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Photons conserve their energy while they propagate through vacuum. However, by interacting with matter they can give rise to higher energy photons through processes such as frequency doubling or two-photon absorption. Thus, two photons can "merge" into a new photon whose energy is the sum of the energies of the initial photons. In fact, it is the non-linearity of the interaction between light and matter that makes this transformation possible.
Conversely, could a photon spontaneously disappear by producing photons of lower energy? While classical physics forbids it, quantum physics allows it. Such a conversion process was used to produce pairs of "entangled photons" (forming a unique quantum state). However, the spontaneous "fission" of a visible or infrared photon is extremely inefficient: it only involves a few out of a million photons crossing a non-linear crystal.
The Josephson junction arrays that connect superconducting islands provide a favorable environment for such experiments in the microwave domain. Electric charges are organized there into Cooper pairs, and the electromagnetic field oscillations (photons) that they generate propagate at microwave frequencies. Moreover, it is easy to introduce an extremely strong non-linearity for these photons by reducing the size of a junction in the array.
Researchers from the IRIG and Yale University (USA) have determined that this non-linearity is particularly effective in "splitting" a resonant photon through a fluorescence process that creates a quasi-resonant photon accompanied by several low-energy photons.
Photons' lifetimes were measured in several superconducting circuits at the University of Maryland (USA). The theory, which attributes the disappearance of resonant photons to their fluorescence into quasi-resonant photons, has made it possible to interpret the results for circuits characterized by an effective impedance of the Josephson junction array greater than the resistance quantum.
Outside of this regime, other processes can compete with the fluorescence. However, no theory exists yet to account for them. From this perspective, the experiments have produced a "quantum simulation" of a "quantum impurity" problem: a small junction coupled to an array of large junctions. Microwave photonics appears to be a promising field for studying other complex quantum systems.