By helping scientists control a strange but useful quantum mechanical phenomenon, an ultra-thin invention could make future computing, sensing and encryption technologies significantly smaller and more powerful. The device is described in a new study recently published in the journal Science.
According to scientists at Sandia National Laboratories and the Max Planck Institute for the Science of Light in the US, the device could replace a roomful of devices that link photons in a bizarre quantum effect known as entanglement. It is a nano-engineered material known as a metasurface that paves the way for entangling photons in a complex way that would not be possible in compact technology.
When photons are said to be entangled, this means that they are linked in such a way that action on one photon affects another, no matter where in the universe those photons are or how far apart they are. This is a bizarre effect of quantum mechanics, a law of physics that governs particles and other very tiny things.
As strange as this phenomenon may seem, researchers have used it to process information in new ways. For example, entanglement helps protect fragile quantum information and correct errors in quantum computing, an area that could one day have sweeping implications for science, finance and national security. Entanglement can also provide advanced new encryption methods for secure communications.
The groundbreaking device is only one hundredth the thickness of a sheet of paper, and its research was conducted in part at the Center for Integrated Nanotechnology, a user facility of the US Department of Energy’s Office of Science, operated by Sandia and Los Alamos National Laboratory. Sandia’s team received funding from the Office of Science’s Basic Energy Sciences Program.
The new metasurface acts as a ‘portal’ for this unusual quantum phenomenon. In some ways it resembles the mirror in Lewis Carroll’s Alice in the Mirror, through which the young protagonist Alice experiences a strange new world.
Instead of walking through their new device, the scientists shine a laser through it. The beam passes through an ultra-thin glass sample covered with a nanoscale structure made of a common semiconductor material called gallium arsenide.
“It disturbs all the light fields,” says Sandia senior scientist Igal Brener. He is an expert in a field called nonlinear optics and leads the Sandia team. Occasionally, he said, a pair of entangled photons of different wavelengths would emerge from the sample in the same direction as the incoming laser beam.
Brener said he was enthusiastic about the device because it was designed to produce a complex network of entangled photons. Instead of producing one pair at a time, it can produce several pairs of photons that are all entangled, and some that can be indistinguishable from each other. Some technologies require these complex so-called multiple entangled varieties to implement complex information processing schemes.
Although other micro technologies based on silicon photonics can also entangle photons, they lack the much-needed level of complex multiple entanglements. Until now, the only way to produce such results has been to use multiple tables filled with lasers, specialized crystals and other optical devices.
When this multiple entanglement requires more than two or three pairs, it is quite complex and somewhat difficult to solve,” Brener said. These nonlinear metasurfaces essentially achieve this task in a single sample, which previously required incredibly complex optical setups.”
The Science paper outlines how the team successfully tuned their metasurfaces to produce entangled photons with different wavelengths. This is a key precursor to producing several pairs of intricately entangled photons at the same time.
However, the scientists note in their paper that the efficiency of their device – the speed at which they were able to produce groups of entangled photons – is lower than other techniques and will need to be improved.
What is a metasurface?
A metasurface is a synthetic material that interacts with light and other electromagnetic waves in ways that conventional materials can’t. Brener says the commercial industry is busy developing metasurfaces because they take up less space and can do more with light than conventional lenses.
“You can now replace lenses and thick optical elements with metasurfaces,” Brener said. “These types of metasurfaces are going to revolutionize consumer products.”
Sandia National Laboratories is one of the world’s leading institutions conducting research on metasurfaces and metamaterials. Between its Microsystems Engineering, Science and Applications Complex, where compound semiconductors are made, and the nearby Integrated Nanotechnology Centre, scientists have access to all the specialized tools they need to design, fabricate and analyze these ambitious new materials.
This work is challenging because it requires precise nanofabrication techniques to obtain the sharp, narrow-band optical resonances that are the seeds of the quantum processes in this work,” says Sylvain Gennaro, a former postdoctoral researcher at Sandia, who was involved in several aspects of the project.
The device was designed, fabricated and tested through a collaboration between Sandia and a research team led by physicist Maria Chekhova. She is an expert in photonic quantum entanglement at the Max Planck Institute for the Science of Light.
Tomás Santiago-Cruz says: “Metasurfaces are leading to a paradigm shift in quantum optics, combining ultra-small quantum light sources with the far-reaching possibilities of quantum state engineering.” He is a member of Max Planck’s team and the first author of the paper.
Brener, who has been studying metamaterials for more than a decade, said this latest research could spark a second revolution – seeing these materials used not only as a new lens but as a technology for quantum information processing and other new applications. There is a wave of metasurface technology that is well established and underway,” he said. There may be a second wave of innovative applications coming up.”
