Quantum Cryptography — BB84 Key Distribution

🔒 Channel secure

Attack

Channel

Photon rate 6/s

Channel noise 0%

Key statistics

Photons sent

Sifted key

QBER

–

Final key

How BB84 works

Alice encodes each key bit onto a single photon's polarization, choosing one of two random bases: rectilinear (+, 0°/90°) or diagonal (×, 45°/135°). Bob measures each photon in a randomly chosen basis of his own. Quantum mechanics guarantees that if Bob's basis matches Alice's, he recovers the bit exactly; if it doesn't, his result is random.

basis match  → P(correct) = 1
basis mismatch → P(correct) = 0.5
QBER = errors / sifted bits

Sifting — after transmission, Alice and Bob publicly compare only their basis choices (never the bits) over a classical channel and keep only photons where both bases matched — about half of all photons sent.
Eavesdropper — an intercept-resend attack forces Eve to guess a basis, measure, and retransmit. Whenever she guesses wrong (half the time), she collapses the photon into a random new state, which shows up as roughly 25% QBER in the sifted key even when Bob's basis matches Alice's — a direct fingerprint of the No-Cloning Theorem: Eve cannot copy an unknown quantum state without disturbing it.
Channel noise — models a lossy fiber; unlike eavesdropping it flips bits independent of basis choice, so distinguishing "noisy" from "attacked" in practice needs the QBER threshold below.
Security threshold — real QKD systems abort key generation if QBER exceeds ≈11%, since above that an eavesdropper could in principle possess enough information to break privacy amplification.

Alice → photon Bob receiver Eve (active) Kept (sifted)

#	Alice bit	Alice basis	Eve basis	Bob basis	Bob bit	Result