Physicists have created a quantum hologram using a metasurface to record a hybrid polarization-holographic state of photons. When the experimenters changed the polarization of an idler photon, part of the hologram was hidden when developed. The scientists proposed using the new technology in the BB84 quantum key distribution algorithm. The results of the study are published in Advanced Photonics.
Hybrid entangled states of photons promise many new possibilities for physicists: for example, it is possible to link the polarization of a single photon and the orbital momentum of another photon in a light beam to perform quantum tomography. However, the study of hybrid quantum entanglement involving a spatial holographic field has become difficult for scientists due to the complex implementation of such states - the classical instrumentation of optical laboratories makes experimental setups too cumbersome and extremely inefficient.
Here, metasurfaces come to the aid of experimenters - structures consisting of an array of elements of subwave sizes. Such materials are capable of modulating light, controlling several characteristics of photons at once: for example, they can emit entangled quanta with a variable wavelength.
Jensen Li of the Hong Kong University of Science and Technology, together with colleagues in 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. The source of photons in the setup was a laser beam with a wavelength of 405 nanometers, and the scientists separated the generated photon pairs using a prism and launched them into two separate arms of the setup - signal and idle. After passing the signal arm, the photon went into a 10-meter-long optical fiber and came out to meet the metasurface, turning it into a hologram. The time that the scientists spent on creating one hologram was about 20 minutes.
The metasurface in the signal arm of the setup generated two different holographic states of photons, which in turn became entangled with two orthogonal polarization states of idler photons. This polarization-holographic entanglement allowed the physicists to remotely control quantum holograms of the signal photon by changing the polarization of the idler. For example, when the researchers measured the hologram of the signal quantum of light without polarization selection of the idler (simply removing the polarizer from the setup), they obtained an image of all four encoded letters "HDVA" at the output. However, after the experimenters returned the polarizer to its place, the signal photon collapsed into a superposition of holographic states - one of the letters of the inscription was erased. To choose which specific letter to hide, the physicists used different polarization angles in the idler arm.
The authors of the paper noted that the quantum holography technology they proposed could be useful in the field of quantum encryption: according to their own estimates, the use of holograms in the BB84 protocol (more details about this protocol can be found in our material “Quantum Technologies. Module 5”) for quantum key distribution gives a bit error rate of only 1.5 percent, which is significantly lower than the required security threshold of 11 percent.
We wrote earlier about how physicists have learned to change everything at once in light pulses using very long metasurfaces.