MDI-QKD: Detector-Blind Quantum Key Distribution

Protocol Control

Pulse rate 3.0/s

Detector efficiency η 70%

Live Status

Idle — press Run

BSM click rate

0.0%

QBER estimate

0.0%

Sifted key rate

0.0 bit/s

Secure key bits

How it works

In MDI-QKD, Alice and Bob each encode a random bit in a random basis (Z or X) on a photon and send it — not to each other — but to an untrusted relay "Charlie" sitting between them, who performs a Bell-state measurement (BSM): the two photons interfere on a 50:50 beamsplitter feeding two detectors.

Hong-Ou-Mandel interference at the beamsplitter:
  same bit, same basis   → photons BUNCH   → no valid coincidence
  diff bit, same basis   → photons ANTI-BUNCH → coincidence possible
  different bases        → partial overlap → discarded (sifting)

P(click | matched basis, a≠b) = ½·η²   (only |Ψ⁻⟩ is linear-optically
                                          identifiable; η = detector eff.)
Sifted-key fraction ≈ ¼ · P(click)   (½ basis match · ½ anti-correlated)

When Charlie announces a valid click, Alice and Bob know their bits are anti-correlated (b = ¬a) — without Charlie ever learning either bit. Because the raw key never passes through Charlie's detectors as classical information, the protocol is provably immune to every detector side-channel attack (blinding, time-shift, etc.) that breaks ordinary BB84 — the whole point of "measurement-device independence".

Security is still verified the ordinary way: Alice and Bob publicly compare a random sample of sifted bits. If the quantum bit error rate (QBER) of that sample exceeds ≈ 11%, an eavesdropper must be present and the key is aborted.

Run — starts photon pairs streaming from Alice and Bob toward the relay at the chosen pulse rate.
Malicious Relay — simulates Charlie's detectors being hacked (e.g. blinded with bright light to fake clicks). Watch the QBER climb past the 11% security threshold and the key generation abort.
η slider — lower detector efficiency lowers the honest click rate and raises the baseline dark-count error floor.

Z-basis photon X-basis photon valid BSM click no click / discarded

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