Microring resonators could become a chip-scale source of entangled photons crucial to quantum computing and encryption. Such resonators are known to be efficient sources of photon pairs, but a 20-µm silicon resonator developed at the University of Pavia in Italy is said to be the first capable of emitting time-energy entangled photons. “The rate at which the entangled photons are generated is unprecedented for a silicon integrated source, and comparable with that available from bulk crystals that must be pumped by very strong lasers,” said professor Dr. Daniele Bajoni. Pumped by a fiber-coupled laser with output power of Optica (doi: 10.1364/optica.2.000088). A diagram of the silicon microring resonator and access waveguide. The green wave at the input represents the laser pump, and the red and blue wave packets at the output represent the generated photon pairs. The infinity symbol linking the two outputs indicates their entanglement. Courtesy of the University of Pavia. To date, entangled photon emitters have been principally made from bulk crystals and could be scaled down to only a few millimeters in size — still many orders of magnitude too large for on-chip applications. These emitters must also be pumped by lasers outputting at hundreds of milliwatts, a large amount of power in the context of telecommunications and computing. Ring resonators, on the other hand, can be easily etched onto silicon wafers in the same manner that other components on semiconductor chips are fashioned. “In the last few years, silicon integrated devices have been developed to filter and route light, mainly for telecommunication applications,” Bajoni said. “Our microring resonators can be readily used alongside these devices, moving us toward the ability to fully harness entanglement on a chip.” Entanglement could be harnessed to increase the power and speed of computations. And because entangled photons can be considered to be a single entity, the phenomenon could allow for new communication protocols that are immune to spying. For more information, visit eecs.unipv.it.