Single Photons From a Silicon Chip

Schematic representation of a single defect in a silicon wafer created by the implantation of carbon atoms, which emits single photons in the telecom O-band coupled to an optical fiber. (Source: Juniks, HZDR)

Just a few years from now, quantum computers are expected to revo­lutionize database searches, AI systems, and compu­tational simu­lations. Today already, quantum crypto­graphy can guarantee absolutely secure data transfer, albeit with limitations. The greatest possible compa­tibility with our current silicon-based elec­tronics will be a key advantage. And that is precisely where physicists from the Helmholtz-Zentrum Dresden-Rossen­dorf (HZDR) and TU Dresden have made remarkable progress: The team has designed a silicon-based light source to generate single photons that propagate well in glass fibers.

Quantum technology relies on the ability to control the behavior of quantum particles as precisely as possible, for example by locking individual atoms in magnetic traps or by sending individual light particles through glass fibers. The latter is the basis of quantum crypto­graphy, a communi­cation method that is, in principle, tap-proof: Any would-be data thief inter­cepting the photons unavoidably destroys their quantum properties. The senders and receivers of the message will notice that and can stop the compromised trans­mission in time. This requires light sources that deliver single photons. Such systems already exist, especially based on diamonds, but they have one flaw: “These diamond sources can only generate photons at frequencies that are not suitable for fiber optic trans­mission,” explains Georgy Astakhov. “Which is a significant limi­tation for practical use.” So Astakhov and his team decided to use a different material – they tried and tested electronic base material silicon.

To make the material generate the infrared photons required for fiber optic communi­cation, the experts subjected it to a special treatment, selec­tively shooting carbon into the silicon with an acce­lerator at the HZDR Ion Beam Center. This created G-centers in the material – two adjacent carbon atoms coupled to a silicon atom forming a sort of artificial atom. When radiated with red laser light, this arti­ficial atom emits the desired infrared photons at a wavelength of 1.3 micrometers, a frequency excellently suited for fiber optic trans­mission. “Our prototype can produce 100,000 single photons per second,” Astakhov reports. “And it is stable. Even after several days of continuous operation, we haven’t observed any deteriora­tion.” However, the system only works in extremely cold conditions – the physicists use liquid helium to cool it down to a tempera­ture of minus 268 degrees Celsius.

“We were able to show for the first time that a silicon-based single-photon source is possible,” Astakhov’s colleague Yonder Berencén is happy to report. “This basically makes it possible to integrate such sources with other optical compo­nents on a chip.” Among other things, it would be of interest to couple the new light source with a resonator to solve the problem that infrared photons largely emerge from the source randomly. For use in quantum communi­cation, however, it would be necessary to generate photons on demand.

This resonator could be tuned to exactly hit the wavelength of the light source, which would make it possible to increase the number of generated photons to the point that they are available at any given time. “It has already been proven that such resonators can be built in silicon,” reports Berencén. “The missing link was a silicon-based source for single photons. And that’s exactly what we’ve now been able to create.” But before they can consider practical appli­cations, the researchers still have to solve some problems – such as a more systematic pro­duction of the new telecom single-photon sources. “We will try to implant the carbon into silicon with greater precision,” explains Georgy Astakhov. “HZDR with its Ion Beam Center provides an ideal infra­structure for realizing ideas like this.” (Source: HZDR)

Reference: M. Hollenbach et al.: Engineering telecom single-photon emitters in silicon for scalable quantum photonics, Opt. Exp. 28, 26111 (2020); DOI: 10.1364/OE.397377

Link: Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany

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