BB84 Intercept-Resend Eavesdropping Detector

Channel Controls

Photon emission rate 2.0 / s

Channel noise (depolarizing) 0%

Eve intercept probability 100%

Sifted-Key Statistics

Collecting samples…

Photons sent

Sifted bits

Quantum Bit Error Rate (QBER)

— %

How it works

BB84 encodes each key bit on a single photon's polarization, chosen from one of two conjugate bases:

Rectilinear (+): |0°⟩ = bit 0,  |90°⟩  = bit 1
Diagonal (×):    |45°⟩ = bit 0,  |135°⟩ = bit 1

Alice picks a random bit and a random basis for every photon. Bob measures each photon in his own random basis. Afterward, Alice and Bob publicly compare (only) their basis choices and keep the bits where the bases matched — this is sifting. Because the two bases are non-orthogonal (conjugate), any measurement in the wrong basis yields a uniformly random result — the no-cloning theorem guarantees an eavesdropper cannot copy the unknown photon state to measure it later without disturbing it.

An intercept-resend attacker (Eve) must measure each photon in a guessed basis and resend a fresh photon in that basis. When her basis happens to differ from Alice's (probability 1/2), she resends the wrong state, so even though Bob's basis matches Alice's, he gets the correct bit only half the time:

P(basis mismatch) = 1/2
P(wrong bit | mismatch) = 1/2
Expected QBER from full intercept-resend ≈ 1/2 × 1/2 = 25%
QBER = errors_sifted / bits_sifted

Emission rate — how many photons per second Alice sends down the channel.
Channel noise — a baseline depolarizing error rate present even with no eavesdropper (real fiber/free-space channels are never perfectly clean).
Eve toggle + intercept probability — switches on a real intercept-resend attacker who measures a fraction of photons in a random basis and resends her own collapsed state.

In real BB84 deployments, Alice and Bob abort key generation and try again if the measured QBER exceeds roughly 11% — the error-correction and privacy-amplification steps can only remove an eavesdropper's information up to that threshold.

Rectilinear basis (+) Diagonal basis (×) Eve intercept event

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