Signal Propagation using relay nodes versus purely optical considerations
Okay, let's compare signal propagation strategies using relay nodes versus purely optical considerations for QKD implementation within the existing NBN infrastructure. We'll focus on the trade-offs between these two approaches, given the diverse technologies present in the NBN.
**1. Relay Nodes (Trusted Nodes):**
* **Concept:** Relay nodes (also known as trusted nodes) are intermediate stations placed between the sender (Alice) and receiver (Bob) in a QKD system. Each node acts as a repeater, performing the following:
1. Receiving the incoming quantum signal.
2. Measuring the qubits.
3. Creating a new key with the previous node.
4. Encrypting the key and sending it to the next node using conventional cryptography over the same or different network links.
5. Generating new qubits using a fresh, locally-generated key and transmitting them to the next node.
* **Advantages:**
* **Distance Extension:** Can extend the reach of QKD systems beyond the distance limitations imposed by fiber attenuation and other impairments.
* **Technology Bridging:** Can bridge different NBN technology segments (e.g., connecting an FTTP segment to an FTTN segment) where direct QKD would be impossible.
* **Easier Implementation (Currently):** Relay nodes are a more mature and readily deployable technology compared to quantum repeaters.
* **Disadvantages:**
* **Security Vulnerability:** The major drawback is that the security of the entire QKD link depends on the security of each trusted node. If a trusted node is compromised, the entire key is compromised.
* **Increased Complexity:** Managing and securing multiple trusted nodes adds complexity to the QKD system.
* **Key Distribution Bottleneck:** The classical key distribution between relay nodes can become a bottleneck, limiting the overall key rate of the system.
* **Trust Assumption:** Relies on the assumption that the nodes are physically secure and not susceptible to compromise by malicious actors.
**2. Purely Optical Considerations (Leveraging Existing NBN):**
This approach aims to maximize the distance and performance of QKD by optimizing the optical parameters and components within the existing NBN infrastructure (specifically on FTTP sections or dedicated fiber spans), without relying on trusted nodes:
* **Strategies:**
* **Wavelength Optimization:** Use the 1550 nm window (or other low-loss wavelengths) for QKD signal transmission.
* **Low-Loss Fiber:** Utilize ultra-low-loss fibers where possible (e.g., dedicated fiber runs).
* **Advanced Detectors:** Employ high-efficiency, low-noise single-photon detectors (e.g., SNSPDs) to maximize the detection rate and reduce the error rate.
* **Polarization Management:** Implement polarization-maintaining fibers (PMF) or active polarization compensation to mitigate polarization drift and maintain qubit fidelity.
* **Dispersion Compensation:** Compensate for chromatic dispersion in the fiber to minimize pulse broadening.
* **Low Optical Power:** Operate at low optical power levels to minimize nonlinear effects in the fiber.
* **Filtering:** Employ optical filters to reduce noise and background light.
* **Advanced QKD Protocols:** Implement QKD protocols that are robust to noise and loss (e.g., differential phase shift keying - DPSK).
* **Advantages:**
* **Higher Security (Potentially):** By avoiding trusted nodes, the security of the QKD system is not compromised by the vulnerability of intermediate stations. The end-to-end link relies on the fundamental principles of quantum mechanics.
* **Lower Complexity:** Simpler system architecture compared to systems with multiple trusted nodes.
* **Scalability (Potentially):** If long-distance transmission can be achieved, it may be easier to scale the system to larger networks.
* **Disadvantages:**
* **Distance Limitations:** The achievable distance is limited by fiber attenuation, detector performance, and other optical impairments. These limitations can be severe, especially within the existing NBN where dedicated fiber runs optimized for QKD may not be available.
* **Technology Constraints:** The existing NBN infrastructure was not designed specifically for QKD. The available fiber types, components, and operating parameters may not be ideal for quantum communication.
* **Cost:** High-performance detectors and polarization management equipment can be expensive.
* **Limited Deployment (Currently):** May be limited to locations with dedicated FTTP links or to very short distances.
* **Fiber Access:** May require access to modify or add components to existing fiber infrastructure which can be complex in a pre-existing national network.
**3. Comparison Table:**
| Feature | Relay Nodes (Trusted Nodes) | Purely Optical Considerations |
| -------------------- | --------------------------- | ---------------------------- |
| Distance Extension | Yes | Limited |
| Security | Lower | Higher (Potentially) |
| Complexity | Higher | Lower |
| Cost | Moderate | High |
| Technology Bridging | Yes | No |
| Implementation | More mature (currently) | More challenging (currently) |
| Fibre Requirements | Lower | High |
**4. NBN-Specific Considerations:**
* **FTTP Dominance:** The best scenario for purely optical approaches is in areas where FTTP connections are dominant and offer high-quality fiber links.
* **FTTN/HFC Limitations:** Relay nodes could, in theory, be used to connect different FTTP areas even if copper sections are in between. However, this comes at the cost of the security of the relay nodes.
* **Existing Infrastructure:** The existing NBN infrastructure may not be ideal for either approach. Retrofitting existing fiber links with high-performance components can be expensive.
* **Network Management:** Integrating QKD systems into the existing NBN network management infrastructure presents challenges.
**5. Hybrid Approach:**
A possible strategy is a hybrid approach that combines the benefits of both methods. For example:
* **Optical "Islands":** Create secure QKD links between critical sites using dedicated FTTP connections and optimized optical parameters. These are "optical islands" of secure quantum communication.
* **Relay Nodes for Limited Connectivity:** Use trusted nodes sparingly to connect these "optical islands" or to extend the reach of QKD to areas with limited FTTP coverage, but be mindful of the associated security risks.
* **Quantum Repeaters (Future):** Quantum repeaters, when available, would replace the need for trusted nodes, enabling long-distance, secure QKD without compromising security.
**6. Conclusion:**
* **Purely Optical:** A "purely optical" approach to signal propagation for QKD, emphasizing advanced optical components and techniques on existing NBN fiber, is preferable from a security standpoint, but faces significant distance and technology limitations within the context of the NBN's mixed-technology architecture. It is currently most feasible only on dedicated FTTP sections.
* **Relay Nodes:** Relay nodes (trusted nodes) offer a more readily deployable solution for extending the reach of QKD and bridging different NBN technologies but introduce a critical security vulnerability that must be carefully considered.
* **Hybrid:** The most practical near-term solution might be a hybrid approach that combines optimized optical links on FTTP with carefully placed (and highly secured) trusted nodes for limited connectivity or for connecting remote FTTP areas that are separated by other tech.
* **Future:** The future of long-distance, secure QKD on the NBN will likely depend on the development and deployment of practical quantum repeaters, which would eliminate the need for trusted nodes and enable end-to-end quantum security. Until then, careful planning and a clear understanding of the trade-offs between security and feasibility are essential.
