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SS2026/Quantum Computing/5. Partial observing and measuring/Partial observing and measuring systems.md

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 A Partial observation is, as the name implies, only an observation of a partial aspect of the result.
 
+First, we need to distinguish between alternatives we want to look at.
+Each alternative is a set A of deterministic possibilities.
+Partial observation yields for each alternative A the probability that the system is in a state in A.
+
+![[Pasted image 20260729122658.png]]
+
+We will often use this, less precise, definition however:
+
+![[Pasted image 20260729122833.png]]
+
+A partial observation of a classical probabilistic system does not change it.
+An example for this:
+
+![[Pasted image 20260729122952.png]]
+![[Pasted image 20260729123015.png]]
+
+
+## Partially measuring a quantum system
+
+![[Pasted image 20260729123551.png]]
+![[Pasted image 20260729123600.png]]
+