Sharing secrets

Alice and Bob want to share a secret. They don’t care what the secret means – the secret doesn’t have to mean anything at all. The secret might be “Bob once spit in Alice’s soup” or it might be “3*%Uw4YM^i44Sq.” The content doesn’t matter, so long as no one else knows it.

So, Alice randomly generates 20 bits and sends them to Bob. (See Figure 5.1.)

Figure 5.1 – Alice sends bits to Bob

What they don’t know is that an eavesdropper (named Eve) listens in on the line. Eve makes copies of the bits and sends them to Bob. (See Figure 5.2.)

Figure 5.2 – Eve snoops

Sorry, Alice. Your bits aren’t secret. Back to the drawing board!

Adding Hadamard gates

Alice knows all about quantum computing. So, before she sends her bits, she applies the Hadamard operator to each one. Since the Hadamard operator is its own inverse, Bob...