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  The Road to the Qubit From Quantum Mechanics to the Physical Architectures Behind Modern Quantum Computers Technical Series: Building the Qubit — Part 1 Quantum computing is often introduced through a deceptively simple statement: A classical computer uses bits. A quantum computer uses qubits. The statement is correct, but it hides the most important engineering question in the entire field: What is a qubit physically? Unlike a transistor, there is no single component called “the qubit.” A qubit is an engineered quantum system capable of providing two controllable quantum states that can be used to encode information. That system might be: a superconducting electrical circuit, an isolated atomic ion, a photon, an electron confined in a semiconductor, a neutral atom held by laser light, a defect inside a diamond, a microwave cavity, or, in more experimental approaches, a topological quantum state. The diversity is not accidental. Each physical implementation solves some engineerin...