2) A secure communication system operates in the following manner:
- Photons are used to establish a cryptographic key between two parties.
- An interceptor attempting to measure the transmitted quantum states introduces detectable disturbances.
- The actual message is subsequently encrypted using the established key.
- The system transmits the quantum signals through the atmosphere rather than through an optical-fibre cable.
Which one of the following best describes this system?
Answer: (b) Free-Space Quantum Key Distribution
Explanation: Quantum Key Distribution uses quantum states, generally carried by photons, to establish a cryptographic key. It does not mean that the actual message itself is transmitted as a quantum state. Once a secure key has been established, conventional encryption can be used for the data.
The security arises because measuring an unknown quantum state disturbs it, while the No-Cloning Theorem prevents creation of a perfect copy of an unknown quantum state. Interception therefore produces detectable errors.
When the quantum signals travel through open air or the atmosphere using optical links rather than fibre, the system is Free-Space QKD. Such systems require line-of-sight and precise pointing and tracking, and can be affected by fog, rain, turbulence and ambient light.
Post-Quantum Cryptography is different. It uses classical mathematical algorithms designed to resist attacks by quantum computers and does not itself depend on transmitting quantum states.
