Polarized light can erase messages encoded in quantum holograms
Hong Liang, Wai Chun Wong, Tailing Ang, Jensen Lee 2024
The quantum evanescence phenomenon makes it possible to embed secure messages in holograms and selectively erase parts of them even after they have been transmitted.
Quantum optical signals are inherently secure information carriers: any interception of the message destroys the vulnerable quantum states that encode it. To exploit this without the need for bulky equipment, Jensen Li of the University of Exeter in the UK and his colleagues have created quantum holograms using metasurfaces, 2D materials engineered to have special properties.
Holograms encode complex information that can be restored when light is shone on it. For example, when light hits a 2D holographic paper card at the right angle, a 3D image appears. To create quantum holograms, researchers encoded information in the quantum state of particles of light, or photons.
First, they used a laser to emit two photons from a special crystal that were tightly bound by quantum entanglement. The photons traveled along separate paths, with only one encountering the metasurface along the way. Thousands of tiny components on the metasurface, like nano-sized bumps, altered the photon’s quantum state in a preprogrammed way, encoding a holographic image into it.
The partner photon encountered a polarizing filter, which controlled which parts of the hologram appeared and which disappeared. The first photon’s state was a superposition of holograms, so it contained different variations of the message at the same time. Because the photons were in an entangled state, polarizing the second photon affected the image the other photon created when it hit the camera. For example, a test hologram contained the letters H, D, V, and A, but adding a filter for horizontal polarization caused the letter H to disappear from the final image.
Li says metasurfaces could be used to encode more complex information into photons, for example as part of quantum cryptography protocols. He presented his work on August 21 at the SPIE Optics + Photonics conference in San Diego, California.
“Everyone dreams of quantum technology going from square metres on a table to being compact enough to fit in a smartphone, and metasurfaces seem like a good way to achieve that,” says Andrew Forbes of the University of Witwatersrand in South Africa. He says that quantum holograms like the one used in this experiment could also be used to image tiny biological structures in the rapidly expanding medical sector.
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