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# Collapsing a Quantum State

## The Prompt

**Using the Stern-Gerlach Experiment, write down the state exiting the spin up port in bra-ket notation.**

(An x-oriented Stern-Gerlach device with an arbitrary quantum state $\vert \phi\rangle$ fired into it is drawn on the board.)

## Context

This SWBQ gives students an opportunity to see the various ways of representing states and how to project quantum states onto other states. Students will also be able to practice explicitly using the projection operator in bra-ket notation to receive the new quantum state of the particle, and students will also practice normalizing the new quantum state.

This SWBQ fits perfectly in lectures between activities within the Stern-Gerlach sequence, particularly after students have experienced several different ways to describe quantum states mathematically.

## Wrap Up

Discussions may arise regarding the difference between the terminology “measurement” and “state”. Occasionally, students will write down that the “state” of the particle is $+ \frac{\hbar}{2}\,$ - this is an excellent opportunity to have a class-wide discussion about the specific usage of both terms.