The Other Answer (and why we're not teaching it)

Post-quantum cryptography, and what QKD buys that it doesn't

Mission 5: Quantum Key Distribution. Part of the Quantum Courier Service, a free course from the Oxford University Quantum Information Society.

Full disclosure before we go further, recruit, because you'll meet a cryptographer eventually and she'll ask.

There are two answers to "quantum computers will one day break our maths." Ours is the one you can see: replace the mathematics with physics, and detect the eavesdropper directly. The other is less cinematic and, right now, far more widely deployed. It is post-quantum cryptography: new mathematical problems, chosen because no known quantum algorithm cracks them, running on the laptop you already own, over the internet that already exists. NIST standardised the first of them in 2024. The UK's NCSC recommends them, and is notably cool on the approach you're about to spend a module learning.

So why are we here? Because QKD's guarantee is of a different kind. PQC's security rests on a problem nobody has solved yet. QKD's rests on a property of the universe, and buys you something PQC cannot: you find out that someone listened. That is worth knowing about even if it never protects your bank. And it requires a dedicated fibre, a trusted relay every hundred kilometres or so, and, awkwardly, a secret you already shared, to authenticate the phone call. Physics gets you a very good lock. It doesn't get you out of doing the engineering.