In a record-setting advance for quantum computing, researchers at Aalto University in Finland have pushed the coherence time of a superconducting transmon qubit beyond the coveted one-millisecond threshold.
The new findings, published July 8 in Nature Communications, mark a turning point for quantum processors, unlocking a longer time window for error-free operations and nudging the field closer to practical, large-scale quantum machines.
Why Coherence Time Matters for Quantum Computing
Quantum bits, or qubits, are notoriously fragile. Their quantum states collapse quickly due to environmental noise, limiting how many operations they can perform before errors take over. Coherence time—the interval during which a qubit retains its quantum state—is thus a key measure of quantum performance.
Most transmon qubits, the workhorse of superconducting quantum devices, top out at energy relaxation and echo dephasing times below 600 microseconds. But in the new study, one of Aalto’s transmons hit a maximum echo dephasing time of over one millisecond and a median of 541 microseconds. The energy relaxation time also peaked at 666 microseconds.
“We have just measured an echo coherence time for a transmon qubit that landed at a millisecond at maximum with a median of half a millisecond,” said Mikko Tuokkola, the Aalto University PhD student who led the measurements.
A Reproducible Recipe for Longer-Lasting Qubits
Behind this leap in coherence lies meticulous engineering. The chip was fabricated at Aalto’s Quantum Computing and Devices (QCD) lab, using ultra-pure materials and processes developed with the Technical Research Centre of Finland (VTT). The device itself was built in Micronova, a cleanroom facility at Finland’s OtaNano national research infrastructure.
Notably, the team didn’t just report the record; they opened the blueprint. Their publication includes extensive technical documentation—from electron-beam lithography to vacuum-level junction deposition—to help others replicate the result.
“We have been able to reproducibly fabricate high-quality transmon qubits,” said Dr. Yoshiki Sunada, who fabricated the chip and now works at Stanford. “The fact that this can be achieved in a cleanroom which is accessible for academic research is a testament to Finland’s leading position in quantum science and technology.”
How They Did It
The qubit—dubbed Q2—was part of a four-qubit chip mounted in a cryogenic chamber and cooled to 10 millikelvin. Using a high-precision setup with a traveling-wave parametric amplifier (TWPA), researchers recorded coherence times over multiple cooldowns and signal configurations. They measured:
- Echo dephasing time (T2): Median 541 μs, max 1057 μs
- Energy relaxation time (T1): Median 425 μs, max 666 μs
- Qubit frequency: 2.9 GHz
Unlike earlier high-coherence records set by alternative qubit types like fluxonium, this achievement stays within the transmon framework, widely used in industrial and academic quantum labs due to its simplicity and scalability.
Implications for Quantum Scale-Up
Longer coherence times ease the burden on quantum error correction. This means fewer physical qubits are needed to reliably encode logical qubits—one of the steepest engineering bottlenecks facing today’s quantum systems.
By crossing the millisecond mark, Aalto’s team has cracked open the door to deeper, more reliable quantum computations. This could improve quantum simulation, optimization, and sensing applications—and give hardware developers a reproducible method for improvement.
“This landmark achievement has strengthened Finland’s standing as a global leader in the field,” said Professor Mikko Möttönen, who leads the QCD group. “It moves the needle forward on what can be made possible with the quantum computers of the future.”
Next Steps: From One Qubit to Many
The researchers acknowledge that one high-performing qubit is just a start. To scale up to dozens or hundreds of millisecond-class qubits, consistency and noise isolation across entire chip arrays will be essential. The group has already opened new postdoctoral positions to accelerate progress.
Yet in a field where a few extra microseconds can make headlines, crossing into the millisecond range is a seismic shift. Other labs are expected to test and build upon Aalto’s fabrication recipe in short order. And with quantum computing’s future resting on ever-quieter, longer-lived qubits, this may not be the last time a Finnish cleanroom makes history.
Journal: Nature Communications
DOI: 10.1038/s41467-025-61126-0
Article Title: Methods to achieve near-millisecond energy relaxation and dephasing times for a superconducting transmon qubit
Publication Date: July 8, 2025
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