Measuring is the moment the quantum part ends. All those amplitudes go in, and a handful of plain 0s and 1s come out.
The rule connecting them is short enough to fit on a sticker: square the amplitude to get the chance.
P(0) = |0.6|² = 0.36 · P(1) = |0.8|² = 0.64 · they sum to 1
Two things happen when you look
You get a random result. Weighted by those squared numbers, but still random. Run the same thing twice and you may get different answers.
Everything else vanishes. Whatever the qubit was holding, it now simply is the thing you saw. Look again and you get the same answer forever.
Which is why you run everything thousands of times
One run gives you one sample. To see the pattern — which is usually where the answer lives — you run the whole thing again and again and count up the results.
A few thousand runs is completely normal. That repetition is a real cost, and it is one of the quiet reasons quantum computers are slower in practice than the headlines suggest.
Where the minus signs go
Look at the rule once more. Squaring throws away the minus sign.
So a qubit holding + and a qubit holding − give you exactly the same results when you look. Identical. You cannot tell them apart.
That is not a flaw — it is the reason the minus signs have to be used before you look. A few more operations can turn an invisible difference into a certainty. That is the next lesson but two.
Worth remembering
- Square the amplitude to get the chance of that result.
- Looking is random, and it wipes out everything you did not see.
- It is the only step you cannot undo.
- You run a circuit thousands of times to see the pattern.
- Squaring hides minus signs, so they must be cashed in before you look.