A qubit, the computational unit of a quantum computer, lives for fractions of a millisecond before forgetting its state. One culprit is a tiny contact inside it: two layers of aluminum sandwiching a layer of aluminum oxide just a couple of nanometers thick. This sandwich is called a Josephson junction, and it's through this junction that superconducting current flows without resistance.
To understand what degrades this contact, physicists shone X-rays through it. The rays reflected off the different layers, and by analyzing the reflection pattern, they could reconstruct the atomic arrangement inside. It turned out the barrier isn't uniform: its bottom boundary, adjacent to the aluminum, is more loosely packed and disordered than the top. The oxide layer itself resembles a mosaic of two different atomic arrangements, tetrahedral and octahedral, and the ratio of these arrangements shifts depending on the oxygen pressure used when growing the film.
Such an uneven structure is a perfect breeding ground for defects known as two-level systems. These defects act like tiny switches that randomly flip their state, and they are thought to steal energy from the qubit, causing it to lose quantum information. Now that we know how fabrication conditions affect the atomic arrangement of the barrier, we can tune those conditions to reduce defects and make qubits live longer.
X-ray reflectometry, the method used in this work, is non-destructive: it allows you to look inside a finished device without taking it apart. It's like taking an X-ray of an engine without turning off the car. For quantum technologies, this tool offers a way to finally connect how we make contacts with how they perform.
Aluminum oxide is the same material that forms on the surface of aluminum foil when you leave it in the air. But in qubits, it's grown in a controlled oxygen atmosphere to achieve the desired thickness and structure.