New chapter on quantum walks

[Stina-A]

Changes made

Have added a chapter on quantum walks which includes a demonstration of the quantum walk search algorithm.

Justification

This tutorial provides a hands-on explanation using Qiskit of a quantum walk algorithm on a 4-dimensional hypercube. First, we provide some theory on quantum walk models and the quantum walk algorithm. Thereafter, we show how we can implement one step of the walk on the 4-dimensional hypercube followed by the implementations of the other two main parts of the algorithm; inverse Fourier transform and phase estimation. Lastly, we show how we can combine the steps to achieve the complete quantum walk search algorithm.

All comments and suggestions are greatly appreciated!

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:white_check_mark: Stina-A
:white_check_mark: frankharkins
:x: Stina Andersson

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[frankharkins]

Thanks for your contribution. I'm closing this pull request as this repository is no longer used and is being archived. You can find the latest version of the Qiskit Textbook online at https://qiskit.org/learn/, and the new content repository at Qiskit/textbook. Please feel free to re-open this pull request there if it's still relevant.