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# Questions tagged [measurement]

For questions related to measurement and its effects as relevant to quantum computation and quantum information.

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### How to compute the energy of a non-eigenstate in a Heisenberg limited form?

Let us have a Hamiltonian $H$ and a state $|\psi\rangle = \sum_i a_i |E_i\rangle$, a linear combination of eigenstates $|E_i\rangle$ of $H$ with eigenvalues $E_i$. What is the best way to achieve a ...
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1 vote
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### Comparative Analysis of Entanglement Measures: Von Neumann Entropy vs. Concurrence

I am seeking clarification on the comparative aspects of two prominent measures of entanglement: Von Neumann entropy and concurrence. My goal is to understand the key differences in how these measures ...
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### Why Measure method doen't work in Colab?

I'm a beginner in Qiskit. When I use Statevector and then measure method in Colab as follows ...
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1 vote
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### Is a quantum while-loop possible?

I was wondering if it's theoretically possible to have a while loop that runs a bit like this: You initialize a set of qbits with the H-gate. You perform a set of operations on them. You check the ...
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### Bell measurement for the $[[5,1,3]]$ code

The $[[5,1,3]]$ code is the smallest quantum error-correcting code (and it is the unique 5 qubit error-correcting code see Corollary 10 of Quantum codes of minimum distance two ), so sometimes it is ...
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### Heisenberg picture for a quantum measurement where the outcome is recorded

I asked a related question here. From the answer by Mateus, we see that one cannot describe a quantum measurement (where the measurement outcome is recorded) as a quantum map. That is because the ...
1 vote
55 views

### What is the quantum map that describes a measurement where the measurement outcome is known?

My question is the other case of this question. Suppose I do a measurement in the $Z$ basis with outcomes $\lambda_0, \lambda_1$ but I don't record the outcome. In this case, I can describe what ...
1 vote
58 views

### Eigenvalues of Pauli Gate and connection to measurement

Suppose I measure a qubit in the $Z$ basis. If I measure and obtain the outcome $+1$, I get the post-measurement state $\vert 0\rangle\langle 0\vert$ and if I measure and obtain the outcome $-1$, I ...
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### Why is a post-measurement state a valid quantum state?

Postulate 3 of the postulates of quantum mechanics in Nielsen & Chuang states that Quantum measurements are described by a collection ${M_m}$ of measurement operators... and the state of the ...
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### What is a measurement that always identifies $|0\rangle$ but can misidentify $|1\rangle$?

Consider a measurement of $\sigma_{z}$ that will correctly identify $|0\rangle$ every time however sometimes incorrectly identifies $|1\rangle$ as $|0\rangle$. I would like to model how such a faulty ...
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1 vote