# Tag Info

## New answers tagged textbook-and-exercises

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### How do you find the possible measurement values of an observable?

Unfortunately, some parts of this question are unclear to me as currently written. I will do my best to try and answer, in the sense of addressing some aspects of your below statement Attempt at ...
• 606
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### Understanding the Bernstein-Vazirani implementation from the qiskit website

The Bernstein-Vazirani algorithm is an oracular problem: you suppose that you are given an oracle that implements a function $f$ such that there is a secret $s$ such that $f(x)=x\cdot s$. Crucially, ...
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### Does applying an operator to a state have nothing to do with taking a measurement?

Both quantum gates and measurements can be represented as matrices. Every quantum gate corresponds to a unitary matrix, and every measurement corresponds to a Hermitian matrix. Some matrices happen to ...
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• 1,429
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### How to prove that ${\rm tr}(A|\psi\rangle\langle\psi|)=\langle\psi| A|\psi\rangle$?

You can use the cyclic property of the trace, ${\rm Tr}(XY) = {\rm Tr}(YX)$. Another way is to note that both sides are linear over $A$. Thus it's enough to prove it for \$A = E_{ij} = |i\rangle\langle ...
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