# Quantum Phase kickback

Every quantum algorithm consists of three major steps: initialization, applying some sequence, and then readout of the result. Although this final step appears to be straightforward, it is not always trivial how to read out the results of all the qubits in your algorithm.

Postdoctoral researcher Ben Criger (QuTech) introduces in this video the concept of quantum phase kickback, which is a powerful tool in extracting information out of your system. To this end, we briefly touch upon relative phases and global phases, which are essential building blocks for the quantum phase kickback to work.

## Prerequisite knowledge

- Basic quantum physics (Bra-ket notation, inner products, operations)
- Two-qubit gates

## Further thinking

Many quantum algorithms are based upon the concept of quantum phase kickback. It is very well possible that you already stumbled across an algorithm that uses it. Can you think of one?

## Further reading

- If you want to read the essentials on this subject in a clear and pleasant way, take a look at this note from Dong Zhou on GitHub. In our opinion, he does an excellent job of treating the essentials of the concept.

https://nosarthur.github.io/quantum%20information%20and%20computation/2018/01/26/kickback.html - For an expert insight on quantum phase kickback, take a look at original papers on the subject:
- Cleve et al. (1997) https://arxiv.org/pdf/quant-ph/9708016.pdf
- Deutsch & Josza (1992) http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.655.5997&rep=rep1&type=pdf