BB84 Quantum Key Distribution — Photon Timeline

BB84 Protocol

Photon rate 6 / s

Photons sent

Sifted key bits

Basis match rate

–

QBER

–

Sifted key (live, last 40 bits)

–

How it works

Alice fires each photon polarized in one of two conjugate bases picked at random: rectilinear (0°/90°) or diagonal (45°/135°), each angle encoding one bit. Bob measures every photon in his own randomly chosen basis. Quantum mechanics gives a hard rule: measuring in the same basis it was prepared in returns the exact bit; measuring in the wrong (conjugate) basis collapses the state and returns a uniformly random bit — 50/50, not a guess about a hidden value. Only photons where Alice's and Bob's bases matched (publicly compared afterward, ~50% of them) survive sifting into the raw key.

basis match  → Bob's bit = Alice's bit   (deterministic)
basis clash  → Bob's bit = random 50/50  (discarded, not sifted)
QBER = errors in sifted key / sifted key length

No Eve: every sifted bit is deterministic, so QBER stays ≈0%.
Eve on: she must guess a basis to measure each photon before resending it. Half the time she guesses right and passes it on undisturbed. The other half she guesses wrong, collapses it into her basis, and resends a photon in the wrong basis — so even when Alice and Bob later agree on a basis, Bob is now measuring Eve's re-prepared state, which mismatches Alice's original 50% of the time. 50% (Eve wrong) × 50% (resulting error) = a real, measurable 25% QBER — the signature Alice and Bob use to detect eavesdropping and abort the key.

⚠ Eve intercepting