Detecting a single particle of light is hard; detecting a single microwave photon is even harder. Microwave photons, the tiny packets of electromagnetic radiation used in current technologies like Wi-Fi and radar, carry far less energy than visible light. They are about 100,000 times weaker than optical photons.
Many existing quantum technologies depend on detecting individual photons with high reliability. For visible light, this is well established using devices that convert incoming light directly into electrical signals. But at microwave frequencies (0.3—30 GHz), this fails because each individual photon doesn't carry enough energy to release an electric charge in a material. This means that detecting single microwave photons requires a completely different strategy.
A long-standing goal has been to realize a simple device capable of continuously detecting microwave photons. Now, scientists at EPFL led by Pasquale Scarlino have developed a semiconductor-based detector that takes an important step in that direction.
Published in Science Advances, the device combines a semiconductor structure called a "double quantum dot" with a superconducting microwave cavity—a tiny resonant circuit that traps and stores microwave photons so they can interact strongly with the device. Together, these components convert incoming microwave photons into a small but measurable electrical current.
"Beyond setting a new benchmark for semiconductor-based microwave photodetectors, the work opens new perspectives for quantum microwave optics, quantum sensing, and scalable quantum information platforms," says Scarlino.
Two main components
The core of the detector is a double quantum dot: two tiny islands of semiconductor material that can each hold a single electron. The researchers defined these islands using metallic gates on a semiconductor chip made on a gallium arsenide/aluminum gallium arsenide (GaAs/AlGaAs) heterostructure that hosts a high-quality two-dimensional electron gas and allows precise control of individual electrons.
One of the metallic gates connects to a superconducting cavity built from an array of Josephson junctions—small superconducting devices made of two superconductors separated by a thin insulating barrier that allow quantum currents to flow and create a highly tunable microwave circuit. This cavity stores microwave photons at frequencies between 3 and 5.2 gigahertz. Because the cavity has a high electrical impedance, that facilitates large electric field induced by the cavity, it interacts strongly with the electron charge in the quantum dots.
Detecting single microwave photons
When a microwave photon enters the cavity and its energy matches the energy splitting of the double quantum dot, the photon can be absorbed by the electron in the double quantum dot. That absorption excites the system and causes the electron to move between the two dots. The electron then tunnels to a nearby reservoir. This motion creates a small direct current. By measuring this current, the researchers can know that a photon has been absorbed.
To measure how well the detector performed, the team first made sure they knew how strong the incoming microwave signal was by measuring the change in the device's energy levels. They then measure the source-drain current flowing through the double quantum dot, between its two electron reservoirs, while gradually increasing the microwave power. When the signal was so weak that fewer than one photon was present at a time, the current rose in direct proportion to the number of incoming photons.
Future quantum applications
Depending on how it is tuned, the system has detected between 55%-67.7% of incoming photons, with the best tuning approaching 70%. That means most photons that enter the system are converted into a measurable electrical signal, a significant step forward for semiconductor-based microwave detectors.
The device also operates continuously. Once a photon is absorbed, the system resets itself within a few nanoseconds as electrons move in and out of the dots, ready for the next event. Its performance is comparative with other state-of-the-art microwave photon detectors. And because the device is built from semiconductor quantum dots, it could in principle sit on the same chip as spin qubits, helping connect microwave photonics with semiconductor-based quantum computing.
Other contributors
- University of Basel
- ETH Zürich
- Lund University
Reference
Fabian Oppliger, Wonjin Jang, Aldo Tarascio, Franco De Palma, Christian Reichl, Werner Wegscheider, Ville F. Maisi, Dominik Zumbühl, Pasquale Scarlino. Tunable high-efficiency microwave photon detector based on a double quantum dot coupled to a superconducting high-impedance cavity. Science Advances 03 April 2026. DOI: 10.1126/sciadv.aeb9784
