The laser clicks ten million times per second, and each click kicks exactly one photon out of a polystyrene film. Not a beam, not a burst, just a single particle of light, neatly packaged in space and time. The source is a molecule you can order from a chemical supply catalog: C60 fullerene, sixty carbon atoms folded into a soccer ball. The very ball whose synthesis earned the Nobel Prize in Chemistry in 1996. Now it's playing a different game.

Until recently, the consensus was that emitting single photons required either cryogenic temperatures and complex semiconductor heterostructures, or colloidal quantum dots that blink and fade. The work by the Barcelona physicists upends that picture. They dissolved commercial C60 in toluene along with polystyrene, dropped the solution onto a gold substrate, and spun it at 4000 rpm. The result is a film where individual fullerene molecules sit isolated from each other, like balls in a vast empty hall. Next comes excitation with a 405 nm laser and detection of the emitted light in a Hanbury Brown and Twiss interferometer. This device measures whether photons arrive in pairs or strictly one by one.

The key quantity is the second-order correlation function g(2)(0). If it's below 0.5, photons don't arrive in pairs: the source is single. For the Barcelona films under continuous excitation, g(2)(0) = 0.304 ± 0.024. Under pulsed excitation at 10 MHz, it's 0.308 ± 0.024. This means photons are emitted on command, with purity sufficient for quantum cryptography. The exciton lifetime, the time from excitation to emission, turned out to be about 4.5 nanoseconds, with a spread from 3 to 27 ns depending on the local environment. The blinking, that is, random switching between bright and dark states, follows a power law with exponent α = 0.369. This points to complex multilevel dynamics, not a simple three-level model.

Where does the light come from? Pure C60 barely emits: its high symmetry forbids optical transitions. But under the influence of light and oxygen, oxygen atoms attach to the molecule, breaking the symmetry and allowing transitions. So the emission spectrum, peaking at 620 nm, is explained by photooxidation of C60, forming C60On. This is both a curse and a blessing. On one hand, oxidation makes the molecule bright. On the other, it introduces instability and blinking. Controlling the degree of oxidation is the key to stable sources.

The practical payoff is clear. Quantum communication needs single-photon sources that work at room temperature, are cheap, and are scalable. No cryogenic setups or complex nanofabrication are needed here. The short exciton lifetime promises high photon generation rates. But the blinking and the spread in lifetimes show that the molecules are sensitive to their environment. The next step is chemical modification of fullerenes, for instance attaching functional groups, to suppress blinking and narrow the lifetime distribution. Embedding them in optical cavities would speed up emission and improve photon collection efficiency.

A soccer ball that spits out exactly one photon on command is no longer a metaphor but a working element for future quantum networks. The same C60 that once amazed chemists with its symmetry now amazes physicists with its ability to serve as a source of quantum light. The game is just beginning.