# Questions tagged [phase-estimation]

Refers to the quantum algorithm used to estimate the eigenvalue corresponding to an eigenvector of a unitary operator. (Wikipedia)

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### How can I understand the notation for the final state of QPE algorithm?

I've been reading about the standard phase estimation from the Qiskit tutorial and got stuck in interpreting the final state representation. What does the state $|2^t\theta\rangle$ mean? Is that the ...
343 views

### Does phase kickback require the system to be in the eigenstate?

I've been watching this video for the introduction to phase kickback. And here's a diagram: I got confused if we really need $|\psi_k\rangle$ to be an eigenstate to make the kickback work. It seems ...
1 vote
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### Understand $U|\psi\rangle = e^{2\pi i\phi}|\psi\rangle$ in phase estimation algorithms

I'm trying to understand the motivations behind $U|\psi\rangle = e^{2\pi i\phi}|\psi\rangle$ in quantum phase estimation. In my interpretation, since $U$ is the unitary operator, this equation wraps ...
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### Why do we start from the least significant bit in phase estimation algorithms?

I've been watching some videos and tutorials for quantum phase estimation. Here's a video I found helpful, which explains the phase estimation in general. I also learned the iterative phase estimation ...
69 views

### What phase should I expect from pauli X?

Here's an example I found that implements the phase estimation algorithm: Here the eigenvector is initialized to the $|+\rangle$ state, which is an eigenvector of Pauli X with eigenvalue 1. The ...
1 vote
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### Writing a Phase Estimation function in Cirq

I am trying to write a phase estimation algorithm using Cirq. The algorithm works for different inputs but I receive a few errors in the estimate_phi(mystery) function. ...
41 views

### Mathematical reasoning of Repeating the Phase Estimation in the Order Finding Algorithm

Repeating the order-finding algorithm of quantum computing twice obtains $\dfrac{s_1'}{r_1'}$ the first time, and $\dfrac{s_2'}{r_2'}$ the second time, where $\dfrac{s_i'}{r_i'}$ is $\dfrac{s_i}{r}$ ...
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### Expectation value of Pauli string for VQE

One approach to get the expectation value $\langle\psi|P\psi\rangle$ of a pauli string $P\in \{I, X, Y, X\}^{\otimes n}$ is the following. Let $(a_i, |\lambda_i\rangle)$ be eigenvalue-eigenvector ...
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