Frontier physics
Quantum cryptography
Simulate the BB84 quantum key distribution principle: send photons prepared in random bases, and detect eavesdropping by the errors an intercept measurement causes. Observe the ~25% error rate eavesdropping introduces.
Equipment
- Interactive Alice–Bob quantum-channel model (panel 1)
- «Parameter» slider (key-sample size) and «Noise» slider (eavesdropping/channel noise)
- Time-evolution plot with the «Run time» checkbox (panel 2)
Procedure
Clean key transfer
Set «Noise» = 0, enable «Run time», and follow the bit string Alice sends/Bob measures. Only bits measured in matching bases are kept; the sample agrees perfectly → clean key. Notice about half the bits are discarded for basis mismatch — the BB84 signature.
Turn on eavesdropping
Raise «Noise» to emulate Eve intercepting and re-measuring photons midway. Since Eve cannot know the basis, half her measurements use the wrong basis and disturb the state; when Bob measures, about 25% of the kept bits are wrong — the eavesdropper's fingerprint.
Measure the quantum bit-error rate
In panel 2, read the error fraction of the publicly compared key portion over time: ~0% on a clean channel, ~25% under full interception. If the QBER passes the threshold (in practice ~11%), Alice and Bob abort — security comes from measurement disturbing the state, not from computational hardness.
Relate security to practical limits
Increase «Noise» gradually and find the threshold where the system «detects» intrusion: errors pass the alarm level. Discuss: real channels carry a few percent of natural noise, so the protocol needs sifting, error correction, and privacy amplification — and must tell channel noise from active eavesdropping.