The quantum river runs deep. We know that the flow of electricity is quantized. And in the last decade we have learned that the flow of heat is too, evidence of which has been recorded in phonon studies. Now researchers have found that at low temperatures, heat quanta can be carried by photons, and that they too respect the universal quantum limit1.

The extension of the theory of quantization to heat transfer was made when scientists recorded discrete units of thermal energy moving through submicrometre-sized dielectric wires. Here, the energy was carried by phonons — arising from the interaction of crystal-lattice vibrations and electrons. Now, Meschke and colleagues from Finland and France have shown for the first time that at low temperatures heat packets can be conducted by photons (specifically through electron–photon interactions), as well as phonons. To observe this, the temperature has to be low enough so that phonon conduction is 'frozen out'.

In their experiments, Meschke and colleagues connected two microscopic islands of gold–palladium metal with aluminium superconducting leads. Using special superconducting switches, they could turn electrical conduction on and off, thereby isolating the role played by electron–photon processes. They found that heat transfer from one island to the other by means of photons, approaches the same quantum-mechanical limit as seen for phonons.

This intriguing result could have important practical implications, for example, for bolometers (far-infrared light detectors) and micro-refrigerators, which rely on weak thermal coupling with their environments. But the work could also teach us about information transfer within quantum systems. Because heat transfer is essentially entropy flow, and entropy is fundamentally linked to information, the fact that heat flow respects a universal quantum limit could say something deeper about the exchange of information.