In laboratories around the world, superconducting qubits behave differently even when they are fabricated according to the same design and from the same materials. Some retain their quantum state longer, others lose it faster — and until recently the reason remained a mystery.
Researchers from the SQMS center at Fermilab conducted the largest blind study to date of 22 transmon qubits created at three institutions. Seven methods of materials analysis and six partner organizations made it possible to link microscopic features to the actual performance of the devices.
Three details turned out to be the key culprits: the thickness of the oxide layer on the surface, the angle of the sidewalls after etching, and the depth of the trenches around the electrodes. Even a difference of one nanometer in oxide thickness noticeably changed the coherence time.
The wall angle affects how strongly the electric field penetrates into the “dirty” oxides. A sharper angle — about 10–15 degrees — reduces losses and improves performance by 20–30 percent compared with a gentler slope.
The depth of the trenches matters too: below roughly 20 nanometers, small deviations strongly affect the result, while at greater depth the effect saturates. These parameters depend directly on the fabrication steps.
Notably, defects visible to the eye — scratches or particles — showed no connection with the spread in performance. What decides the outcome are precisely the nanoscale features of surface chemistry and geometry.
The results give manufacturers clear guidelines: control the oxide thickness, the etch angle, and the trench depth. This is a step toward reproducible and reliable quantum processors.
The work was published in Applied Physics Reviews and opens the way to targeted improvement of materials for future quantum computers.
