Fault-Tolerant Quantum Computation

To protect quantum information using a quantum error correcting code, one has to build more elementary physical systems and do some more processing on them in order to detect and correct errors. Although in any realistic set-up all these new elements and new operations will also be noisy which could very well just introduce more noise instead of decreasing it. That is why all these new elements have to be added in a fault-tolerant way. That is to say in a way which permits to correct more errors than it introduces.

Another crucial concern when building a quantum computer is of course to be able to compute. Which means one should be able to do operations on the protected information and this without lowering the protection. This is called fault-tolerant quantum computation.