A photonics research group co-led by Gregor Weihs of the University of Innsbruck has developed a new technique for generating multi-photon states from quantum dots that overcomes the limitations of conventional approaches. This has immediate applications in secure quantum key distribution protocols, where it can enable simultaneous secure communication with different parties.

Quantum dots—semiconductor nanostructures that can emit single photons on demand—are considered among the most promising sources for photonic quantum computing. However, every quantum dot is slightly different and may emit a slightly different color. This means that to produce multi-photon states, we cannot use multiple quantum dots.

Usually, researchers use a single quantum dot and multiplex the emission into different spatial and temporal modes, using a fast electro-optic modulator. The technological challenge is that faster electro-optic modulators are expensive and often require very customized engineering. To add to that, they may not be very efficient, which introduces unwanted losses into the system.