  We study effects of the physical realization of quantum computers on their
logical operation. Through simulation of physical models of quantum computer
hardware, we analyze the difficulties that are encountered in programming
physical realizations of quantum computers. Examples of logically identical
implementations of the controlled-NOT operation and Grover's database search
algorithm are used to demonstrate that the results of a quantum computation are
unstable with respect to the physical realization of the quantum computer. We
discuss the origin of these instabilities and discuss possibilities to overcome
this, for practical purposes, fundamental limitation of quantum computers.
