Significant debate exists in the literature with regards to the permissibility of feedback in reversible computing nanotechnologies. Feedback allows for reuse of logical subroutines, which is a desired functionality of any computational device. Determining whether loop back is allowed is paramount to assessing the robustness of reversible logic in any quantum design. In this paper, the fundamental discoveries in entropy and quantum mechanics that serve as the foundations for reversible logic are reviewed. The fundamentals for implementation of reversibility in computing are shown. Then, definitions are presented for a sequential reversible logic structure. A sequential reversible logic structure is proven to have an identical number of feedback-dependent inputs and feedback-producing outputs, and new metrics for measuring the probability of each output state are presented. Using these metrics, the reversibility of each clock cycle of such a device is verified. Therefore, we demonstrate that any reversible logic structure with feedback is physically reversible.
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