High-dimensional entangled states offer the promise of increased data transfer and better security than conventional two-dimensional (qubit) systems, and thus realising and detecting such states is a topical and actively researched field. Yet despite the promise, few studies have managed to extend the tools beyond qubits. In this talk I will review the present state-of-the-art in creating and detecting high-dimensional quantum states based on the spatial modes of light. In particular, I will consider tools to engineer such states, present new results on their propagation through turbulence, and then discuss the challenges in the detection schemes that presently are used. Finally, I will show how it is possible to perform entanglement swapping and teleportation of such states, essential tools to cover long distances in a secure quantum communication network.
High-dimensional entangled states offer the promise of increased data transfer and better security than conventional two-dimensional (qubit) systems, and thus realising and detecting such states is a topical and actively researched field. Yet despite the promise, few studies have managed to extend the tools beyond qubits. In this talk I will review the present state-of-the-art in creating and detecting high-dimensional quantum states based on the spatial modes of light. In particular, I will consider tools to engineer such states, present new results on their propagation through turbulence, and then discuss the challenges in the detection schemes that presently are used. Finally, I will show how it is possible to perform entanglement swapping and teleportation of such states, essential tools to cover long distances in a secure quantum communication network.
Quantum mechanics is now a mature topic dating back more than a century. During its scientific development, it fostered many technological advances that now are integrated into our everyday lives. More recently, over the past few decades, the authors have seen the emergence of a second quantum revolution, ushering in control of quantum states. Here, the spatial modes of light, “patterns of light,” hold tremendous potential: light is weakly interacting and so an attractive avenue for exploring entanglement preservation in open systems, while spatial modes of light offer a route to high dimensional Hilbert spaces for larger encoding alphabets, promising higher information capacity per photon, better security, and enhanced robustness to noise. Yet, progress in harnessing high dimensional spatial mode entanglement remains in its infancy. Here, the authors review the recent progress in this regard, outlining the core concepts in a tutorial manner before delving into the advances made in creation, manipulation, and detection of such quantum states. The authors cover advances in using orbital angular momentum as well as vectorial states that are hybrid entangled, combining spatial modes with polarization to form an infinite set of two-dimensional spaces: multidimensional entanglement. The authors highlight the exciting work in pushing the boundaries in both the dimension and the photon number, before finally summarizing the open challenges, and the questions that remain unanswered.
The field of high-dimensional quantum photonics involves the use of multimode photonic degrees-of-freedom such as the spatial, temporal, or spectral structure of light to encode multi-level quantum states. Recent years have seen rapid progress in the development of methods to generate, manipulate, and distribute such quantum states of light and their use in a range of quantum technology applications that offer practical advantages over conventional qubit-based approaches. High-dimensional quantum states of light encoded in photonic time-bins, frequency-bins, transverse-spatial modes, waveguide paths, and temporal modes have enabled noise-robust fundamental tests of quantum mechanics, error-resilient and high-capacity quantum communication protocols, andas well as efficient approaches for quantum information processing, to name just a few examples. However, research in this field has progressed fairly independently, with little exchange across different photonic degrees-of-freedom or between experiment and theory and no comprehensive comparison between degrees-of-freedom. This roadmap aims to bridge this gap by surveying progress in each area and identifying shared challenges and opportunities that cut across two or more photonic degrees-of-freedoms. We review early work and state-of-the-art experimental techniques under development for high-dimensional quantum states encoded in single and entangled photons, as well as theoretical tools for their measurement and certification. We outline the main outstanding challenges for theory and each experimental degree-of-freedom, identifying promising future directions of research that may enable these to be overcome. We end by discussing interconnections and shared challenges centered around their distribution, measurement, and manipulation, with a view towards their integration into next-generation quantum technology platforms and applications.
