Measurement
January 4, 2025
Measurement connects quantum formalism to observable outcomes. In the simplest picture, measuring a qubit in the computational basis yields 0 or 1 with probabilities given by the squared magnitudes of the amplitudes.
Collapse (operational view)
After a projective measurement whose outcome you record, the post-measurement state lies in the subspace consistent with that outcome. This is why intermediate measurements change the rest of a circuit’s behavior.
In Qiskit,
measure_all() records outcomes into classical bits. Always think about when you measure relative to gates you apply afterward.Next
Move on to Linear algebra for the mathematical toolkit.