Towering above the crowd helps in basketball. Now investigators at Cambridge-based Toshiba Research Europe Ltd. and at Cambridge University, both in the UK, have shown that it also is the key to an on-demand single-photon source for telecom applications. Such a source could be used to transmit quantum bits over networks, resulting in completely secure communications. The single-photon source uses semiconductor quantum dots that are 45 nm across and about 10 nm high.The new source uses semiconductor quantum dots that are 45 nm across and about 10 nm high. The researchers created the dots by depositing InAs on a GaAs substrate.The dots self-assemble because of strain after a critical InAs coverage value is reached. The resulting dots typically have a height of 5.7 nm and an emission of about 900 nm until a second critical coverage point is passed. Then the taller dots appear."By controlling the deposition around this second critical coverage, we were able to form a low density of these large dots, emitting close to 1300 nm," said Andrew J. Shields, head of the quantum information group at Toshiba Research.After filtering out the small-dot emissions, the scientists created a single-photon source and used optics to couple the quantum dot output to a fiber. Because the emission from the dots has a narrow linewidth, the single photons can be sent down a fiber for distances of 100 km or more without substantial degradation of the signal.Single-photon sources could be used in quantum key distribution, in quantum cryptography or in other areas of quantum optics, such as quantum metrology and computing. Today, they are approximated in commercial quantum key distribution systems by attenuating lasers so that, on average, only a single photon is found in every time slice. The problem is that this technique reduces the total number of photons, limiting the system's range or data rate.Toshiba's single-photon technique does not suffer from this drawback and so could offer a better source for these applications. Shields expects the tall quantum dots to appear in products fairly soon."We plan to integrate the single-photon source into our [quantum key distribution] system over the next year and to commercialize in around three years," he explained. "The single-photon source may be available as a module before then."