Scientists at Aalto University have developed an ultra-sensitive calorimeter capable of measuring microwave energy pulses smaller than a zeptojoule—roughly the energy needed to lift a red blood cell one nanometre against gravity. The device uses a gold-palladium nanowire cooled to 20 millikelvin and sandwiched between superconducting aluminum islands, leveraging the quantum proximity effect to create Josephson junctions. When energy is absorbed, it shifts the resonance frequency of a built-in oscillator, allowing precise readout of tiny temperature changes.

The team improved signal detection by applying matched filtering, a radar-inspired technique that enhanced their signal-to-noise ratio by 30%. They resolved pulses of about 150 microwave photons at 8.4 gigahertz with an energy resolution of 0.83 zeptojoules. While impressive, this is still above the thermodynamic noise limit of 0.13 zeptojoules for the current design. The main constraint now lies in the amplification electronics, not the sensor.

The technology has two promising applications. In quantum computing, it could enable less disruptive qubit readouts by operating at the same ultra-cold temperatures as qubits. For fundamental physics, it may one day detect single microwave photons from hypothetical dark matter particles called axions. Future improvements could involve graphene-based absorbers, which theoretically allow resolutions down to 0.05 zeptojoules. The work, published in Nature Electronics, marks a significant step toward single-photon detection in the microwave range.