Physicists created a quantum hologram using a metasurface to record the hybrid polarization-holographic state of photons. When the experimenters changed the polarization of an idler photon, part of the hologram was hidden upon development. The scientists proposed using this new technology in the BB84 quantum key distribution algorithm. The results of the study were published in Advanced Photonics.
Hybrid entangled states of photons promise physicists many new possibilities: for example, they can couple the polarization of a single photon with the orbital momentum of another photon in a light beam to perform quantum tomography. However, studying hybrid quantum entanglement involving a spatial holographic field has proven challenging for scientists due to the complex implementation of such states—the traditional instrumentation of optical laboratories makes experimental setups too cumbersome and extremely inefficient.
Here, metasurfaces—structures consisting of an array of subwavelength elements—come to the aid of experimenters. Such materials can modulate light by controlling several photon characteristics simultaneously: for example, they can emit entangled quanta with variable wavelengths.
Jensen Li of the Hong Kong University of Science and Technology, together with colleagues from the UK and China, used a metasurface to create a quantum hologram based on polarization-holographic entanglement – a hybrid state in which the polarization and complex spatial modes of photons are coupled.
To do this, the physicists used a metasurface with a common amplitude profile but different phase profiles in the image plane. A laser beam with a wavelength of 405 nanometers served as the photon source in the setup. The generated photon pairs were separated using a prism and launched into two separate arms of the setup—a signal arm and an idler arm. After passing through the signal arm, the photon traveled through a 10-meter-long optical fiber and emerged toward the metasurface, transforming it into a hologram. The time it took the scientists to create one hologram was approximately 20 minutes.
A metasurface in the signal arm of the setup generated two different holographic photon states, which in turn became entangled with two orthogonal polarization states of the idler photons. This polarization-holographic entanglement allowed the physicists to remotely control the quantum holograms of the signal photon by changing the polarization of the idler. For example, when the researchers measured the hologram of the signal photon without polarization selection of the idler (simply removing the polarizer from the setup), they obtained an image of all four encoded letters "HDVA." However, after the experimenters replaced the polarizer, the signal photon collapsed into a superposition of holographic states—one of the letters in the inscription was erased. To select which letter to obscure, the physicists used different polarization angles in the idler arm.
The authors of the paper noted that their proposed quantum holography technology could prove useful in the field of quantum encryption: according to their own estimates, the use of holograms in the BB84 protocol (more details on this protocol can be found in our material "Quantum Technologies. Module 5") for quantum key distribution yields a bit error rate of only 1.5 percent, which is significantly lower than the required security threshold of 11 percent.
We previously wrote about how physicists have learned to change everything in light pulses at once using very long metasurfaces.
