A 15-member research team from the UK, Germany, and Japan has developed a new method for generating and detecting quantum-entangled photons at a wavelength of 2.1 μm.1 Entangled photons are used in encryption methods such as quantum key distribution to completely secure telecommunications between two partners against eavesdropping attempts.
Until now, it has been only technically possible to implement such encryption mechanisms with entangled photons in the near-infrared range of 700 to 1550 nm. However, these shorter wavelengths have disadvantages, especially in satellite-based communication: They are disturbed by light-absorbing gases in the atmosphere as well as by the background radiation of the sun. Using existing technology, end-to-end encryption of transmitted data can be guaranteed at night, but not on sunny and cloudy days.
The international team, led by Matteo Clerici from the University of Glasgow (Glasgow, Scotland), wants to solve this problem with its discovery. The photon pairs entangled at a 2.1 μm wavelength would be significantly less influenced by the solar background radiation, says Michael Kues from the PhoenixD Cluster of Excellence at Leibniz University of Hannover (Hannover, Germany). In addition, transmission windows exist in the earth's atmosphere, especially for wavelengths at around two micrometers.
For their experiment, the researchers used a lithium niobate nonlinear crystal. They sent ultrashort laser pulses into the crystal; a nonlinear optical interaction produced the entangled photon pairs with the new wavelength of 2.1 μm. "The next crucial step will be to miniaturize this system by converting it into photonic integrated devices, making it suitable for mass production and for use in other application scenarios," says Kues.