Stern–Gerlach Sequential Measurements

Follow spin measurement branches and distinguish forgetting outcomes from postselection.

Experiment

Open the Stern–Gerlach Sequential Measurements module to interact with the apparatus.

1. Set the Intermediate axis degrees to 90°.
2. Set the Intermediate operation to Keep plus only.
3. Observe the intermediate + probability becomes 0.5, and the retained fraction drops to 0.5, because we discard the |−n> branch.
4. The final + conditional probability is 0.5, leaving a joint retained and final + probability of 0.25.
5. Compare this with setting the operation to No measurement, where the state remains undisturbed |+z> and the final + conditional probability is 1.