Qubits, Quantum Gates and Quantum Circuits – a Computer Science Perspective

Summary

Quantum computers use the abstract notion of a qubit as a logical unit for storing and processing information. The processing of information is done using quantum gates, of which four are introduced here: the Pauli-X gate, the identity gate, the Hadamard gate and the CNOT gate. The qubits and the gates are presented using vectors (and the bra-ket notation) and matrices respectively. They are assembled using visual programming as well as IBM’s software Qiskit. For this, the basic concepts of visual programming and the library Qiskit are introduced.

This three‑part teaching unit introducing secondary‑school students to the foundational concepts of quantum computing. The material includes explanatory texts, exercises, and Qiskit activities, as well as a final set of 12 cumulative tasks and student‑friendly information material, which can be accessed separately.

Required materials

• computers/tablets and internet access

Part 1: Qubits and their Representations

This part introduces students to the concept of information in computing and guides them toward understanding what makes quantum information special. Through introductory research tasks, students recall how classical computers represent and process information before transitioning to the definition of a qubit.
Teachers are provided with clear explanations and examples of how qubits can be written using bra–ket notation and vector representations, giving students the mathematical tools they will need later. This part serves as a foundational first lesson, preparing learners for quantum gates and circuits.

Part 2: Quantum Gates and Quantum Circuits

Part 2 begins with short research tasks on classical logic gates—AND, OR, XOR, and NOT—to build conceptual bridges between classical and quantum logic. It then introduces students to quantum gates and quantum circuits, explaining their purpose and how they manipulate the state of a qubit.
Key examples include the Pauli‑X gate (as a quantum version of NOT), the identity gate, and the Hadamard gate. The section also offers a practical introduction to building circuits using IBM’s Qiskit. Teachers can use this lesson to combine conceptual understanding with optional hands‑on coding activities, depending on classroom needs.

Part 3: The CNOT Gate and Entanglement

In Part 3, students will explore the CNOT gate, a two-qubit gate. This section demonstrates how the gate operates and interacts with previous concepts. A key focus is generating entanglement by using a Hadamard gate followed by a CNOT gate, which enables students to observe how quantum circuits can produce genuine quantum states.
This final section also includes 12 cumulative exercises that reinforce concepts from all three parts. Students can access separate student‑friendly information sheets (definitions, explanations, representations) and a standalone version of the exercises for independent use.