Classical computers rely on binary logic where the fundamental unit of information, the bit, exists in one of two deterministic states: 0 or 1. Quantum computing introduces a distinct unit of information known as the qubit. Unlike a classical switch that is either off or on, a qubit operates according to the laws of quantum mechanics, allowing for states that are not strictly binary until a measurement occurs.

This chapter establishes the theoretical and practical definitions required to engineer quantum systems. We begin by examining the physical and mathematical limitations of classical binary processing. You will then define the qubit and learn to represent its state using Dirac notation (bra-ket syntax). While a classical bit state is a scalar value, a qubit state $|\psi\rangle$ is described as a vector in a complex vector space:

$|\psi\rangle = \alpha|0\rangle + \beta|1\rangle$

Here, $\alpha$ and $\beta$ represent probability amplitudes rather than definite values. To visualize these properties, we will utilize the Bloch sphere, a geometric representation that maps the state of a qubit to the surface of a unit sphere.

The content concludes with a practical setup of the Python environment. You will install the necessary software development kits (SDKs) and libraries that serve as the interface for simulating quantum circuits in later chapters.