“This is a crucial advance,” says Ramón Aguado, a CSIC researcher at the Madrid Institute of Materials Science (ICMM) and co author of the study. He explains that the team has shown it is possible to retrieve information stored in Majorana qubits using a technique known as quantum capacitance. According to Aguado, this method works as “a global probe sensitive to the overall state of the system,” allowing researchers to detect properties that were previously out of reach.

Why Topological Qubits Are So Hard to Measure
Aguado compares topological qubits to “safe boxes for quantum information.” Instead of keeping data in a single, fixed location, these qubits spread information across two linked quantum states called Majorana zero modes. Because the information is distributed in this non local way, it is naturally shielded from small, local disturbances that typically disrupt fragile quantum systems.

This built in protection is what makes topological qubits so appealing for quantum computing. “They are inherently robust against local noise that produces decoherence, since to corrupt the information, a failure would have to affect the system globally,” Aguado explains. But that same strength has created a major experimental challenge. If the information does not sit in one specific place, how can scientists actually detect or measure it? As Aguado puts it, “this same virtue had become their experimental Achilles’ heel: how do you “read” or “detect” a property that doesn’t reside at any specific point?”

Building a Kitaev Minimal Chain
To solve this problem, the researchers constructed a carefully designed nanostructure known as a Kitaev minimal chain. Aguado likens the process to assembling Lego pieces. The device consists of two semiconductor quantum dots connected through a superconductor, forming a small but precisely controlled system.