Tiny semiconductor islands grown right on silicon fire off single photons, like a machine gun that shoots exactly one bullet at a time. These islands are called quantum dots. They can emit light in discrete packets, which is needed for secure information in quantum networks. But until now, they have been difficult to integrate with silicon electronics and to make work at not-too-low temperatures.

In the new work, quantum dots made of indium gallium arsenide were placed in ring resonators — structures that gather light into a narrow beam, like a mirrored well. Each dot can be tuned with an electric field: voltage shifts the emission color by about 16 nanometers, like switching radio stations without touching the transmitter itself. At the same time, the purity of single photons remains high: the probability of emitting two photons at once is less than one percent.

Such sources can be placed side by side on a single chip and tuned to the same wavelength so that photons from different dots are indistinguishable. This paves the way for quantum repeaters and secure communication over ordinary fiber-optic lines.