Tomás Santiago-Cruz and Maria Chekhova from the Max Planck Institute for the Science of Light and the Friedrich-Alexander-Universität Erlangen-Nürnberg in cooperation with Sandia National Laboratories successfully used resonant metasurfaces to create photon pairs at several different frequencies.

A photon is the quantum (the minimum amount involved in an interaction) of any form of electromagnetic radiation, such as light. Photons are essential to a number of contemporary research fields and technologies, including quantum state engineering, which in turn represents the cornerstone of all quantum photonic technologies. With the help of quantum photonics, engineers and scientists are working to create new technologies such as new types of supercomputers and new forms of encryption for highly secure channels of communication.

The creation of photon pairs is one of the key requirements for quantum state engineering. This has traditionally been achieved through the use of one of the two nonlinear effects, spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing (SFWM), in bulk optical elements. The nonlinear effects cause one or two pump photons to spontaneously decay into a photon pair.

These effects, however, require strict momentum conservation for the involved photons. Any material, which the photons have to travel through, has dispersion properties, preventing momentum conservation. There are techniques that still achieve the needed conservation, but those severely limit the versatility of the states in which the photon pairs can be produced. As a consequence, even though traditional optical elements like nonlinear crystals and waveguides have successfully produced many photonic quantum states, their use is limited and cumbersome. Therefore, researchers have recently focused their attention on so-called optical metasurfaces.

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