One intriguing method that could be used to form the qubits needed for quantum computers involves electrons hovering above liquid helium. But it wasn't clear how data in this form can be read easily-now RIKEN researchers may have found a solution1.
Conventional computers work by performing operations on bits encoded in silicon. But no one is really sure how qubits will be encoded in the quantum computers of the future. Half a dozen or so platforms are currently being pursued, including superconductors, silicon, light and trapped ions.
One of the most intriguing ideas for qubits is electrons that float above the surface of liquid helium at a temperature of about four degrees Celsius above absolute zero.
The great advantage of such a system is that it provides a very clean environment, with minimum interference from spins of nearby particles. This will allow qubits to maintain their quantum state for much longer than in noisier environments.
"For an electron floating in a vacuum above helium, the only thing close to it is helium atoms, which are highly inert," explains Asher Jennings of the RIKEN Center for Quantum Computation (RQC). "That means the electron is very well protected, making it an excellent system for storing quantum information."
But for such qubits to be used in quantum computers, the data they store needs to be readable. The small magnetic moment of an electron above helium makes direct readout of its spin infeasible. And so, scientists are exploring indirect schemes for reading out the electron spin.
A promising way to do that is to detect the transition of the electron from its lowest energy state to a higher one, known as the Rydberg state.
Now, scientists including Jennings and Erika Kawakami, also of RQC, have demonstrated it should be possible to detect the Rydberg transition of a single electron by measuring the change in capacitance.
They did this by using 10 million electrons floating above liquid helium, thereby creating a system that acts as a capacitor. The team were able to detect the change in the quantum capacitance that occurred when the electrons were promoted to the Rydberg state through changes in the microwave frequency.
While the system needs to be scaled down by a factor of 10,000 times, it demonstrates that such a scaled-down device should be able to detect the signal from a single qubit.
"Our measurements of the capacitance change in a large system indicate that it should be easily measurable for a single electron in a single-electron device," says Jennings.
The team is now working on performing the same measurement in a one-electron system.
Asher Jennings (second from left in right row) and Erika Kawakami (second from left in left row) and co-workers have used frequency modulation of microwaves to probe the quantum capacitance of Rydberg transitions of electrons on the surface of liquid helium. © 2026 RIKEN
