Quantum entanglement is a fundamental property of quantum mechanics, highly valued for information processing and communication. Photonic quantum states, leveraging the weak interaction of photons, are widely explored. However, it's recognized that composite approaches are likely necessary for a comprehensive system.
Structured light involves tailoring light in all its degrees of freedom (DoFs), spatial and temporal, creating complex optical fields in classical and quantum domains. Combining DoFs has led to novel states of light in 2D, 3D, and even 4D fields. This field emerged recently, particularly with seminal work on the orbital angular momentum (OAM) of light as a basis, leading to a surge in activity and depth.
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A Brief Overview of Structured Light
In a typical visible laser beam, millions of transverse modes exist per square millimeter, presenting an extraordinary resource for potential exploitation. Historically, the focus was on Gaussian beams, with efforts directed towards eliminating undesired transverse structures.
An article published in Nature Photonics reveals that comparing an unstructured plane wave with sinusoidal fringes reveals crucial principles. While a plane wave's structure lies in its uniform phase gradient (observable only with interferometry), sinusoidal fringes structure the light's intensity, visible to the naked eye.
Immediate three-dimensional control is achievable in systems that confine light, such as photonic crystals. The synergy of structured matter and structured light in such systems has unlocked numerous exciting possibilities. This approach proves powerful for executing unitary operations on multimodal classical light and high-dimensional quantum states. Another prevalent method involves controlling light's structure in three dimensions by strategically configuring the degrees of freedom (DoFs) of the initial 2D field. Utilizing wave interference, the desired structure, exemplified by the Talbot effect and fractal light, can be created.
How Can Structured Light Revolutionize the Communication Networking Systems?
The potential offered by the multitude of modes within a small cross-sectional area of light can revolutionize communication networks, enhancing speed. In the quantum realm, structured light's high-dimensional quantum states hold promise for significantly improving security.
The rapid advancement in the research domain of structured light in the past decade has been primarily propelled by optical communications, aiming for faster communication through an increased number of modes. Challenges in this domain include turbulence and divergence, limiting long-distance demonstrations to propagation without data transfer. Although developments in custom optical fiber for structured light have shown promise, achieving distances of 50 km with eight modes, error correction is still required. These distances remain modest when compared to using Gaussian beams in single-mode fiber.
What is Hybrid Entanglement, and How Does it Form a Connection Between Quantum Mechanics and Light?
Advancements in spatial mode development have been facilitated by the precise control of individual degrees of freedom (DoF). Moreover, more exotic forms of entangled photonic states have emerged, incorporating various DoFs and, in some instances, utilizing all available DoFs. One notable example of these exotic quantum states is hybrid entanglement, as per the article published in AVS Quantum Science. Hybrid entanglement entails the entanglement of two particles spatially separated, each existing in a distinct degree of freedom.
Significant progress in the field occurred following the introduction of the spin–orbit (SO) coupling optics, allowing the interfacing of polarization (spin) and orbital angular momentum (OAM) at the single-photon level.
Hybrid states find intriguing applications, including the abstract concept of the path in fundamental tests of quantum mechanics, such as the quantum eraser experiment. These states also show great potential for practical applications, with notable use cases in quantum communication. For instance, hybrid modes have been utilized to implement high-dimensional single-photon quantum key distribution (QKD) based on the "BB84" protocol in both free-space and fiber environments.
Quantum Transport of Light with a Non-Linear Detector
The exchange of information between two distant parties, where data is shared without physical transportation, is a vital element in the development of future quantum networks. Leveraging high-dimensional states for such exchanges holds the potential for increased information capacity and enhanced resilience to noise. However, progress in this area has been constrained thus far.
Remote state preparation enables the exchange of information between parties without physically transmitting the information across the link, requiring the sender to possess knowledge of the information to be sent. Teleportation facilitates secure information exchange between distant parties without the necessity for a physical link.
Researchers published an article in Nature Communications in which they described an experimental setup to present a nonlinear spatial quantum transport system that operates in arbitrary dimensions. The scheme utilized two entangled photons to establish the quantum channel, while a bright, coherent source was employed for information encoding.
One of the entangled photons underwent upconversion in a nonlinear crystal, utilizing the coherent beam for both information carrier and efficiency enhancement. Successful single photon detection led to the transportation of information to the other photon, facilitated by a bi-photon coincidence measurement. The system is dimension and basis-independent, and the modal capacity of the quantum channel can be easily controlled by adjusting parameters such as beam size and crystal properties.
By expressing information across various spatial bases, including orbital angular momentum (OAM), Hermite-Gaussian, and their superpositions, the new experimental approach demonstrated the transfer of information across many spatial modes. The experimental results were substantiated by a comprehensive theoretical treatment, offering a novel method for leveraging high-dimensional structured quantum states through nonlinear optical control and detection.
This novel approach to quantum transport of unknown high-dimensional spatial states opens up exciting prospects for the future. The potential extension of this method to mixed degrees of freedom, such as hybrid entangled states involving polarization and space, as well as hyper-entangled states combining space and time, holds promise for achieving multi-degree-of-freedom and high-dimensional quantum control. This innovation marks a significant step forward in the field, offering new possibilities for advanced quantum information processing and communication applications.
References and Further Reading
Forbes, A. et. al. (2021). Structured light. Nat. Photonics 15, 253–262. Available at: https://doi.org/10.1038/s41566-021-00780-4
Nape I. et. al. (2019) Quantum mechanics with patterns of light: Progress in high dimensional and multidimensional entanglement with structured light. AVS Quantum Sci. 1 (1). 011701. Available at: https://doi.org/10.1116/1.5112027
Sephton, B. et al. (2023). Quantum transport of high-dimensional spatial information with a nonlinear detector. Nat Commun 14, 8243. Available at: https://doi.org/10.1038/s41467-023-43949-x