As part of our work in the Allegro Project, we’ve taken a deeper step into enabling secure quantum key distribution (QKD) between multiple users in realistic network conditions — even in cases of basis mismatch.
1️⃣ Each user randomly selects a bit value b ∈ {0,1} and a basis B ∈ {X,Y}.
They encode this choice into the phase of a coherent state pulse and send it to a central Measurement Unit (MU).
(b=0, B=X)→ φ = 0(b=1, B=X)→ φ = π(b=0, B=Y)→ φ = π/2(b=1, B=Y)→ φ = 3π/2
2️⃣ The encoded pulses travel through optical fibers and arrive simultaneously at the MU.
The MU performs an interference measurement via detectors. A detection is successful when:
- One detector fires, or
- Two detectors fire simultaneously.
3️⃣ Users then reveal their encoding bases.
- If the detection type matches the announced bases, then one or more pairs can extract a shared key bit.
- If not, the round is discarded and repeated.
🔁 This cycle continues until the users generate keys of sufficient length. Finally, they estimate two important parameters:
- Bit Error Rate (BER)
- Phase Error Rate
📉 Our findings (see Figures 3-6 and 3-7 in the report) highlight the trade-off between hardware complexity and user distance. While more advanced MUs enable broader functionality, they also reduce the average achievable distance compared to simpler schemes — a key design consideration for quantum networks.
This protocol not only enables pairwise key generation in mismatched bases but also supports key distribution to all three user pairs simultaneously in ideal conditions — paving the way for scalable, multi-party quantum-secure networks.
🔗 Exciting steps forward in the direction of quantum-safe communication!
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