What is a Qubit?

foundationsReading-focused

3 min read

Classical bits vs. qubits, superposition, and how Qiskit represents quantum state.

Suggested before this lesson

Tensor Products and Multi-Qubit State Space

Classical Bits vs. Qubits

A classical bit is always 0 or 1. A qubit can exist in a superposition of both states at once:

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

In Qiskit, a qubit always starts in the $|0\rangle$ state:

Visualizing State: the Bloch Sphere

Any single-qubit pure state can be drawn as a point on the surface of the Bloch sphere. The north pole is $|0\rangle$ , the south pole is $|1\rangle$ , and the equator holds equal-superposition states such as $|+\rangle$ and $|-\rangle$ .

The visualization below shows how the starting state sits at the north pole before any gates are applied.

The

∣0 ⟩

State on the Bloch Sphere

An untouched qubit, fresh from qc = QuantumCircuit(1), always starts at the north pole.

State Amplitudes

Measurement Probabilities

$∣0 ⟩$

100.0%

$∣1 ⟩$

0.0%

Bloch Sphere

$q$

Check your understanding

|\alpha|^2 = 0.25

for a qubit in state

\alpha|0\rangle + \beta|1\rangle

, what is the probability of measuring

|1\rangle

0.25

0.75

0.5

Cannot be determined

Classical Bits vs. Qubits

A classical bit is always 0 or 1. A qubit can exist in a superposition of both states at once:

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

where

|\alpha|^2 + |\beta|^2 = 1

. Measuring the qubit collapses it to

|0\rangle

with probability

|\alpha|^2

|1\rangle