Instead of watching photons fly down a 3D channel, this view puts BB84's math directly on two 2D plots. Each key bit lives on a polarization angle from one of two conjugate bases:
Rectilinear (+): 0° = bit 0, 90° = bit 1
Diagonal (×): 45° = bit 0, 135° = bit 1
Top — polarization phase circle. The most recent photon's angle is drawn as a vector from the centre of a polar dial: blue for Alice's transmitted state, red (dashed) for Eve's resent state when she intercepts, green for the axis Bob measured along.
Middle — sifted-key event strip. Every resolved photon scrolls across a strip chart, plotted at its Alice-basis angle on the vertical axis. A filled dot means Bob's basis matched Alice's and the bit was sifted into the key; a dim ring means the bases disagreed and the round was discarded; a red cross marks a sifted bit that came out wrong.
Bottom — live QBER graph. The measured quantum bit error rate is plotted against event count, with reference lines at the real-world 11% abort threshold and the theoretical 25% ceiling of a full intercept-resend attack.
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.
- Channel noise — baseline depolarizing error rate even without Eve.
- Eve toggle + intercept probability — a real intercept-resend attacker measuring a fraction of photons in a random basis and resending her own collapsed state.
Deployed QKD systems abort key generation once measured QBER exceeds roughly 11% — beyond that, error correction and privacy amplification can no longer remove an eavesdropper's information.