Spin & Stern–Gerlach

Measuring a quantum arrow

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• Spin measurements give only two results, up or down, along any axis.
• An atom up along one axis is up along an axis tilted by $\theta$ with probability $\cos^2(\theta/2)$.
• Measuring spin along one axis scrambles it along the others.

Steps

1. Two spots, not a smear
2. Sorting again
3. Tilt the second magnet
4. Sideways
5. Up, sideways, up
6. Two small turns