QUANTUM 07
How BB84 sends a key on photons and exposes eavesdroppers
Sinclair College - Dr. Mike Libassi
TODAY
01 · BASES AND PHOTONS
| Basis | Bit 0 | Bit 1 | Read in the other basis |
|---|---|---|---|
| + rectilinear (Z) | → |0⟩ | ↑ |1⟩ | 50/50 random |
| × diagonal (X) | ↗ |+⟩ | ↖ |−⟩ | 50/50 random |
Measuring in the wrong basis gives a random result and destroys the original polarization.
02 · THE PROTOCOL
Alice picks a random bit and basis for each photon
Bob picks his own random basis for each
They compare bases publicly (never bits) and keep matches, about half
They reveal a sample of kept bits and measure the error rate
If the error rate is low, the unrevealed bits become the key. If it's high, they discard everything.
03 · CATCHING EVE
Real channels have a few percent of natural noise. Protocols commonly abort above roughly 11%.
03 · CATCHING EVE
An eavesdropper can't look without leaving fingerprints.
04 · REALITY CHECK
| Limit | Why it matters |
|---|---|
| Distance | Photons are lost in fiber, and quantum signals can't be amplified (amplifying means copying) |
| Trusted nodes | Long links relay through stations that see the key, a classic weak point |
| Authentication | The public channel still needs conventional crypto or pre-shared keys |
| Hardware | Real detectors have been attacked through side channels |
The U.S. NSA does not recommend QKD for national security systems and points to post-quantum cryptography instead.
WRAP-UP
BB84 encodes bits in two random bases: + and ×
Alice and Bob sift to matching bases, then check a sample
An intercepting eavesdropper causes about 25% errors
Practical limits make PQC the broader-scale answer today
DISCUSSION
BB84 is secure by the laws of physics. Why do governments still prefer post-quantum cryptography?
Weigh distance, cost, trusted relays, and what already runs on today's networks.