A review of ‘classical entanglement’ blurring the quantum-classical divide

Entanglement or non-separability constitutes a cornerstone of quantum mechanics from which many of its unique characteristics arise. For example, non-separability in entangled particle pairs leads to apparent instantaneous transfer of information and counterintuitive states of matter. Such phenomena find applications in diverse areas, such as quantum computing or quantum cryptography.

Nevertheless, non-separability is also ubiquitous in the classical domain. Indeed, even prism dispersion of light as observed by Newton over three centuries ago can be considered as an example of non-separable light. However, non-separability in classical systems, or “classical entanglement” is little explored and only in a fragmented fashion, while its potential is certainly not fully exploited.

Over the last few years, there has been a surge of interest in non-separable optical systems, typically involving free-space propagating beams and pulses. To this end, the on-demand design and generation of non-separable classical states of light using its various degrees of freedom, such as space, polarization, frequency, and propagation path has become crucial. The concept of non-separability in optics is now being extended to space-time non-separable pulses and ray-wave coupled geometric light.

Recently, a review published in Laser & Photonics Reviews proposes a comprehensive review of non-separability in classical light providing a perspective on the opportunities for both fundamental science and applications. This review provides a bird’s eye view on the rapidly growing, but incoherent, body of work on non-separable classical states involving different degrees of freedom of light and will introduce a unified framework for their classification.