  Certain physical aspects of quantum error correction are discussed for a
quantum computer (n-qubit register) in contact with a decohering environment.
Under rather plausible assumptions upon the form of the computer-environment
interaction, the efficiency of a general correcting procedure is evaluated as a
function of the spontaneous-decay duration and the rank of errors covered by
the procedure. It is proved that the probability of errors can be made
arbitrarily small by enhancing the correction method, provided the decohering
interaction is represented by a bounded operator.
