Quantum repeaters: From quantum networks to the quantum internet (2212.10820v2)

Abstract: A quantum internet is the holy grail of quantum information processing, enabling the deployment of a broad range of quantum technologies and protocols on a global scale. However, numerous challenges exist before the quantum internet can become a reality. Perhaps the most crucial of these is the realization of a quantum repeater, an essential component in the long-distance transmission of quantum information. As the analog of a classical repeater, extender, or booster, the quantum repeater works to overcome loss and noise in the quantum channels comprising a quantum network. Here, we review the conceptual frameworks and architectures for quantum repeaters, as well as the experimental progress towards their realization. We also discuss the various near-term proposals to overcome the limits to the communication rates set by point-to-point quantum communication. Finally, we overview how quantum repeaters fit within the broader challenge of designing and implementing a quantum internet.

Quantum Repeaters: From Quantum Networks to the Quantum Internet

The concept of the quantum internet represents a significant ambition in the field of quantum information processing, promising the deployment of quantum technologies and protocols on a global scale. At the heart of this vision lies the quantum repeater, an indispensable component for the long-distance transmission of quantum information. Quantum repeaters are essential to overcoming the loss and noise inherent in quantum channels, effectively serving as the quantum analogs of classical repeaters or amplifiers in optical networks. This paper provides a comprehensive overview of the theoretical frameworks and architectural designs proposed for quantum repeaters, while also detailing the experimental progress made towards their realization.

The classical internet, despite its vast reach and transformative impact on society, faces sustainability challenges, particularly in scaling and energy consumption. With an ever-increasing number of connected devices and the energy demands of optical communication contributing to climate change, innovative solutions like distributed intelligence and distributed trust are necessary. In this context, the quantum internet, which promises provably secure communication and enhanced computational capabilities, is positioned as a complementary extension of the classical internet.

A quantum internet would involve the global transmission of quantum information, distinct in its purpose and potential impacts from the classical internet. The realization of such a network hinges critically on addressing the issue of photon loss in optical fibers. Unlike classical information, quantum information (qubits) cannot be amplified directly due to the no-cloning theorem, necessitating the development of quantum repeaters.

Quantum repeaters typically employ modular approaches by dividing communication channels into shorter, manageable segments interconnected by intermediate nodes. In these nodes, errors are handled through reliable quantum operations and error suppression techniques, including entanglement purification and quantum error correction. The paper categorizes quantum repeaters into three generations based on their methods for error suppression:

1. First-generation repeaters employ probabilistic error suppression, using protocols like entanglement purification to handle operation errors.
2. Second-generation repeaters incorporate deterministic error correction mechanisms, significantly improving communication rates by removing the need for two-way classical signaling delays.
3. Third-generation repeaters offer seamless, deterministic error suppression, allowing for loss corrections and invariably high communication rates across longer distances.

The paper also explores all-photonic repeater architectures, which leverage photonic cluster states to facilitate quantum communication without relying on lengthy-lived quantum memories. Such schemes focus on employing graph states, where the quantum information processing is achieved through measurement-based quantum computations.

Practical implementation challenges for quantum repeaters include the development of resilient quantum memories, efficient light-matter interactions for generating photon-memory entanglements, and the mitigation of losses in optical fibers. Significant advancements have been made in various quantum memory platforms, such as trapped ions and defect centers in diamonds, pointing toward feasible routes for first-generation repeater networks.

Milestones in quantum communication, such as advancements in measurement-device-independent quantum key distribution (MDI-QKD) and twin-field (TF) QKD protocols, illustrate progress toward beating the fundamental rate limits for point-to-point quantum transmission. These achievements are critical stepping stones towards comprehensive quantum repeater networks and ultimately, a fully operational quantum internet.

The paper concludes with a discussion on integrating quantum repeaters within the broader quantum internet architecture, emphasizing the importance of designing scalable and efficient quantum networks. From advancing cryptographic protocols to facilitating distributed quantum computing and sensing, the potential applications of a quantum internet highlight its conceivable impact on technology and society. The realization of a quantum internet remains a complex yet fruitful endeavor, promising significant improvements in secure global communication and quantum computational capabilities.