Light is often described as a transverse wave, with its electric field confined to the plane perpendicular to propagation. Under tight focusing or subwavelength confinement, however, longitudinal field components emerge and the electric field acquires a genuinely three-dimensional vectorial structure. Rather than being merely a consequence of non-paraxial propagation, this additional degree of freedom can be tailored and exploited to control light–matter interactions and encode information.

In this talk, I will explore how the three-dimensional vectorial structure of the electric field can be engineered and measured in nanoscale and quantum optical systems. I will first discuss how the full 3D coherence matrix and degree of 3D polarization provide tools for characterizing focal, spatially varying electric fields and their interactions with nanoscale systems. I will show how these quantities can provide insights into the structural properties of extended nanostructures, as well as the orientation and dynamics of individual emitters and scatterers. I will further discuss how tailored vectorial fields can be used to control and characterize the response of such nanostructures.

I will then turn to the use of these additional field degrees of freedom for quantum information. Paraxial scalar and vectorial structured fields have been established as resources for encoding information in higher-dimensional optical states. However, the third dimension of the electric field has largely been neglected, despite its natural emergence in, for example, nanophotonic devices. I will present an inverse-designed, on-chip decoder for quantum key distribution that enables the generation and measurement of high-dimensional states encoded in three-dimensionally polarized spatial modes.

Together, these examples illustrate how the three-dimensional vectorial nature of confined light can be transformed from a feature that is often neglected into a resource for probing and controlling nanoscale matter, as well as for encoding and processing quantum information.