Researchers Build First Fully Connected Quantum Network with Microcombs

Researchers in China have built a large-scale fully connected quantum network, meaning every user can connect to every other user simultaneously, overcoming a major scalability challenge in quantum communication. The network spans over 200 kilometers and supports 200 users while guaranteeing user-to-user security even with an untrusted network provider. This large-scale system utilizes integrated soliton microcombs to achieve precise frequency generation and locking, enabling high-visibility Hong-Ou-Mandel interferences and measurement-device-independent quantum key distribution. The implemented architecture, the researchers state, “paves the way for realizing large-scale fully connected MDI quantum networks across metropolitan and intercity regions.”

Microcomb-Driven Fully Connected Network Architecture

This architecture, detailed in recent findings, departs from traditional point-to-point quantum key distribution by enabling a system where each of the 200 users can establish a secure connection with any other user within the network without relying on a trusted intermediary. This eliminates a key vulnerability present in earlier designs and has substantial implications for secure communication. Central to this advancement is the utilization of ‘microcombs’, a relatively new technology in optical physics, to achieve this connectivity, offering a potentially more stable and scalable approach than previous methods.

Some of the authors are affiliated with the Xi’an Institute of Optics and Precision Mechanics. These microcombs, specifically soliton microcombs (SMCs), function as compact frequency sources capable of generating hundreds of parallel frequency channels. The researchers demonstrate that by locking the seed laser frequency and repetition rate, massive parallel Hong-Ou-Mandel (HOM) interferences can be achieved between independent SMC chips.

This eliminates the need for complex, wavelength-specific laser locking across the entire network, a significant hurdle in previous attempts at scaling quantum networks. The network involves collaboration across eight institutions in China, including the Hefei National Laboratory and the National University of Defense Technology, demonstrating a significant national effort to advance quantum networking capabilities. This collaborative approach underscores the complexity of building such a system, requiring expertise in quantum information, integrated photonics, and long-distance fiber optic communication.

The experiment was conducted over 200 kilometers. “A massively parallel fully connected MDI-QKD network is constructed using SMC and MDI-QKD transmitter chips,” the researchers state, outlining the core components of their system. The design prioritizes security by employing measurement-device-independent quantum key distribution (MDI-QKD). MDI-QKD closes detection loopholes, enhancing the security level in practical implementation by ensuring that even a compromised network provider cannot eavesdrop on the communication.

The system operates by distributing quantum states across hundreds of frequency channels connecting and sharing information with each other simultaneously, executing post-selected Bell state measurements (BSMs) at the network provider, and then publicly announcing the results. “According to the fundamental principles of quantum mechanics, the quantum relay in the network cannot obtain any information shared between end users during Bell state measurements,” the team explains, highlighting the inherent security of the approach.

The implementation utilizes silicon-photonic-chip-based MDI-QKD transmitters operating at 2.5 GHz, achieving an average secure key rate of 62 bits per second for each user-to-user connection over the 200-kilometer span. This represents a substantial improvement over previous fully connected quantum networks, with the user scale and secure key rate improved by one and three orders of magnitude, respectively, compared to those of previous reported fully connected quantum networks.

Each user requires only a single, frequency-locked seed laser, simplifying the infrastructure requirements at each node. The researchers emphasize that the frequency locking of SMC chips is fulfilled locally, demanding no remote operation from the network provider or other end users. The team acknowledges the challenges of scaling quantum networks, noting that a fully connected network of N users traditionally requires precise frequency locking between O(N^2) laser pairs.

Their microcomb-driven approach circumvents this issue, offering a more practical pathway towards large-scale deployment. “The proposed architecture significantly simplifies the implementation complexity and expands the user scale of a fully connected quantum network,” they assert.

Two-Photon Interference and MDI-QKD Security

The pursuit of unconditionally secure communication has driven rapid advancements in quantum key distribution, or QKD, over the past two decades. Initial demonstrations focused on point-to-point links, gradually extending distances and improving key rates. However, true network functionality demands the ability to connect multiple users, leading to the concept of fully connected quantum networks where every terminal can communicate directly with every other terminal simultaneously.

Achieving this scalability has proven remarkably difficult, but a newly constructed system in China represents a significant leap forward, leveraging the subtle phenomenon of two-photon interference to guarantee security even with an untrusted intermediary. Central to this network’s operation is measurement-device-independent quantum key distribution, or MDI-QKD. This protocol addresses a critical vulnerability in standard QKD systems: potential eavesdropping via compromised detectors at the receiving end. By employing two-photon Hong-Ou-Mandel (HOM) interference and post-selected Bell state measurement, MDI-QKD effectively eliminates this threat.

Integrated Soliton Microcombs Enable Parallel Frequency Locking

The demand for absolutely secure communication has spurred rapid development in quantum key distribution (QKD) technologies, but scaling these systems beyond a handful of users has remained a significant hurdle. This achievement, published this year, bypasses limitations of previous designs through the innovative use of integrated soliton microcombs (SMCs) for precise frequency management. Traditional multi-user QKD networks face a scaling problem rooted in the need for precise frequency locking between a rapidly increasing number of laser pairs.

An N-user network conventionally requires locking O(N^2) lasers, a logistical and technical challenge that has previously limited demonstrations to only three nodes. The team’s approach circumvents this issue by employing silicon-photonic-chip-based transmitters operating at 2.5 GHz, coupled with the unique capabilities of SMCs. The core innovation lies in the localized control afforded by the SMCs.

Each user requires only a single, precisely locked seed laser; the SMC then generates the necessary multitude of frequencies for communication. “We utilize SMC and transmitter chips to implement the massively parallel MDI-QKD, where the seed laser frequency and repeating-rate are locked to align all the soliton comb lines,” the researchers explain in their published work. This is a deliberate design choice, intended to demonstrate that secure communication can be achieved even if the intermediary network is compromised.

User Network Performance Over 200 Kilometers

The expectation that scaling quantum networks demands ever more complex infrastructure is being challenged by a newly demonstrated system in China. While conventional wisdom suggests a tangled web of precisely tuned lasers for each connection, researchers have successfully built a fully connected quantum network over 200 kilometers and supporting 200 users with a surprisingly streamlined approach. This isn’t simply an incremental improvement; it represents a fundamental shift in how large-scale quantum communication might be achieved, prioritizing localized control over centralized coordination.

This localized control is a key differentiator, simplifying the implementation considerably. The network operates on the principle of measurement-device-independent quantum key distribution (MDI-QKD), leveraging two-photon Hong-Ou-Mandel (HOM) interference to guarantee security even if the network provider is untrusted. This deliberate design choice addresses a critical vulnerability in many quantum communication systems, ensuring that eavesdropping attempts are detectable regardless of the intermediary’s integrity.

The system doesn’t rely on the trustworthiness of the infrastructure itself, but on the laws of quantum mechanics to safeguard information. By locking the seed laser and repetition rate, the team was able to achieve high-visibility HOM interferences, a crucial requirement for efficient MDI-QKD.

This precise control over the quantum states allows for reliable key distribution even over significant distances and with a large number of users. The authors state that the user scale and secure key rate are improved by one and three orders of magnitude compared to those of previous reported fully connected quantum networks.