Start by defining what “quantum-secure” means for your organization. It could mean protecting a conventional connection with post-quantum cryptography (PQC), using quantum key distribution (QKD) over fiber, or combining the two. If you choose QKD, plan for more than a fiber run: you need quantum and classical channels, compatible endpoint equipment, a way to deliver keys to the systems that use them, and evidence that the complete route works as intended.
Choose the security approach before planning the fiber
QKD and PQC address quantum-related security in different ways. QKD uses quantum optical signals to establish shared keys; the keys themselves are classical strings used by encryption equipment or other cryptographic applications. QKD does not replace the network connection or encrypt traffic by itself. ETSI describes QKD as complementary to PQC within a layered cybersecurity strategy, and its quantum-safe VPN guidance recommends combining quantum-safe and classical key-establishment techniques.
| Approach | What it does | What to assess between buildings |
|---|---|---|
| Post-quantum cryptography (PQC) | Uses cryptographic techniques intended to resist attacks by quantum computers on a conventional network connection. | Migration of the relevant VPN or other cryptographic systems, compatibility with existing equipment, and applicable current standards and jurisdictional policy. |
| QKD over fiber | Uses quantum optical signals to generate shared keys for cryptographic endpoints; it requires a quantum channel and a classical channel. | Actual route loss and stability, fiber-sharing options, endpoint and key-management interoperability, authentication, security evaluation, and operations. |
| Hybrid design | Combines quantum-safe and classical key-establishment techniques; QKD may be one component of a broader design. | How the methods work together, how keys reach the encryptors, and how the service behaves during outages or migration. |
There is no basis for assuming every organization needs QKD. Compare the options against the information to protect, the length of time it must remain confidential, the threat model, regulatory requirements, operational constraints, and lifecycle cost. ETSI’s 2018 VPN report is useful background on migration complexity, but implementation decisions should also account for current cryptographic standards and jurisdictional policy.
What a building-to-building QKD link contains
A QKD link has at least two logical channels. ITU-T Recommendation X.1711, dated March 2026, describes a quantum channel that transmits quantum signals and a classical channel used for synchronization and key distillation. QKD modules sit at the endpoints. Keys generated there must reach the encryptors or other applications that will consume them through an interoperable key-management path.
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This makes the project both an optical engineering task and a systems-integration task. ITU-T Y.3800 provides an overview framework for QKD-network design, deployment, operation, and maintenance. ETSI’s QKD work covers optical characterization, implementation security, authentication, application and key-delivery interfaces, and interoperable key-management interfaces. Its listed ETSI GS QKD 020 V1.1.1, dated June 2026, specifies a REST-based interoperable KMS API; confirm that the products under consideration implement the interfaces you need.
Plan the link in six stages
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Write down the security requirement
Identify which information needs protection, how long it must remain confidential, which traffic crosses between the buildings, and what threat or regulatory requirement drives the project. Compare a PQC transition or hybrid VPN with QKD rather than assuming a quantum-generated key is required.
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Map the endpoints and the real fiber route
Inventory the route between the two sites: measured or verified length, fiber type and ownership, patch panels, connector types, intermediate sites, available strands, rights of way, and whether a physically diverse path is possible. These details are inputs to feasibility and resilience planning; the cited standards do not establish a route-specific specification for your site.
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Characterize the optical path
Plan calibrated measurements of fiber and connector loss, polarization stability, background noise, and timing and synchronization, followed by system-level validation. NIST IR 8483 identifies these as quantum-network characterization areas. Because quantum signals cannot simply be amplified like ordinary data, distance alone does not establish feasibility; optical loss and the actual system and route matter.
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Validate fiber sharing before choosing it
Assess dedicated fiber and shared-fiber designs on the actual route. NIST describes research into coexistence of quantum and classical signals on one fiber, including O-band/C-band multiplexing while avoiding severe background noise. NIST also describes new dark fiber as a high-cost approach. Neither option should be treated as a default without route-specific performance and cost assessment.
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Specify key delivery and system security
Require suppliers to explain how endpoint modules authenticate, how generated keys reach encryptors, how key rates and outages are handled, and whether the key-management and application interfaces interoperate with the security equipment you selected. Check the scope of implementation-security evaluation as well as standards conformance.
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Require operational evidence and recovery behavior
Ask for measured performance on the actual route under intended traffic and operating conditions; monitoring and alarm behavior; maintenance responsibilities; and failover behavior when the quantum link or key service is unavailable. NIST’s work on measurement planes, network stability, synchronization, and performance evaluation illustrates why observability and recovery need to be specified, not assumed.
Can QKD use existing fiber, and how far can it work?
Existing fiber may be a candidate, but its availability does not establish that it will support the planned QKD system. Fiber loss, connector loss, noise from classical signals, polarization behavior, synchronization, and the equipment design all affect feasibility. Shared fiber may reduce the need for a separate route, but coexistence has to be validated; dedicated fiber also brings infrastructure and cost considerations.
No universal distance limit, key rate, or deployment cost for an inter-building QKD link is established here. Ask vendors to characterize the actual route and provide measured results for the proposed system. Treat a distance figure without its system, fiber, loss, and test conditions as insufficient for a site decision.
What to put in a procurement or design brief
- Security objective: information and traffic in scope, confidentiality lifetime, threat model, and applicable policy.
- Route evidence: route map, ownership and access, fiber and connector inventory, measured losses, noise and stability observations, and available diverse paths.
- System interfaces: endpoint modules, authentication method, key-management interface, encryptor integration, and interoperability evidence.
- Performance and availability: measured performance on the intended route, expected behavior under operating conditions, alarms, maintenance ownership, and recovery when a channel or key service is unavailable.
- Security evidence: implementation-security evaluation scope and the division of responsibility among QKD modules, key-management systems, encryptors, and network operators.
- Whole-life assessment: installation, operations, maintenance, fiber access, and lifecycle costs compared with PQC or a hybrid VPN approach.
ETSI’s QKD standards listings were current to June 2026. Its 2018 VPN report is dated background guidance, while NIST IR 8483 was published in September 2023; confirm the current standards and policy applicable to the intended deployment.
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