11.10 Summary

In this chapter we connected the ‘‘logical’’ idea of qubits and circuits with the ‘‘physical’’ idea of how you might build a quantum computer. We looked at the polarization of light to show a physical quantum system, and used our ket notation to understand the unusual effect we observe when using three filters. Through this we could see that the theory of quantum mechanics does seem to provide a good model for what we experimentally observe, at least in this case.

Real qubits don’t survive forever and decoherence explains the several ways in which quantum states wander over time. While we do not yet have large enough systems to implement fault tolerance, we examined what error correction might be able to do in the quantum realm.

We need to be able to measure the progress we are making and answer questions like ‘‘how powerful is your quantum computer?’’. The definition...