Final answer:
In Quantum Cryptography, if Alice and Bob have different values for their extensions, the secure key exchange fails, which is essential for secure communication. This is because they would not share the same encryption key, reflecting an error in the key exchange protocol.
Step-by-step explanation:
In Quantum Cryptography, if Alice and Bob have different values for their extensions, the correct answer is c) Secure key exchange fails. Quantum cryptography relies on the principles of quantum mechanics to secure communication channels. Unlike classical bits, quantum bits or qubits exist in a superposition of states until measured. If a secure key exchange is attempted, a discrepancy in the values of the qubits between Alice and Bob would mean they do not share the same encryption key, leading to the failure of secure communication.
Quantum entanglement, an essential phenomenon used in some quantum communication protocols, entails that a pair of particles or qubits affect each other's state, no matter the distance between them, so that the state of one (when measured) instantly determines the state of the other. Quantum decoherence refers to the loss of quantum coherence, where the system in question loses its quantum behavior and behaves classically due to interaction with the environment. Quantum superposition, as illustrated by Schrödinger's cat thought experiment, is the concept that a quantum system can be in multiple states at once, and it is not until an observation is made that the system collapses into one of the possible states. In the context of quantum cryptography, if Alice and Bob have different values for the extensions, it simply indicates an error in the key exchange process.