See what a qubit looks like before you measure it
What does a qubit look like before you measure it?
A qubit always starts in the state, which you can think of as a coin sitting heads-up on a table. In this experiment you will apply a single gate called the Hadamard (H) that puts the qubit into an equal mix of and . This mixed state is called superposition.
Before measurement, a qubit in superposition is not secretly 0 or secretly 1. It genuinely holds both possibilities at once, each with a probability amplitude. The Hadamard gate creates this by splitting the state into two equal branches. When you finally measure, each branch has a 50% chance of being the outcome you see. Run the experiment many times and watch the histogram settle toward a balanced split.
The H gate transforms into an equal superposition of and . Watch the Bloch sphere: the arrow moves from the north pole (pure to the equator, where it points along the +X direction. The histogram now shows equal weight on both outcomes.
It is tempting to think superposition means the qubit reads out both 0 and 1 at the same time. That is not what happens. Each measurement returns one ordinary classical outcome. Superposition describes the probabilities before measurement, not the result after it.
The active qubit produces 0 and 1 with roughly equal frequency. Over 1000 shots you should see counts near 500 for each outcome. Any untouched qubits stay in .
This experiment is a hands-on companion to the Superposition and Measurement lesson. That lesson explains the mathematical framework (amplitudes, Born rule, state vectors) behind what you see here. After running this experiment, the formula should feel concrete rather than abstract.
Read the full lesson →After applying H to |0⟩, you run a single measurement. What do you see?