BB84 Protocol Capstone

BB84 Capstone

Time to build the whole thing. In the previous lesson you walked through BB84 on paper; in this project you implement every role in the protocol, Alice, Bob, and Eve, and watch the security argument come true in your own measurement statistics.

Your Project

The starter code gives you the protocol skeleton: random bit and basis generation, the per-qubit exchange loop, and the reporting. You implement the four pieces that matter, one per checkpoint.

Checkpoint 1, Alice encodes. Complete encode_qubit(bit, basis). Bit 1 means apply X; basis 1 (the X basis) means apply H afterward, turning |0⟩/|1⟩ into |+⟩/|−⟩. A built-in self-test checks all four encodings and prints the checkpoint marker.

Checkpoint 2, Bob measures. Complete measure_qubit(qc, basis). Bob cannot read the X basis directly: when his basis is 1 he must apply H before the Z measurement, rotating |+⟩/|−⟩ back to |0⟩/|1⟩. The self-test confirms that matching bases round-trip every bit perfectly.

Checkpoint 3, basis reconciliation. Complete sift(...): keep exactly the positions where Alice's and Bob's bases agree. On a quiet channel the sifted keys must match bit for bit, and about half the qubits survive.

Checkpoint 4, eavesdropper detection. Complete eve_intercept(qc): Eve measures the incoming qubit in a random basis of her own and resends what she found, the classic intercept-resend attack. Then run the protocol with Eve on the line. Theory says she corrupts 25% of the sifted key; the abort rule fires whenever the measured error rate (QBER) exceeds 11%.

What Success Looks Like

Run 1 (no Eve): sifted keys match, QBER 0%, a usable shared secret. Run 2 (Eve on every qubit): QBER lands near 25% and the program prints the abort. The protocol works not because Eve is clumsy, but because physics forces her to leave statistical fingerprints.