BB84 Quantum Key Distribution

What is it?

Imagine Alice and Bob want to share a secret key. A spy named Eve is listening. In BB84, Alice sends polarized photons — each one encodes a bit either as rectilinear (+) or diagonal (×). Bob guesses the basis for each photon. Where their bases match, Bob gets the correct bit. Where they don't, Bob gets random noise. After transmission, they compare bases publicly — the matching positions become their shared secret key!

Concrete Example

Alice wants to send bits: 1 0 1 1 0

Alice's bits:   1   0   1   1   0
Alice's bases:  +   ×   +   ×   +
Photons sent:  [↕] [⤢] [↕] [⤢] [↕]

Bob's bases:    +   +   ×   ×   +
Bob measures:  1   ?   ?   1   0

Compare bases:  ✓   ✗   ✗   ✓   ✓
Sifted key:    1   -   -   1   0  →  "110"

How It Works

1. The answer is converted to bits (8 per character, but shown grouped for clarity)
2. Alice chooses a random encoding basis (rectilinear or diagonal) for each bit
3. Bob chooses random measurement bases — the puzzle provides Alice's bases and Bob's results
4. You (as Bob) must determine where bases match by comparing Alice's bases with your own decisions
5. Where bases match, the bit is correct → sifted key
6. Convert sifted key bits back to ASCII → the answer

Step-by-Step Solving

def recover_key(alice_bases, bob_results):
    """Bob compares his bases with Alice's to sift the key."""
    # Bob would compare his chosen bases with Alice's
    # But in this puzzle, it's simulated: you're given
    # Alice's bases and Bob's measurements
    sifted = []
    for i in range(len(alice_bases)):
        # If Bob guessed the same basis as Alice...
        # (For the puzzle, the agent must determine this)
        pass
    return sifted

Real-World Applications

• Secure communication: BB84 is the most widely implemented QKD protocol. Commercial systems run over fiber optic links between banks and government agencies.
• China's Micius satellite: First quantum communication satellite, BB84-based key distribution between space and ground stations
• Quantum networks: The backbone of future quantum internet — secure against any future quantum computer attack
• Post-quantum security: Unlike RSA or ECC, QKD can't be broken by Shor's algorithm on a quantum computer