Nonseparable States of Light: From Quantum to Classical

Abstract

Controlling the various degrees‐of‐freedom (DoFs) of structured light at both quantum and classical levels is of paramount importance in optics. It is a conventional paradigm to treat diverse DoFs separately in light shaping. While, the more general case of nonseparable states of light, in which two or more DoFs are coupled in a nonseparable way, has become topical recently. Importantly, classical nonseparable states of light are mathematically analogue to quantum entangled states. Such similarity has hatched attractive studies in structured light, for example, the spin‐orbit coupling in vector beams. However, nonseparable classical states of light are still treated in a fragmented fashion, while its forms are not limited by vector beams and its potential is certainly not fully exploited. For instance, exotic space‐time coupled pulses open nontrivial light shaping toward ultrafast time scales, and ray‐wave geometric beams provide new dimensions in optical manipulations. Here, a bird's eye view on the rapidly growing but incoherent body of work on general nonseparable states involving various DoFs of light is provided and a unified framework for their classification and tailoring, providing a perspective on new opportunities for both fundamental science and applications, is introduced.

Topological quasiparticles of light: Optical skyrmions and beyond

“…polarity and vorticity, which may access exotic biparticle hybridentangled state, |ψ = 1 5c). Hyper and hybrid entangled states are very hot topic in quantum community with many promised applications [107], but still lack practical system to realize the complex entangled states, while optical skyrmions may provide a good solution to this challenge.…”

Self-healing of structured light: a review

“…10: Self-healing of classical entanglement: (a) setup; (b) experimental results of self-healing vector beams; (c) experimental results of self-healing classical entanglement [115]. Copyright by American Physical Society separability is extremely similar to the non-separability of quantum entangled photon pairs, widely termed classical entanglement, thus many quantum analogue methods can be exploited to characterize the properties of vector beams [111][112][113]. Vector beams have gained interest, due to their ability to produce tight focal spots when focused by high numerical aperture objectives [114].…”

“…It follows that the nonseparability is also maintained when measured after the shadow region [129] but should still be preserved at any location where the field has nonzero amplitude. Importantly, the self-healing of classical entanglement enables the quantum-like classical technology of communication and cryptography towards higher security [113,130].…”

High-dimensional orbital angular momentum entanglement from an ultrathin nonlinear film

“…In the classical domain, the OAM can increase the capacity of optical communication links ranging from implementation in fibre [19,20], over-city links [21] and free-space [22], and can also operate in mm-wave [23]. Actually, each OAM mode can be considered as an individual quantum degree of freedom [24,25]. Quantum mechanically, the OAM occurs in discrete values of mZ per photon, where m is in principle an unbounded integer.…”