# Tag Info

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### How to perform quantum state tomography on two qubits?

Preliminary I would like to rewrite the equation that you have in a slightly different manner. Since a density matrix can be written as a matrix, we can also write it down as a linear combination of ...
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### What is the relationship between Choi and Chi matrix in Qiskit?

( I copied some text from a previous answer of mine) Defining the Choi and $\chi$ matrix The Choi matrix is a direct result of the Choi-Jamiolkowski isomorphism. Some intuition on what this is can be ...
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### How to perform Quantum Process Tomography for three qubit gates?

I am sure that since you are asking this question you probably already understand this, but for future & other's reference let me give a quick recap of what we are trying to achieve. Quantum ...
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### Why is the complexity of $n$-qubit state tomography not upper bounded as $O(3^n)$?

Let's say you have a magical machine that gives you $\langle P_{p} \rangle$ (which are expectation values and therefor, well, numbers) and only the $\langle P_{p} \rangle$. It does this for all the $3$...
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### Is there a circuit to compare two quantum states?

The task that you describe in your question — a circuit which flips a single qubit, if and only if the two input states are different — is not possible. We can show this as follows. First, there is no ...
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### In shadow tomography, how is the state reconstructed from its shadows?

Under the assumption that the ensemble $\mathcal{U}$ faithfully produces the Haar expectations at least to the second moment, the inversion can be performed as suggested in the last paragraph of the ...
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### Using Classical Shadow to predict quantum state's fidelity has nothing to do with the dimension of the density matrix?

Without really checking your arguments, there is a fundamental reason why the scaling could be fine, but it is still not strange at all. The point is that you estimate with additive precision, but ...
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### Are SIC-POVMs optimal for quantum state reconstruction?

First of all, here's a short disclaimer: I'm not an in-depth expert in this field, I'm just currently getting in contact with tomography more and more often :) So take the following with a grain of ...