# Measuring in qiskit collapses onto the Z-Plane. Why do any rotations other than Y matter?

We have the situation where a 50/50 split between $$|1\rangle$$ and $$|0\rangle$$. This was done using a $$H$$-Gate. Now, when measuring in qiskit, if my understanding is correct, we collapse onto the Z-plane which then results in either a 0 or a 1.

So my question now would be, what's the practical usage of any rotation that isn't a Y-Rotation? As far as I can see, the Y-Rotation changes the amplitudes/probabilities of the measurement, where as a Z-Rotation does not.

• A $Z$ rotation affects the phase of the state, which can be useful to get constructive/destructive interference later on. Oct 25, 2021 at 9:09
• Does that have any implications on quantum circuits? From my understanding it allows us to use even more of the complete state-space, but that isn't per se directly measurable, and there for, usable to construct algorithms with. Oct 25, 2021 at 10:17
• Yes it has. Take for example the quantum Fourier transform, it uses the controlled Phase gate, which is a controlled rotation around the Z axis (in Qiskit’s implementation, it differs from the normal CRZ by a relative phase) Oct 25, 2021 at 10:25