Practical Guide to Using a Quantum Computer (Based on Materials from the International Quantum Center at CERN, Switzerland) # 2

Abstract This laboratory work presents a practical implementation of the BB84 quantum key distribution protocol using the Qiskit framework. The material is based on educational resources developed by the International Quantum Center at CERN (Switzerland) and is intended to provide hands-on experience with quantum circuit construction, quantum state preparation, and quantum cryptographic principles. In Laboratory Work No. 2, participants model the BB84 protocol within a quantum computing environment, emphasizing both theoretical understanding and executable circuit design. The experiment employs quantum circuits to prepare Bell states and to generate random bits as part of the quantum key distribution process. Visual representations of Bell states in the computational and Hadamard bases are analyzed to illustrate their orthonormal structure and physical interpretation. The quantum system is initialized and transformed into Bell states using the Composer interface by applying the Hadamard (H) gate to the first qubit q[0]q[0]q[0] and subsequently entangling it with the second qubit q[1]q[1]q[1]. This procedure produces orthonormal state vectors that form the basis for secure quantum communication. The laboratory work demonstrates how entanglement, basis selection, and measurement outcomes contribute to the security properties of BB84. The guide highlights the hybrid quantum-classical workflow implemented in Qiskit and provides a step-by-step framework for simulating quantum key distribution protocols on cloud-accessible quantum hardware and simulators. Through this practical exercise, participants gain insight into the physical foundations of quantum cryptography, probabilistic measurement, and the implementation of quantum communication protocols in modern quantum computing ecosystems.