Fluctuation and Noise Letters Vol. 3, No. 3 (2003) C3–C7 c  World Scientific Publishing Company COMPARISON OF ENERGY REQUIREMENTS FOR CLASSICAL AND QUANTUM INFORMATION PROCESSING JULIO GEA-BANACLOCHE Department of Physics, University of Arkansas Fayetteville, AR 72701, USA LASZLO B. KISH Texas A&M University, Department of Electrical Engineering College Station, TX 77843-3128, USA Received 5 August 2003 Revised 11 August 2003 Accepted 11 August 2003 By considering the energy requirements of quantum and classical computers we propose a criterion to separate the classical from the quantum regime and show that the classical scaling laws are much more favorable for conventional, general purpose computation. Keywords : Energy efficiency; information versus energy dissipation; microprocessors; CMOS; quantum computing. It is sometimes suggested, especially in the popular press, that quantum computers [1,2] might be, in some sense, the natural successors of today’s conventional digital computers, as the current trends in miniaturization reach the atomic level. While it is true that there are a few special tasks (notably, integer factoring [3,4]) which a quantum computer could, exploiting some unique quantum mechanical effects, perform much faster than a classical computer, this does not address the question of whether it would actually make any sense to push conventional computers into the quantum domain, for anything other than these very special purpose tasks. The energy dissipation and its relation to error-free operation has recently been identified as one of the most important problems in classical microprocessors [5]. Accordingly, in this note, we address the above issue by comparing the ultimate en- ergy requirements of quantum and classical computers, based on our recent studies on classical [5] and quantum [6] systems, respectively. It was shown in [6] that the minimum error ǫ q that is to be expected when an elementary logical operation is performed on a quantum computer satisfies ǫ q > h Eτ . (1) C3