# Chapter 2 Qubits

About quantum bits and quantum circuits, including the “impossible” square root of \texttt{NOT}, as well as an introduction to single-qubit unitaries and rotations of the Bloch sphere, and the implications concerning universal gates.

When studying classical information theory, one single bit isn’t usually the most interesting object to think about — it’s either 0 or 1. Yet in the quantum case, just working with one “quantum bit” (which we call a qubit) opens up a whole world of interesting mathematics. In fact, single-qubit interference is arguably the fundamental building block for quantum computing, and so deserves to be thoroughly investigated and understood.