A laser pulse enters a tiny cavity made of a transparent crystal. Inside, light bounces between mirrors, and occasionally one photon splits into a pair: one goes one way, the other goes another. One becomes the signal, and the other acts as a 'bell' announcing that a single photon is ready. Such a source doesn't emit photons on a schedule, but it does tell you exactly when a single photon appears.

The researchers made the cavity from a crystal that, when illuminated by a laser, generates pairs of photons. They directed light into the cavity and measured how many single photons were produced. It turned out that 70% of the 'rings' are accompanied by a true single photon. This is a record for this type of source. However, without additional filtering, the photons aren't perfectly clean: sometimes two photons come out instead of one. The scientists placed a special filter, an etalon, at the output. It only lets through photons of the desired frequency, like a narrow door letting people through one at a time. After the filter, the purity rose to 96%.

Such a source can be used in quantum cryptography, where each photon carries a secret key. If there are two photons, an eavesdropper could intercept the extra one and crack the cipher. That's why purity is so important. In the future, similar sources will help build quantum networks where information is transmitted with absolute security.