# Quantum Instrument - Description

Description

Usually it is implemented as a weighted collection of completely positive maps the sum of which is trace preserving.

Quantum Instrument: collection acts as: $rho^{AB} rightarrow tilde{rho}^{AA'B} := sum_k mathcal{E}_k left( rho^{AB} right)otimes vert k rangle langle kvert^{A'}$

A quantum instrument is more general than a quantum operation because it records the outcome k of which operator acted on the state.

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Henry David Thoreau (1817–1862)

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John Locke (1632–1704)

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Paul Tillich (1886–1965)