High-dimensional quantum communication: benefits, progress, and future challenges (1910.07220v1)

Abstract: In recent years, there has been a rising interest in high-dimensional quantum states and their impact on quantum communication. Indeed, the availability of an enlarged Hilbert space offers multiple advantages, from larger information capacity and increased noise resilience, to novel fundamental research possibilities in quantum physics. Multiple photonic degrees of freedom have been explored to generate high-dimensional quantum states, both with bulk optics and integrated photonics. Furthermore, these quantum states have been propagated through various channels, \textit{e.g.} free-space links, single-mode, multicore, and multimode fibers and also aquatic channels, experimentally demonstrating the theoretical advantages over two-dimensional systems. Here, we review the state of the art on the generation, the propagation and the detection of high-dimensional quantum states.

• The paper demonstrates that high-dimensional states significantly boost information capacity and noise resilience compared to traditional qubits.
• The study reviews experimental methodologies such as orbital angular momentum and integrated photonics for generating and manipulating qudits.
• The paper highlights future challenges including integration with quantum memories and robust certification of high-dimensional entanglement for secure quantum networks.

High-Dimensional Quantum Communication: Insights and Challenges

The paper "High-dimensional quantum communication: benefits, progress and future challenges" provides a comprehensive examination of the current landscape and future prospects in the field of high-dimensional quantum communication. This work discusses the substantial advancements and the intrinsic benefits associated with using high-dimensional quantum states, often referred to as qudits, compared to the conventional two-level quantum systems, or qubits, in various quantum applications.

Core Advantages of High-Dimensional Quantum States

High-dimensional quantum states offer significant advantages over traditional qubits, which are thoroughly explored in this review:

1. Information Capacity: High-dimensional states can encode more information per quantum system, enhancing communication efficiency. For instance, a quantum system described by a four-level state (ququart) can encode 2 bits of information, doubling the capacity compared to a qubit system.
2. Noise Resilience: The robustness to noise of qudits increases with their dimensionality, allowing for higher error thresholds in quantum communication protocols, such as Quantum Key Distribution (QKD). The theoretical studies and practical implementations highlight that systems using qudits can achieve higher secret key rates and resist higher noise levels than qubit-based systems.
3. Quantum Cloning Resistance: According to the no-cloning theorem, perfect cloning of an unknown quantum state is impossible. Higher dimensions further decrease the fidelity of cloned states, thereby offering enhanced security for quantum communication protocols by effectively resisting cloning-based eavesdropping attacks.
4. Violation of Local Theories: Experiments have shown that high-dimensional entangled states violate local realism more significantly than lower-dimensional systems, offering stronger non-local correlations. This feature can be particularly beneficial for applications requiring the certification of non-classical correlations, such as device-independent QKD.
5. Communication Complexity: High-dimensional quantum systems can reduce communication complexity, specifically in scenarios where quantum correlations enable more efficient protocol execution compared to classical strategies.

Implementation of High-Dimensional Quantum Communication

The paper outlines various methodologies for generating qudits, discussing both bulk optics approaches, such as utilizing orbital angular momentum and time-bin encoding, and integrated photonic platforms. Notably, advancements in silicon photonics and other integrated technologies facilitate scalable and stable generation and manipulation of high-dimensional states.

Transmission Platforms for High-Dimensional States

Different communication channels for transmitting high-dimensional states are explored, including free-space and fiber-based links, and emerging research on underwater communication channels. Free-space links benefit from the inherent spatial structure of qudits, while fiber links offer practical advantages due to existing infrastructure. Multicore and multimode fibers are identified as promising candidates for fiber-based high-dimensional state transmission, albeit with challenges such as maintaining phase stability and minimizing intermodal dispersion.

Future Directions and Challenges

The paper concludes by identifying key areas for future research, such as improving protocols for entanglement distribution and storage, which are critical for the development of a functional quantum internet. Integrating qudits with quantum memories remains a significant challenge yet is crucial for widespread deployment of high-dimensional quantum communication systems. Moreover, the continued development of methods for certifying high-dimensional entanglement will enhance the reliability of these systems.

This comprehensive review underscores the promise of high-dimensional quantum communication while highlighting the experimental and theoretical challenges that must be overcome to realize its full potential. The ongoing exploration of these high-dimensional states is expected to play a pivotal role in the advancement of quantum technologies.