The goals of the proposed research are to clarify the relationships between all the quantum mechanical constraints on the energy needed to carry out qubit logical operations in physical systems, to determine whether there is a particular scaling, or ultimate limit, that applies universally; and, if so, to ascertain its form. The key requirement to be emphasized is that the system used to switch the interaction on and off be quantum mechanical. There are two aspects to the proposed research, one practical, and one fundamental. From a practical viewpoint, it would be useful to the quantum computing community to determine the form of the ultimate energy/accuracy tradeoff allowed by quantum mechanics. From a fundamental viewpoint, the question being investigated is under what circumstances one can use a quantum system to carry out approximately a unitary operation on another, coupled, system. The minimum-energy requirement may be seen as quantifying, in some way, the degree to which the control system must be approximately classical. This is a question of basic interest which goes back to the foundational days of quantum mechanics.
