Since the beginning of the 20th century, quantum mechanics has brought many subversive new concepts to people. Quantum entanglement is one of the most representative. After some initial processing, two or more particles can enter a state that looks like "super spatiotemporal correlation": even if they are separated by hundreds of thousands of light-years, the change of one particle will affect other particles instantaneously. This phenomenon was once called by Einstein as "ghostly over distance interaction", so quantum entanglement is often known as "Einstein's ghost". Photon system is one of the platforms which are often used to show quantum entanglement in practice. Nonlocal correlation properties can be constructed based on many degrees of freedom of photons. In recent years, more and more attention has been paid to the entangled state of photon space-time mode, because there are a lot of controllable degrees of freedom in space-time, such as photon orbital angular momentum. The development of these degrees of freedom can greatly improve the information carrying capacity of photons, which is very conducive to applications such as quantum communication, quantum computing and so on.
As an important platform to realize effective and fine control of the spatial characteristics of light field, the excellent ability of metamaterials has been fully demonstrated in the field of classical optics before, but its application potential in quantum optics has not been explored. For the metamaterials composed of micro and nano antenna type "artificial atoms", the interaction between them and photons can lead to the entanglement of spatial degrees of freedom. Not only that, the exploration of this direction directly establishes the relationship between the properties of entangled photon states and mesoscopic structure, or it can provide some thinking angles for further solving the perception contradiction between micro randomness and macro certainty that has puzzled people for a long time.
Professor Liu Yushen, associate professor of Changshu Institute of technology, and Professor Lu Yanqing of Nanjing University worked together to study the generation and regulation of entangled photon states based on the process of parametric down conversion in metamaterials with ideal optical nonlinearity. The material, geometry, spatial orientation and lattice type of the artificial atoms can affect the overall response of the electromagnetic field. Therefore, the nonlinear interaction process of photons in metamaterials can be effectively controlled by the flexible design of micro and nano structures, and the spatial characteristics of photon states can be planned according to the needs. The theoretical framework for describing the system is based on the combination of nonlinear Huygens Fresnel principle and equivalent beam splitter model: for the former, each artificial atom is regarded as a secondary down conversion wave source, and its local interaction is described by Hamiltonian density, and then the whole form of entangled state is obtained by integrating the whole space; for the latter, the intrinsic loss of the system is dealt with Consumption. As a specific consideration, the metamaterials with the transverse spatial distribution of the micro / nano antenna in the shape of a forked grating are used to generate the orbital angular momentum entanglement, and their effectiveness is verified by simulation.
Relevant research results were published in Laser & Photonics reviews (DOI: 10.1002/lpor.201900146) and selected as the current inner cover.
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