Where it shows up
Datacenter metro links
Dark-fiber spans between facilities; distances 5–80 km are common.
Utility control networks
Backbone links for grid/SCADA with strict monitoring and known routes.
Campus backbones
Research/health campuses with a few well-characterized fiber runs.
Link budget explorer (semi-log)
Total loss: — dB
ηch (linear): —
QBER: —%
Sifted: — bit/s
Secret: — bit/s
Toy model for DV-QKD intuition (decoy-BB84 spirit). Channel loss: ηch=10^{-(αL + L_patch)/10}.
Click probability per gate: p_click ≈ µ·ηdet·ηch + p_dark + p_coex.
Approx. QBER ≈ (p_dark + p_coex)/(2·p_click). Sifted rate: 0.5·f·p_click. Secret fraction (very rough): max(0, 1 − 2·h₂(QBER)), secret rate = sifted × fraction.
This is a sanity-check curve, not a certification proof.
What limits metro deployments (the real list)
- Fiber attenuation ~0.2 dB/km at 1550 nm ⇒ key rate falls roughly exponentially with distance.
- Patch/connector/splice loss each panel or splice adds ~0.2–0.5 dB; several hops can cost multiple dB.
- Coexistence noise if sharing fiber with classical WDM, Raman scattering and leakage add background counts; many deployments require dark fiber or heavy filtering/time-division.
- Detector limits efficiency η and dark counts (10⁻⁶–10⁻⁵ per gate), plus afterpulsing and dead time, all lift QBER and cap the clock rate.
- Timing & jitter gate width vs jitter limits how narrow you can gate without losing signal.
- Polarization/phase drift reduces interference visibility and raises QBER; requires active compensation on metro fibers.
- Finite-key effects short accumulation windows reduce the privacy-amplified secret fraction compared to asymptotic formulas.
- Sifting & EC leakage half of detections are discarded (basis mismatch), and error-correction leaks information (overhead factor).
- Trusted nodes beyond ~100–200 km without repeaters, you relay keys via trusted sites; true quantum repeaters aren’t production-ready yet.
- Ops reality fiber route control, patch inventory, environmental drift, monitoring hooks, and HSM/KMS integration dominate project success.
How to read a link budget (step-by-step)
- Sum all optical losses:
Loss = α·L + L_patch(dB). - Convert to transmittance:
ηch=10^{−Loss/10}. - Find per-gate signal click:
p_sig ≈ µ·ηdet·ηch. - Add noise:
p_click = p_sig + p_dark + p_coex. - Estimate QBER (random noise assumption):
QBER ≈ (p_dark + p_coex)/(2·p_click). - Sifted rate:
R_sift ≈ 0.5·f·p_click; Secret fraction:r ≈ max(0, 1−2·h₂(QBER)); Secret rate:R = R_sift·r.
RFI questions
- Guaranteed total optical loss & jitter budget at the target distance; coexistence strategy (dark fiber vs WDM filters)
- Measured QBER vs distance and 24-hour stability (with patch panels in the loop)
- HSM/KMS integration (key injection into TLS/IPsec; audit & alarms); monitoring hooks for drift/faults