Quantum photonic platforms have proven to be essential in realizing fundamentally secure quantum transfer of information, with commercially ready systems already deployed in municipal and terrestrial links. The drive toward higher bit rates and robustness to eavesdropping and noisy channels has focused attention on moving from the present two-dimensional quantum states of polarization, to harnessing all of light’s degrees of freedom for multi-dimensional quantum coding with structured photons. In this Perspective, we outline the present state-of-the-art in achieving this control with spatial modes of light, both as single photon and entangled states, highlight the open challenges that remain, and consider the roadmap that might see its full potential realized.

Light can be tailored to carry angular momentum well beyond the restriction of its two spin states, left- and right-circularly polarized light, by imbuing it with orbital angular momentum (OAM). OAM is controlled by imparting finer and finer azimuthal phase gradients, twisting the wavefront ever tighter in one of two helicities, clockwise or anticlockwise. This can be done directly within a laser — OAM lasers — by imprinting an intracavity twist on the circulating light, but it requires judicious laser cavity design to break nature’s angular momentum degeneracy. Without this, the laser produces equal measures of both helicities, for no net OAM. We review the physics of OAM lasers, covering diverse symmetry-breaking approaches such as gain or loss control, asymmetric cavity geometries and geometric phase control. Structured matter allows this symmetry breaking to be done at the microscale and nanoscale, for OAM lasers based on topological matter, photonic crystals and optical breaking of chiral symmetry in microring cavities, as well as leveraging non-Hermitian photonic design at exceptional points. The exciting prospect of using structured matter to engineer twisted light is discussed along with the opportunities and challenges ahead.

Information exchange between two distant parties, where information is shared without physically transporting it, is a crucial resource in future quantum networks. Doing so with high-dimensional states offers the promise of higher information capacity and improved resilience to noise, but progress to date has been limited. Here we demonstrate how a nonlinear parametric process allows for arbitrary high-dimensional state projections in the spatial degree of freedom, where a strong coherent field enhances the probability of the process. This allows us to experimentally realise quantum transport of high-dimensional spatial information facilitated by a quantum channel with a single entangled pair and a nonlinear spatial mode detector. Using sum frequency generation we upconvert one of the photons from an entangled pair resulting in high-dimensional spatial information transported to the other. We realise a d = 15 quantum channel for arbitrary photonic spatial modes which we demonstrate by faithfully transferring information encoded into orbital angular momentum, Hermite-Gaussian and arbitrary spatial mode superpositions, without requiring knowledge of the state to be sent. Our demonstration merges the nascent fields of nonlinear control of structured light with quantum processes, offering a new approach to harnessing high-dimensional quantum states, and may be extended to other degrees of freedom too.

In their Review (article number 2300939), Chané Moodley and Andrew Forbes explore the capabilities of the intersection between quantum imaging and artificial intelligence. With technological and AI advancements, we expect faster, higher-quality quantum images. This review explores key progress and future directions in this dynamic and interdisciplinary field.