"This is a crucial advance," explains Ramón Aguado, a CSIC researcher at the Madrid Institute of Materials Science (ICMM) and one of the study's authors. "Our work is pioneering because we demonstrate that we can access the information stored in Majorana qubits using a new technique called quantum capacitance," continues the scientist, who explains that this technique "acts as a global probe sensitive to the overall state of the system."
To better understand this achievement, Aguado explains that topological qubits are "like safe boxes for quantum information," only that, instead of storing data in a specific location, "they distribute it non-locally across a pair of special states, known as Majorana zero modes." This characteristic is what makes them so valuable for quantum computers: "they are inherently robust against local noise that produces decoherence, since to corrupt the information, a failure would have to affect the system globally." However, "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?."
To achieve this, the team has created a modular nanostructure of small pieces, like playing with Lego, called the Kitaev minimal chain: "the experimental team is able to create a chain with two semiconductor quantum dots coupled through a superconductor," describes Aguado, who indicates that, in this way, "instead of acting blindly on a combination of materials, as in previous experiments, we create it bottom-up and are able to generate Majorana modes in a controlled manner, which is in fact the main idea of our QuKit project ."
Once the minimal Kitaev chain was created, and using this Quantum Capacitance probe, it was possible, for the first time, to discriminate in real time and in a single measurement whether the non-local quantum state formed by the two Majorana modes is even or odd—that is, whether it is full or empty, which is considered the basis of the qubit. "The experiment elegantly confirms the protection principle: while local charge measurements are blind to this information, the global probe reveals it clearly," says researcher Gorm Steffensen, also part of the team at the ICMM-CSIC.
Another "highly relevant" result of this experiment is the observation of "random parity jumps," which allows for the measurement of "parity coherence exceeding one millisecond, a very promising value for future operations of a topological qubit based on Majorana modes."
This study combines a novel experimental methodology, developed primarily at Delft University of Technology, with the theoretical contribution of the ICMM-CSIC, which has been "crucial for understanding this highly sophisticated experiment," the authors conclude.
