Quantum Local Area Networks (QLANs)

• Quantum Local Area Networks are localized infrastructures that connect quantum processors, memory modules, and sensors using protocols like entanglement distribution, QKD, and teleportation.
• They employ flexible topologies—such as ring and star—and advanced photonic integrated circuits to achieve high entanglement fidelity (CHSH = 2.700) under robust environmental controls.
• QLANs integrate quantum and classical signals with precise sub-nanosecond synchronization and dynamic environmental adaptations, paving the way for scalable quantum internets.

Quantum Local Area Networks (QLANs) are localized quantum communication infrastructures designed to connect quantum processors, memory modules, and sensors within laboratories, campuses, or metropolitan areas. Distinguished by their integration of quantum hardware with layered network control, QLANs support key quantum networking protocols—such as entanglement distribution, quantum key distribution (QKD), and quantum teleportation—while often operating alongside or within conventional classical networks. QLANs enable the scaling of quantum information processing systems and act as foundational elements for future quantum internetworks.

1. Network Topologies and Physical Implementation

QLANs leverage a range of physical topologies tailored to specific deployment environments, supported by advanced link characterization and network reconfigurability. Three prototypical QLANs are:

• Griffiss QLAN: Implements a ring topology with four nodes interconnected via coarse wavelength-division multiplexers (CWDM OADMs), enabling closed-loop entanglement distribution. The network utilizes duplex single-mode fibers with both indoor spooled loops and up to 15 km of buried fiber, and includes reconfigurable optical switching for rapid topology adaptation. Free-space links supplement fiber channels for laboratory-scale validation.
• Stockbridge QLAN: Adopts a hub-and-spoke star topology, with a central “Quantum Pad” distributing photons to remote “Pad” nodes via reconfigurable optical switches and buried or aerial fibers in a forested environment. This architecture enables dynamic mesh connectivity, integration of free-space optical (FSO) links, and geographic resilience.
• Rome Research Site (RRS) QLAN: Connects three adjacent laboratories via fiber conduits and towers, combining indoor and aerial links. Physical topology is mutable between ring and star motifs, giving experimental flexibility.

Link integrity is maintained by continuous optical time-domain reflectometry (OTDR) measurements (with observed losses of ~0.2 dB/km for well-installed fibers, and up to 6–8 dB in long buried loops), automated polarization drift stabilization (maintaining SOP fidelity near 99% over multiple days), and sub-nanosecond synchronization using White Rabbit (WR) timing (enabling coincidence timing fluctuations with near-zero drift per kilometer). Environmental parameters such as temperature, wind, and air turbulence are closely monitored, allowing for robust adaptation of network operation across different deployment scenarios (Sheridan et al., 1 Aug 2025).

2. Entanglement Distribution and Source Technology

Entanglement distribution within QLANs is achieved using photonic integrated circuit (PIC) based sources and advanced optical filtering.

• Photonic Integrated Circuit Spiral Source: A CMOS-compatible silicon PIC in a spiral configuration is pumped with a 1550 nm CW laser to generate time-energy entangled photon pairs via four-wave mixing (FWM). Output channels (typically 1530 nm and 1570 nm) are isolated by CWDM filters, and the joint spectral intensity is engineered for high entanglement fidelity.
• Characterization via Bell Inequality Violation: Time-energy Bell states are generated and analyzed using programmable Franson interferometers (e.g., Finisar WaveShapers for phase stabilization and dispersion control). A measured Clauser-Horne-Shimony-Holt (CHSH) value of $S=2.700$ confirms entanglement quality close to the Tsirelson bound of $S=2.828$, even over deployed fiber links requiring active stabilization. Programmatically applied quadratic spectral phase via optical filtering compensates for deployed-fiber chromatic dispersion and maintains narrow coincidence peaks (Sheridan et al., 1 Aug 2025).
• Stabilization Measures: Automated polarization compensators (e.g., QU-APC modules) and real-time environmental feedback are essential for sustained entanglement distribution, especially in aerial or long buried fibers affected by weather and mechanical stresses. Time-of-flight drift remains under 700 ps over four-day measurements across 15 km of buried fiber, ensuring fidelity for time-bin qubit protocols.

3. Classical–Quantum Coexistence and Synchronization

QLANs are engineered for coexistence of low-power quantum signals with high-power classical channels:

• Co-propagation Strategies: Quantum and classical signals are multiplexed (typically across O-band and C-band) to separate high-power data from fragile quantum channels, minimizing spontaneous Raman scattering and four-wave mixing noise.
• White Rabbit (WR) Synchronization: WR links distribute precise timing signals over shared fiber, enabling distributed time tagger synchronization with sub-nanosecond precision. Coincidence timing accuracy is verified by drift histograms showing near-zero-center deviations, even for kilometer-spanning links.
• Network Flexibility: Topologies are reconfigurable according to experimental requirements and environmental conditions (e.g., toggling between ring and star, or enabling/disabling FSO links). Control-plane traffic (for network management, state monitoring, or WR signals) often traverses dedicated LAN fibers, preserving quantum channel performance.

4. Environmental Adaptation and Field Deployment

• Laboratory vs. Rugged Field Sites: While ring networks in temperature-controlled laboratories benefit from environmental insulation, field deployments (such as Stockbridge) must account for rapid SOP drift due to wind, sunlight, and air turbulence. FSO links in wooded, temperate forests demonstrate that polarization stabilization remains effective despite turbulent conditions.
• Continuous Monitoring: Weather stations, disdrometers, and scintillometers provide real-time feedback for fine-tuning stabilization parameters, aligning with trends in time-of-flight or polarization drift. This monitoring is essential for deploying QLANs in harsh or changing outdoor environments with buried, aerial, and FSO links.

5. Toward Heterogeneous and Scalable Quantum Networks

• Matter-based System Integration: Future work aims to integrate disparate quantum hardware—e.g., superconducting qubits, trapped ions, and optically interfaced quantum memories—as network nodes within a QLAN. Telecom-band quantum links, in conjunction with quantum frequency conversion and transduction, provide a path to qubit-agnostic networking.
• Quantum Metropolitan Area Network Expansion: Plans include interconnecting Griffiss, Stockbridge, and RRS QLANs (as well as additional emerging sites) into a multi-environment quantum metropolitan area network (QMAN). This requires scalable control architectures, advanced stabilization (esp. for polarization and phase), and dynamic topology reconfiguration.
• Ongoing Research Directions: Continued development focuses on quantum-classical coexistence (e.g., refined multiplexing of timing channels), optimized compensation for environmental instabilities, and enhancing automated network management. A plausible implication is that cohabitation of quantum and classical signals alongside robust network reconfigurability will be critical for the deployment of real-world QLANs supporting distributed quantum computing, secure quantum-enhanced sensing, and scalable QKD (Sheridan et al., 1 Aug 2025).

6. Significance and Outlook

The multi-node, reconfigurable QLANs described—featuring fielded fiber and FSO links, PIC-based entangled photon sources, and advanced synchronization and stabilization—demonstrate the practical feasibility of deploying high-fidelity quantum networks in real environments. The observed CHSH violation ($S=2.700$) approaches theoretical maxima, confirming the integrity of distributed entanglement even under environmental stress.

The anticipated integration of heterogeneous matter-based quantum systems marks a major step toward building broadly compatible, scalable quantum internet infrastructure. This heterogeneous, field-rugged, and reconfigurable QLAN paradigm supports both current experimentation and lays the groundwork for connecting quantum systems across metropolitan areas and, ultimately, globally.