Curriculum / Quantum Networking / Metropolitan QKD Networks
Metropolitan QKD Networks
Design a metropolitan QKD network and compute its key distribution capacity.
Metropolitan QKD Networks
Metropolitan quantum key distribution networks connect multiple users in a city-scale area, 10 to 100 km, using star, ring, or mesh topologies. Unlike point-to-point QKD (Alice and Bob connected by one fiber), metro QKD networks serve many users simultaneously, requiring network switching, wavelength multiplexing, and key management infrastructure.
Metro QKD Architecture
Trusted node star topology: All users connect to a central trusted node. The central node generates BB84 keys with each user separately, then relays keys between user pairs (decrypting with Alice's key, re-encrypting with Bob's key). Security: relies on the central node being physically and operationally secure. Cost: O(N) fiber connections for N users.
Ring topology: Nodes are connected in a ring. Keys are generated on each link separately; routing sends key material around the ring. More resilient than star (no single point of failure). Used in Beijing and Tokyo metropolitan QKD networks.
Wavelength-division multiplexed (WDM) star: Multiple QKD pairs operate on different wavelengths in the same fiber, directed by a passive wavelength router (like an arrayed waveguide grating, AWG). No trusted node needed for switching: the AWG is purely passive. Supports O(N²) simultaneous QKD sessions over O(N) fibers.
Key Rate vs Distance in Metro Networks
The BB84 secret key rate over fiber depends on:
- •Fiber loss: 0.2 dB/km at 1550 nm → channel transmissivity
- •Detector dark count rate: False positive counts that appear as signal photons
- •QBER from dark counts: QBER ≈ dark count rate / (signal rate × η)
- •Information-theoretic efficiency: where h is binary entropy and e is the QBER: one factor of h(e) is spent on error correction and one on privacy amplification
A typical metropolitan QKD system (ID Quantique Clavis3, Toshiba QKD system) achieves:
- •10 km: ~10-100 kbps secure key rate
- •50 km: ~1-10 kbps
- •100 km: ~0.1-1 kbps
This is the opening of the lesson. The full walkthrough, the interactive circuit, and the graded challenge continue inside myqubit.
How this lesson works
A hands-on coding challenge. You write Qiskit-compatible Python in the browser editor, run it instantly via WebAssembly, watch the circuit and Bloch sphere react, and pass automatic output checks. The AI tutor Qubitus gives Socratic hints if you get stuck.
Part of: Quantum Networking
Build quantum networks from the ground up, entanglement distribution, quantum repeaters, the quantum internet, and satellite-based QKD.
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