  The so-called "threshold" theorem says that, once the error rate per qubit
per gate is below a certain value, indefinitely long quantum computation
becomes feasible, even if all of the qubits involved are subject to relaxation
processes, and all the manipulations with qubits are not exact. The purpose of
this article, intended for physicists, is to outline the ideas of quantum error
correction and to take a look at the proposed technical instruction for
fault-tolerant quantum computation. It seems that the mathematics behind the
threshold theorem is somewhat detached from the physical reality, and that some
ideal elements are always present in the construction. This raises serious
doubts about the possibility of large scale quantum computations, even as a
matter of principle.
