Microwaves offer a new way to detect electrons as quantum bits

Researchers at Japan’s RIKEN Center for Quantum Computation have developed a novel method to read quantum information stored in electrons suspended above liquid helium, potentially overcoming a major challenge in this unconventional quantum computing platform. Unlike traditional silicon-based qubits, electrons above liquid helium exist in an extremely clean and isolated environment, free from magnetic and chemical disturbances, which could allow qubits to maintain their quantum states for significantly longer durations. However, directly measuring the electron’s spin state is difficult due to its very small magnetic moment, prompting the team to explore indirect readout techniques. The researchers demonstrated that by monitoring transitions of electrons from their ground state to higher-energy Rydberg states, it is possible to infer the quantum state indirectly through changes in quantum capacitance. Using a large ensemble of about 10 million electrons forming a capacitor, they detected measurable shifts in microwave frequency corresponding to these transitions. This experimental validation suggests that similar capacitance changes could be detected at the single-electron level, providing a feasible and non-invasive