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

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.

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We will often use this, less precise, definition however:

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A partial observation of a classical probabilistic system does not change it. An example for this:

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Partially measuring a quantum system

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As with a complete measurement, the partial measurement changes the system!! Also called "projective measurement in the computational basis"

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