Quantum Key Distribution Loss Calculator

1) Total loss budget (dB)

• Fiber loss = Length(km) × Attenuation(dB/km)
• Connector loss = Nconnectors × Loss/connector
• Splice loss = Nsplices × Loss/splice
• Total loss = sum of fiber + connectors + splices + splitter + other + margin

2) Convert loss to transmittance

The optical channel transmittance is computed as:

η_channel = 10^(−Loss_dB / 10)

3) Optional rate estimate (planning)

• η_total = η_channel × η_detector × η_system
• p_click ≈ 1 − exp(−μ·η_total) + p_dark
• R_sift ≈ PulseRate · p_click · SiftingFactor
• R_secret ≈ R_sift · max(0, 1 − (1+f)·H2(QBER))

1. Enter link length and attenuation based on your fiber type and wavelength.
2. Add connector and splice counts from your as-built path or drawings.
3. Select a splitter ratio if you plan passive distribution.
4. Include “Other fixed losses” for bends, filters, and patch panels.
5. Set a safety margin to cover aging, repairs, and temperature variation.
6. Press Calculate to see total loss and transmittance.
7. Optionally refine rate settings to estimate key throughput.
8. Use Download CSV or Download PDF for reporting.

Quantum key distribution (QKD) links behave like very sensitive optical measurement paths. In construction projects—campus networks, inter-building trunks, security upgrades, or data-center corridors— the same physical practices that protect fiber performance also protect your key budget. This calculator helps you translate drawings and as-built counts into a repeatable loss estimate, so decisions about routing, terminations, and passive distribution can be made early.

Start with the route length and wavelength, then apply the fiber attenuation typical for your cable and environment. Next, model the “touch points” created by patch panels, equipment ports, and cross-connects. Connectors are convenient for installation and maintenance, but each mating pair adds insertion loss and variability. Splices are usually lower loss than connectors, yet excessive splice count can accumulate quickly on long runs. If your design uses passive splitters for shared distribution, include their insertion loss (and any custom measured value). Finally, add a safety margin to cover aging, temperature effects, cleaning cycles, and future re-terminations.

Example: a campus backbone between Building A and Building B is 5 km at 1550 nm. Using 0.20 dB/km fiber attenuation, four connectors at 0.35 dB each, six splices at 0.10 dB each, a 1:4 splitter, 0.80 dB fixed losses, and a 3.00 dB margin yields roughly 13 dB total loss. That value supports a quick check against vendor budgets and helps teams justify cleaner routing, fewer patch points, or a higher grade of components if the loss is tight.

For construction execution, treat loss as a deliverable: track connector counts in drawings, label patch panels to avoid unnecessary jumpers, enforce bend-radius limits, protect ducts from crush and micro-bends, and schedule acceptance testing. If you collect OTDR traces and insertion-loss measurements during commissioning, you can replace assumptions with measured values and keep a living record for maintenance teams. Use the CSV and PDF exports to capture each revision, attach it to method statements, and reduce rework.

When planning materials, prefer standardized, low-loss connector families and verify polish type and cleanliness controls. Test critical links bidirectionally to catch poor splices and hidden bends. If the site includes vibration, heat, or frequent patching, increase the margin and consider protected pathways, fewer interconnect points, and stricter handover criteria.

• Design stage: compare alternative routes and termination strategies.
• Installation stage: validate counts and keep connector quality consistent.
• Commissioning stage: update inputs with measured insertion loss results.

1) What does “total loss” represent in this tool?

Total loss is the sum of fiber attenuation, connector losses, splice losses, splitter insertion loss, other fixed losses, and a safety margin. It estimates how much optical power is reduced across the link.

2) Why should I include a safety margin?

A margin covers real-world variability: connector cleanliness, temperature changes, future re-terminations, aging, and route adjustments. Using a margin reduces the risk that a design passes on paper but fails after installation.

3) When should I override the splitter loss value?

Override the splitter loss when you have a datasheet value, a lab measurement, or a field-tested component with known insertion loss. This is common when splitters include connectors or additional packaging losses.

4) Are the key-rate numbers guaranteed?

No. The key-rate outputs are planning estimates based on simplified assumptions. Actual throughput depends on protocol settings, detector behavior, synchronization, background noise, and vendor-specific post-processing performance.

5) What loss range is typically workable for site links?

Many practical links target moderate loss budgets, often well below 30 dB, but limits depend on equipment and protocol. Use vendor specifications and measured insertion-loss tests to confirm feasibility for your project.

6) How do I reduce loss in a construction environment?

Reduce connector count, keep connectors clean, prefer fusion splices where appropriate, avoid tight bends, choose lower-loss components, and minimize unnecessary patching. Good labeling and route control prevent costly rework and added losses.

7) What should I document for handover?

Record route length, component counts, measurement results, and the final loss budget used for acceptance. Attach CSV/PDF outputs, OTDR traces, and insertion-loss test reports so operations teams can troubleshoot quickly later.