Quantum cryptography is a method of securing communication by applying the principles of quantum mechanics. Unlike traditional encryption that relies on mathematical complexity, quantum cryptography uses the physical properties of particles — specifically photons — to create and distribute encryption keys that are theoretically unbreakable.
The most well-known protocol is BB84 (Bennett-Brassard 1984). It works like this:
Quantum cryptography provides a security guarantee rooted in the laws of physics rather than computational assumptions. Any attempt by an eavesdropper to intercept and measure the photons inevitably disturbs their quantum state, alerting both parties to the breach. This makes it resistant to attacks from future quantum computers that could break today's mathematical encryption.
In 2017, China launched the Micius satellite, the world's first quantum communication satellite. It successfully demonstrated quantum key distribution (QKD) between ground stations over 1,200 km apart, proving that quantum-secured communication can work at intercontinental distances. Banks and government agencies in several countries are now piloting QKD networks for their most sensitive data transmissions.
When an exam question mentions photons, polarization filters (vertical, horizontal, forward slash, backslash), or spinning particles used for encryption — the answer is always Quantum Cryptography. It is the only encryption method that operates at the quantum physics level using photon behavior to secure keys.