Quantum key distribution
Agreeing on an encryption key with security grounded in physics rather than in mathematical difficulty.
All ordinary key exchange rests on certain computations being impractically heavy. QKD rests instead on measurement inevitably disturbing what is measured, which makes eavesdropping detectable. That holds regardless of how much computing power the attacker has, quantum computer included.
The limitations are practical. The signal cannot be amplified, since the no-cloning theorem forbids copying, which caps range at a few hundred kilometres in fibre. China demonstrated satellite-based QKD with Micius in 2017. Several security agencies, the UK's NCSC among them, nonetheless advise against QKD in practice and recommend post-quantum cryptography — which requires no new physical infrastructure.