Metasurfaces present new opportunities for quantum research

An ultrathin device could make future computing, sensing and encryption technologies smaller and more powerful by helping scientists control the phenomenon of quantum mechanics, according to research published in the journal Science.

Scientists at Sandia National Laboratories and the Max Planck Institute for the Science of Light have developed a device that could replace the equipment needed to link photons in a quantum effect called entanglement. This device — a kind of nano-engineered material called a metasurface — paves the way for entangling photons in complex ways that have not been possible with compact technologies.

A metasurface is a synthetic material that interacts with light and other electromagnetic waves in ways conventional materials can’t. According to researchers from Sandia, metasurfaces take up less space and can do more with light than, for instance, a traditional lens. In the artist rendering of a metasurface that accompanies this article, light passes through tiny, rectangular structures — the building blocks of the metasurface —  and creates pairs of entangled photons at different wavelengths.

When scientists say photons are entangled, they mean they are linked in such a way that actions on one affect the other, no matter where or how far apart the photons are in the universe. It is an effect of quantum mechanics, the laws of physics that govern particles and other very tiny things. Although the phenomenon of entanglement may seem odd, scientists have harnessed it to process information in new ways. For example, entanglement helps protect quantum information and correct errors in quantum computing; it is also enabling new, advanced encryption methods for secure communication.

Research on the device, which is reportedly a hundred times thinner than a sheet of paper, was performed, in part, at the Center for Integrated Nanotechnologies. Sandia’s team received funding from the Office of Science, Basic Energy Sciences program.

The new metasurface acts as a ‘doorway’ to this unusual quantum phenomenon. Scientists shone a laser through the metasurface; the beam of light passed through an ultrathin sample of glass covered in nanoscale structures made of a common semiconductor material called gallium arsenide. Sandia senior scientist Igal Brener, an expert in the field of nonlinear optics who led the Sandia team, said it “scrambles” all the optical fields. Occasionally, Brener said, a pair of entangled photons at different wavelengths emerge from the sample in the same direction as the incoming laser beam.

Brener said he is excited about this device because it is designed to produce complex webs of entangled photons — not just one pair at a time, but several pairs all entangled together, and some that can be indistinguishable from each other. Some technologies need these complex varieties of so-called multi-entanglement for sophisticated information-processing schemes. Other miniature technologies based on silicon photonics can also entangle photons, without the much-needed level of multi-entanglement. Until now, the only way to produce such results was with multiple tables full of lasers, specialised crystals and other optical equipment.

“It is quite complicated and kind of intractable when this multi-entanglement needs more than two or three pairs. These nonlinear metasurfaces essentially achieve this task in one sample when before it would have required incredibly complex optical setups,” Brener said.

The Science paper outlines how scientists tuned their metasurface to produce entangled photons with varying wavelengths, a critical precursor to generating several pairs of intricately entangled photons simultaneously. However, the scientists noted in their paper that the efficiency of their device — the rate at which they can generate groups of entangled photons — is lower than that of other techniques and needs to be improved.

Sandia is among the leading institutions performing research in metasurfaces and metamaterials. Between its Microsystems Engineering, Science and Applications complex, which manufactures compound semiconductors, and the nearby Center for Integrated Nanotechnologies, researchers have access to a range of specialised tools to design, fabricate and analyse these new materials.

“The work was challenging as it required precise nanofabrication technology to obtain the sharp, narrowband optical resonances that seeds the quantum process of the work,” said Sylvain Gennaro, a former postdoctoral researcher at Sandia who worked on several aspects of the project.

The device was designed, fabricated and tested through a partnership between Sandia and a research group led by physicist Maria Chekhova, an expert in the quantum entanglement of photons at the Max Planck Institute for the Science of Light. “Metasurfaces are leading to a paradigm shift in quantum optics, combining ultra-small sources of quantum light with far-reaching possibilities for quantum state engineering,” said Tomás Santiago-Cruz, a member of the Max Planck team and first author on the paper.

Brener said this newest research could spark a revolution that sees these materials developed as a new kind of lens and also as a technology for quantum information processing and other new applications. “There was one wave with metasurfaces that is already well established and on its way. Maybe there is a second wave of innovative applications coming,” Brener said.

Image caption: In this artist rendering of a metasurface, light passes through tiny, rectangular structures — the building blocks of the metasurface — and creates pairs of entangled photons at different wavelengths. Image credit: Sandia National Laboratories.