Quantum teleportation allows protected information exchange between distant parties, a crucial resource in future quantum networks. Using high-dimensional quantum states for teleportation offers the promise of higher information capacity channels, protection against optimal cloning machines and improved resilience to noise. However, such promising advantages are limited by the commonly used linear optical detection schemes that require ancillary photons, where the number of entangled photons grows with the dimension of the quantum state to be teleported. Here we overcome this restriction and experimentally realise the teleportation of high-dimensional states with just three photons, enabled by nonlinear optical detection. To create high-dimensional quantum states we employ photonic spatial modes and demonstrate a ten-dimensional teleportation channel, exceeding the state-of-the-art of two spatial modes for qubit teleportation and qutrit teleportation with four and five photons. Our experimental scheme is not basis dependent and easily scaled in dimension without changing its three-photon configuration, making high-dimensional teleportation practically feasible for future high-bandwidth robust quantum networks.
Quantum structured light offers a roadmap to high-dimensional and multi-dimensional quantum states by exploiting all of light’s many degrees of freedom at the quantum level. In this talk, I will review the recent progress in quantum entanglement of photons in their spatial degree of freedom: quantum structured light. I will explain how to create high-dimensional quantum states in the laboratory, how to measure them, and review the present state of the art in the field. I will show how the patterns can be abstracted, mixing degrees of freedom to produce quantum topologies in light, introducing new paradigms for quantum state observables and classification. Finally, I will outline the advantages and disadvantages of using such single photon and entangled states in real-world applications, offering a perspective on the present challenges and exciting opportunities in the field.
Photonic entanglement is a fundamental resource in quantum information processing and its distribution between distant parties is a key challenge in quantum communications. Optical satellite links allow transmitting entangled photons over longer distances than currently possible on ground and could provide a path towards global quantum communication networks. The majority of current photonic free-space quantum communication systems, including two recent quantum satellite experiments, use two-dimensional polarization encoding, where each photon can carry at most a single bit of quantum information. Encoding additional qubits in the high-dimensional spatio-temporal degrees of freedom could not only increase channel capacities but also improve robustness with respect to noise and eavesdropping in quantum cryptography. Here, we give an overview of our efforts towards exploiting high-dimensional entanglement in long-distance free-space quantum communications. We report on the results of a first feasibility study, in which we used polarization / energy-time hyperentanglement to increase the dimensionality of the state space and transmit genuine 4-dimensional entanglement via an intra-city free-space link. We discuss how we intend to extend this approach to applications in noise-tolerant large-alphabet quantum key distribution via free-space links, and, ultimately, experiments with satellites.
Over the last few decades, quantum cryptography has been one of the most important quantum technologies developed. It holds the promise of communicating secure information over untrusted channels. In particular, quantum cryptography is immune to the eventual threats posed by large-scale quantum computers, which is not the case for our current classical cryptography infrastructures. In a world where information, communication and privacy are of paramount importance, the development of secure quantum communication infrastructures becomes imperative. So far, most of the quantum cryptographic systems developed to date are based on two-dimensional encoding schemes, analogous to classical bits, known as qubits. Nevertheless, other than for the polarization of light, photonic degrees of freedom, i.e. position, momentum, time and frequency, naturally occur as high-dimensional quantum systems. Moreover, by considering high-dimensional encoding schemes beyond qubits, advantages in terms of information capacity and noise tolerance are predicted in theory. In this thesis, the basic components of a high-dimensional quantum cryptographic system are investigated. We begin by reviewing important developments in quantum cryptography and high-dimensional quantum information. As a starting point, two different quantum information tasks, i.e. optimal quantum cloning and quantum metrology, are experimentally investigated for high-dimensional quantum systems in order to demonstrate the numerous advantages of performing quantum tasks beyond qubits. The photonic degree of freedom used in these experiments, known as transverse spatial modes, is obtained by combining the position and momentum photonics degrees of freedom. Thus, we develop a novel method to measure high-dimensional quantum states encoded in arbitrary spatial modes. Using a similar characterization technique, several high-dimensional quantum communication channels are characterized to perform high-dimensional quantum cryptography. The tools developed in the laboratory are then brought to realistic conditions in order to test the feasibility of high-dimensional quantum cryptography. Two different types of quantum channels are experimentally investigated, namely an intra-city 300 m long free-space link and a short 3 m outdoor underwater link. Finally, we investigate and compare novel high-dimensional protocols to overcome current limitations.