No-Cloning Theorem

The Eavesdropper’s Dilemma

Eve intercepts a photon. She wants to:

1. Copy the photon
2. Measure her copy to learn the bit
3. Forward the original to Bob, undetected

This would be a perfect attack. But physics says no.

The No-Cloning Theorem

You cannot perfectly copy an unknown quantum state.

This isn’t a technology limitation. It’s a fundamental law of quantum mechanics.

If Eve doesn’t know the photon’s polarization, she cannot create a perfect copy of it.

Why It Matters

Eve has only two options:

1. Measure the photon → disturbs it → Bob gets errors → Eve detected
2. Forward without measuring → learns nothing

There is no third option. She cannot copy-then-measure.

The no-cloning theorem makes quantum eavesdropping fundamentally detectable.