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Violation of Bell’s inequality for helical Mathieu–Gauss vector modes
Vector beams display varying polarisation over planes transversal to their direction of propagation. The variation of polarisation implies that the electric field cannot be expressed as a product of a spatial mode and its polarisation. This non-separability has been analysed for particular vector beams in terms of non–quantum entanglement between the spatial and the polarisation-degrees of freedom, and equivalently, with respect to the degree of polarisation of light. Here we demonstrate theoretically and experimentally that Mathieu–Gauss vector modes violate a Bell-like inequality known as the Clauser–Horn–Shimony–Holt–Bell inequality. This demonstration provides new insights on the violation of Bell inequalities by a more general class of vector modes with elliptical symmetry.
Light correcting light with nonlinear optics
Structured light, where complex optical fields are tailored in all their degrees of freedom, has become highly topical of late, advanced by a sophisticated toolkit comprising both linear and nonlinear optics. Removing undesired structure from light is far less developed, leveraging mostly on inverting the distortion, e.g., with adaptive optics or the inverse transmission matrix of a complex channel, both requiring that the distortion be fully characterized through appropriate measurement. We show that distortions in spatially structured light can be corrected through difference-frequency generation in a nonlinear crystal without any need for the distortion to be known. We demonstrate the versatility of our approach using a wide range of aberrations and structured light modes, including higher-order orbital angular momentum (OAM) beams, showing excellent recovery of the original undistorted field. To highlight the efficacy of this process, we deploy the system in a prepare-and-measure communications link with OAM, showing minimal cross talk even when the transmission channel is highly aberrated, and outline how the approach could be extended to alternative experimental modalities and nonlinear processes. Our demonstration of light-correcting light without the need for measurement opens an approach to measurement-free error correction for classical and quantum structured light, with direct applications in imaging, sensing, and communication.
Quantum cryptography with structured photons
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.
Orbital angular momentum lasers
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.
Quantum transport of high-dimensional spatial information with a nonlinear detector
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.
Advances in Quantum Imaging with Machine Intelligence
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.