  Near zero-energy computing describes the concept of executing logic
operations below the (kBT ln 2) energy limit. Landauer discussed that it is
impossible to break this limit as long as the computations are performed in the
conventional, non-reversible way. But even if reversible computations were
performed, the basic energy needed for operating circuits realized in
conventional technologies is still far above the (kBT ln 2) energy limit, i.e.
the circuits do not operate physically reversible. In contrast, novel
nanotechnologies like Quantum-dot Cellular Automata (QCA) allow for
computations with very low energy dissipation and, hence, are promising
candidates for breaking this limit. Accordingly, the design of reversible QCA
circuits is an active field of research. But whether QCA in general (and the
proposed circuits in particular) are indeed able to operate in a logically and
physically reversible fashion is unknown thus far, because neither physical
realizations nor appropriate simulation approaches were available yet. In this
work, we address this gap by utilizing an established theoretical model that
has been implemented in a physics simulator enabling a precise consideration of
how energy is dissipated in QCA designs. Our results provide strong evidence
that QCA is indeed a suitable technology for near zero-energy computing.
Further, the first design of a logically and physically reversible adder
circuit is presented which serves as proof-of-concept for future circuits with
the ability of near zero-energy computing.
