The satellite sends one polarized photon per pulse, each encoded in a randomly chosen basis (rectilinear + or diagonal ×). The ground station measures every photon in its own random basis. Only pulses where both bases match are kept — that subset is the raw key.
basis match → keep bit
basis mismatch → discard (public sifting)
QBER = mismatched kept-bits / kept-bits
- No eavesdropper: QBER stays near the natural noise floor — mostly detector/atmosphere noise.
- Eavesdropper (intercept–resend): Eve must measure and retransmit each photon. Whenever she guesses the wrong basis (50% of the time), her retransmitted photon carries a random bit — an unavoidable consequence of the no-cloning theorem. That injects roughly 25% QBER into the sifted key, far above the abort threshold, so Alice and Bob detect the intrusion and discard the key.
- Classical channel: switch the channel mode to see the same eavesdropper simply copy every bit perfectly in transit — with no physical mechanism to reveal that a copy was ever made.