ fabricated on three separate chips; this variation decreased as qubit frequency increased, a range comparable to theoretical predictions regarding energy loss into the surrounding electromagnetic environment.

Electrostatic tuning demonstrated a substantial bandwidth for controlling qubit operation, spanning several gigahertz. While promising, current coherence figures do not yet clarify how well many interconnected qubits would function together in a larger system, representing an ongoing challenge towards building practical quantum computers.

Representative Coherence Data Needed to Validate Device Consistency

The team’s fabrication of gate-tunable qubits using indium arsenide antimonide/aluminium junctions represents a strong advance by avoiding damaging etching processes common to other approaches; however, limitations beyond simply scaling up production are acknowledged. Only representative coherence times were presented without detailed distributions across tested devices, raising questions about consistency between individual qubit performance on each chip and whether observed values accurately reflect the potential of this *in situ* growth method.

Coherent quantum building blocks grown directly during deposition avoid damaging fabrication steps typical in other designs, potentially unlocking scalable manufacturing within the decade and allowing exploration of novel device architectures.

*In situ* fabrication allows precise electrical control over qubit properties with bandwidth exceeding several gigahertz, confirmed through observations of coherent quantum behaviour exhibiting relaxation times up to 0.91 microseconds. At Niels Bohr Institute and University of Copenhagen, they have created these coherent building blocks while simplifying manufacturing by eliminating complex etching processes. This approach offers a pathway towards more efficient production methods for future quantum technologies.

This research successfully demonstrated coherent control of qubits fabricated from indium arsenide antimonide/aluminium junctions grown directly during deposition, avoiding damaging processing steps used in other designs. The resulting ‘gatemons’ exhibited gate-tunable operation spanning several gigahertz with representative coherence times reaching 0.91 microseconds across five devices on three chips. Measured relaxation times ranged approximately from 0.2 to 1 microsecond and align with theoretical estimates of energy loss into the electromagnetic environment, establishing these *in situ* junctions as functional circuit elements. This work introduces a method for characterizing this type of weak link using microwave spectroscopy.

👉 More information
🗞 Gatemon qubits based on in-situ ternary InAsSb/Al nanowire Josephson junctions
✍️ Zhenhai Sun, David Feldstein-Bofill, Sangeeth Kallatt, Sabbir A. Khan and Morten Kjaergaard (University of Copenhagen)
🧠 ArXiv: https://arxiv.org/abs/2611.16595

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