Metasurfaces entangle light and information in new study

Quantum entanglement is a fundamental phenomenon in nature and one of the most intriguing aspects of quantum mechanics. It describes a correlation between two particles, such that measuring the properties of one instantly reveals those of the other, no matter how far apart they are. This unique property has been harnessed in applications such as quantum computing and quantum communication.

A common method for generating entanglement is through a nonlinear crystal, which produces photon pairs with entangled polarizations via spontaneous parametric down-conversion (SPDC): if one photon is measured to be horizontally polarized, the other will always be vertically polarized, and vice versa.

Meanwhile, metasurfaces—ultrathin optical devices—are known for their ability to encode vast amounts of information, allowing the creation of high-resolution holograms. By combining metasurfaces with nonlinear crystals, researchers can explore a promising approach to enhancing the generation and control of entangled photon states.

In a recent study published in Advanced Photonics, researchers from Hong Kong and the UK introduced a novel approach to creating quantum holograms using metasurfaces. By carefully designing the orientations of nanostructures within the metasurface, they enabled the generation of a quantum hologram, where polarization and holographic information become entangled.

“We have demonstrated that metasurfaces serve as a versatile platform for generating quantum holograms. The entanglement property of these quantum holograms is further revealed by projecting one photon onto various polarization states corresponding to interference effects observed elsewhere,” explained Jensen Li, professor of Computational Engineering and Metamaterials at the University of Exeter, and senior author of the report.

The approach offers a compact yet flexible method that is difficult to achieve with conventional materials. To demonstrate, the researchers successfully created four holographic letters—”H,” “V,” “D,” and “A”—entangled with the polarization of the paired photons. By selecting different polarizer orientations for one photon, specific letters in the hologram could be selectively erased, demonstrating precise control over the entangled holographic information.

Beyond its fundamental significance, this research also holds promise for practical applications, such as quantum communication, by encoding information in both the letters and polarization states. “With more complex entanglement patterns, we may be able to increase the information capacity for quantum key distribution, which is a secure way to communicate,” said Hong Liang, a co-author of the study. “We believe that metasurfaces can significantly reduce the size of quantum optical systems, making this technology much more practical for everyday use.”

Additionally, the researchers suggest that metasurfaces can be used for anti-counterfeiting technology. Beyond the inherent difficulty of replicating the metasurface itself, the intricate relationship between the letters and polarization states and the relative phase profile between different holograms creates a unique pattern that is extremely challenging to reproduce, enhancing security against forgery.

“Our demonstration can also be interpreted as a quantum eraser at the holographic level. Compared to the traditional double-slit quantum eraser, our setup replaces two slits with two holograms and obtains ‘which-hologram information’ from the polarization of entangled photons. Erasing this information now has a concrete effect, as illustrated by erasing holographic letters,” added Wai Chun Wong, another co-author of the paper.

This research highlights how modern nanofabrication technology can harness quantum effects for practical applications. Despite their ultrathin nature, metasurfaces enable complex quantum operations that would traditionally require intricate optical setups. This achievement not only deepens our collective understanding of quantum mechanics but also provides further general insight for quantum information processing.