This result is part of the research programme of the Quantum Computing and Technologies (QCT) group at the Institut de Física d’Altes Energies (IFAE), which focuses on the experimental study of superconducting quantum circuits, engineered light–matter interaction, and technologies relevant for quantum information processing.

The study, entitled Direct Detection of Down-Converted Photons Spontaneously Produced at a Single Josephson Junction , authored by Alba Torras and collaborators, has been published in Physical Review Letters. Using a carefully engineered superconducting circuit and a detection system based on a quantum-limited amplifier, the researchers directly observed the decay of a single microwave photon into three photons, each carrying one third of the original photon’s energy.

In this experiment, the interaction between photons is mediated by a strongly nonlinear superconducting element—a small Josephson junction—that couples two high-impedance resonators. Remarkably, the generation of photon triplets occurs spontaneously, without the need for strong classical driving fields, placing the system in a previously unexplored operational regime.

Beyond its fundamental relevance for many-body quantum optics, this work identifies new dissipation and loss mechanisms in superconducting circuits. A detailed understanding of these processes is essential for improving the performance, coherence and scalability of superconducting qubits, which are a leading platform for the development of quantum computing technologies.