Measurement Strategies
A normal measurement asks each qubit, "Are you 0 or 1?" This is called the Z basis question. Put an H right before measuring, and you ask the X basis question instead. The question you choose decides what your counts can show about the state.
What question does a measurement ask?
A measurement reads a qubit and gives back one ordinary bit. But a measurement is not neutral. It asks a certain question. Different questions show different facts about the same state.
The standard question is "Are you 0 or 1?" It is asked in the computational basis, also called the Z basis. A basis is just the pair of answers a measurement tells apart. Real machines and the Lab both measure only in this basis.
That sounds limiting, but it isn't. To ask a different question, you turn the state first. Then the question you want lines up with the readout you have. A gate placed right before measurement changes what is being asked.
Think of a sundial that can only tell north from south. To learn east versus west, you turn the whole dial a quarter turn first. The example stops working here: turning a sundial doesn't change the sun, but the gate really does change the qubit's state.
- Worked example: what happens when I ask one state two questions?
- Run the X-basis questionINTERACTIVE
- Compare: the Z question on the same stateINTERACTIVE
- Worked example: what do I see if I read only one qubit of a pair?
- Run the Bell pair and read the columnsINTERACTIVE
- Try this: what if I ask the two qubits different questions?INTERACTIVE
- How do I choose a measurement strategy?
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