EPB Bohr IV Quantum Network

• EPB Bohr IV Quantum Network is a metropolitan-scale quantum platform built on commercial fiber that enables polarization-controlled multiphoton entanglement generation.
• It employs all-fiber linear optics and classical synchronization protocols to achieve low-loss, on-demand quantum state distribution with both Bell and N00N state capabilities.
• Integration with telecom infrastructure paves the way for secure quantum cryptography, distributed quantum computation, and scalable multi-node quantum networking.

The EPB Bohr IV Quantum Network is a metropolitan-scale quantum networking platform implemented in Chattanooga, TN, notable for its deployment of advanced fiber-based quantum communication protocols, active distribution of multi-photon entangled states, and compatibility with commercial telecom infrastructure. Leveraging polarization-controlled entanglement generation, all-fiber linear optics, and integration-ready synchronization and control architectures, the network serves as a critical testbed for multi-node quantum information distribution and future quantum internet deployments.

1. Architectural Design and Network Platform

The EPB Bohr IV Quantum Network is architected atop commercial metro-scale fiber infrastructure, linking multiple quantum nodes via standard telecom optical fibers (Reaz et al., 3 Sep 2025). The network uses a fully fiber-coupled linear optic experimental platform, emphasizing low-loss connectivity at standard telecom wavelengths. Fiber-coupled switches, beam splitters, polarization controllers, and polarizers are deployed throughout the network to route and manipulate quantum signals with minimal insertion loss, enabling direct interfacing with classical data channels and networking equipment (Reaz et al., 3 Sep 2025).

Synchronization across nodes is provisionally handled through classical timing protocols. A plausible implication is that future upgrades may adopt White Rabbit timing synchronization, yielding sub-nanosecond precision and supporting true independent multi-node entanglement protocols (Alshowkan et al., 2021). Overall, the platform is designed for on-demand distribution of quantum states, robust to channel losses, and upgradable as quantum networking technology advances.

2. Polarization-Controlled Multiphoton Entanglement Generation

At the heart of the EPB Bohr IV implementation is a four-photon entangled state produced centrally using a spatially degenerate, continuous-wave type-II spontaneous parametric down-conversion (SPDC) biphoton source (Reaz et al., 3 Sep 2025). The SPDC source emits pairs of photons in orthogonal polarization states within the same spatial mode, facilitating efficient and stable fiber coupling.

Two independent SPDC Bell pairs are generated and fused via interference at fiber-based beam splitters. Through projective polarization measurements on two locally retained photons, the system probabilistically heralds the remaining two photons into either a Bell state or a N00N state. The heralding operation is achieved entirely in-fiber using polarization controllers and polarizing beam splitters.

Key controlled states are:

• Bell Singlet State:

$$|\Psi^-\rangle = (|HV\rangle - |VH\rangle)/\sqrt{2}$$

This state manifests particle-particle entanglement.

• Two-Photon N00N State:

$$|N00N\rangle = (|2,0\rangle - |0,2\rangle)/\sqrt{2}$$

This state is a superposition of two photons in one spatial mode and zero photons in the other (mode-mode entanglement).

Projective measurements act as a switch: heralding photons with orthogonal polarization projects the remote pair into the Bell state; identical polarization projects into the N00N state.

3. Entanglement Distribution and Verification

Following heralding, the remaining entangled photons are distributed over metro-scale fiber paths to two spatially separated network nodes (Reaz et al., 3 Sep 2025). Single-photon detectors and time-tagging electronics—often utilizing superconducting nanowire technology—record four-photon coincidence events, sweeping optical delays and adjusting polarization settings to observe high-visibility interference patterns (e.g., Hong-Ou-Mandel dips) and verify entanglement quality.

Bell inequality tests and fidelity measurements confirm that the remote nodes share quantum correlations exceeding classical thresholds. While experimental channel losses (fiber attenuation, switching elements) and limited source fidelity reduce raw entanglement fidelity, post-selection and extended integration compensate, demonstrating robust entanglement distribution in real-world conditions.

4. Integration with Telecom Infrastructure and Network Scalability

The experimental design demonstrates compatibility with commercial telecom infrastructure, integrating seamlessly with conventional fiber routing, switching, and multiplexing hardware (Reaz et al., 3 Sep 2025). Photon routing and active polarization control mitigate environmental disturbances and polarization drift, critical for metro-scale operation. This suggests that on-demand heralded entanglement can be provided as a service across existing fiber networks, leveraging telecom-grade optical components and future upgrades (such as nondegenerate photon sources for optimized wavelength selection and increased fusion probabilities).

Scalability is inherent in the fiber platform architecture. Fiber-coupled switches and polarization controllers allow flexible node additions and reconfiguration. Prospective upgrades—such as White Rabbit timing, active feedback polarization stabilization, and improved SPDC sources—would directly enhance network throughput, entanglement fidelity, and multi-node distribution capacity (Alshowkan et al., 2021).

5. Quantum Information Applications

Dual-state entanglement distribution on EPB Bohr IV supports particle–particle entanglement (Bell states) for quantum cryptography and distributed quantum computation, as well as mode–mode entanglement (N00N states) for quantum metrology and phase-sensing. The ability to select between Bell and N00N states via polarization projective measurement offers significant flexibility for diverse quantum networking applications and protocols.

Experimental key rates, entanglement fidelities, and performance under loss conditions are not explicitly quantified in (Reaz et al., 3 Sep 2025); nonetheless, successful metro-scale entanglement is confirmed via fourfold coincidences and interference signatures. A plausible implication is that future deployments may optimize protocols for increased key rates and multi-user entanglement swapping.

6. Limitations, Prospects, and Future Directions

Current limitations include moderate SPDC source fidelity, significant channel loss over metropolitan fiber, and the necessity of centralized heralding measurements (rather than truly distributed multi-node entanglement). Planned improvements are:

• Implementation of nondegenerate photon sources for wavelength-optimized heralding and transmission.
• Deployment of White Rabbit timing synchronization for sub-nanosecond precision and autonomous multi-node operation (Alshowkan et al., 2021).
• Active polarization control and feedback stabilization for long-term fidelity and operational stability.
• Integration with advanced quantum internet architectures (stack-based design, SDN optical switching) as demonstrated in (Du et al., 2021, Alia et al., 2021), and (Berrevoets et al., 2021).

7. Theoretical Context and Quantum Network Implications

The EPB Bohr IV Quantum Network sits at the intersection of operational quantum networking and foundational quantum mechanics. Its use of deterministic control over entanglement generation and distribution is directly linked to the “entanglement molecule” and “entanglon” concepts introduced in formal analyses of EPR-Bohr scenarios (Floyd, 2010). Quantum information transfer exploits nonlocal correlations and action-at-a-distance mediated by polarization-controlled multi-photon states, echoing the theoretical structures required for robust instantaneous quantum networking.

The network provides evidence that high-dimensional, multi-photon entanglement distribution is feasible across commercial fiber networks, laying groundwork for global quantum information systems leveraging classical and quantum co-propagation (Berrevoets et al., 2021). The dual-state heralded distribution protocol demonstrates versatility that will likely underpin future secure communication, distributed quantum sensing, and scalable quantum computing over real-world infrastructure.

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In summary, the EPB Bohr IV Quantum Network establishes a practical framework for multi-photon, polarization-controlled entanglement distribution over commercial metropolitan fiber, combining foundational quantum information science with telecommunications engineering to propel quantum networks from laboratory concepts to operational platforms.