What is a qubit?

Bits vs qubits

A classical bit is the simplest possible piece of information: it is either 0 or 1, like a light switch that is off or on. Every photo, song and video game is ultimately a long string of these switches.

A qubit (quantum bit) also gives you 0 or 1 when you look at it — but before you look, it can exist in a combination of both at once. We write the two possible outcomes as |0⟩ and |1⟩ (the funny brackets are called 'kets', from physicist Paul Dirac's notation).

The state of a qubit is written α|0⟩ + β|1⟩, where α and β are numbers called amplitudes. They tell you how much '0-ness' and how much '1-ness' the qubit has.

Amplitudes and probabilities

When you measure a qubit, you get 0 with probability |α|² and 1 with probability |β|². Because the two probabilities must add to 1, we always have |α|² + |β|² = 1.

For example, the state (|0⟩ + |1⟩)/√2 has α = β = 1/√2, so both outcomes have probability (1/√2)² = 1/2 — a perfect coin flip.

Amplitudes are richer than probabilities: they can be negative or even complex numbers. That extra structure is what makes quantum computing more powerful than flipping coins, as you'll see in the interference lesson.

The Bloch sphere

Every possible state of one qubit can be drawn as an arrow from the center of a sphere: |0⟩ points to the north pole, |1⟩ to the south pole, and superpositions live in between — the equator holds the perfect 50/50 mixes.

This picture, called the Bloch sphere, turns abstract algebra into geometry. Quantum gates simply rotate the arrow. In our simulator, every qubit gets its own Bloch sphere so you can watch the rotations happen.