Act as a theoretical physicist and cryptographer. Provide a deep-dive explanation of the BB84 Quantum Key Distribution protocol. Your explanation must cover: 1. The role of Heisenberg's Uncertainty Principle in detecting eavesdropping. 2. The mathematical basis for the error rate calculation to determine the presence of an interceptor (Eve). 3. A step-by-step walkthrough of the sifting and reconciliation phases. 4. A comparison of BB84 with the Ekert91 protocol, highlighting the differences in entanglement usage. Ensure the explanation is technically rigorous but accessible to a graduate student in computer science.