Coplanar QCA reversible 4 to 2 encoder architectures for ultra-dense nanoscale systems

Quantum-dot Cellular Automata provides an ultra dense and low power alternative to CMOS by encoding binary information through electron polarization rather than charge transport. However, existing reversible encoder architectures often suffer from excessive cell counts, large layout areas, long clocking delays, and limited robustness under fabrication defects. This work introduces a reusable coplanar reversible multiplexer that enables systematic construction of compact and fault-tolerant QCA encoders. Using this framework, two 4 to 2 architectures are developed: a reversible priority encoder and a hybrid multiplexer–majority-gate encoder. Both designs employ optimized majority-based primitives to minimize spatial complexity and garbage outputs. Simulations performed in QCADesigner 2.0.3 demonstrate that the proposed priority encoder requires only 28 cells and 0.06 μm², while the hybrid encoder achieves 19 cells and 0.03 μm², with both completing computation within two clock zones. Deposition-fault analysis confirms correct functionality across all input combinations. Compared with the most recent reversible 4 to 2 encoder, the proposed designs reduce cell count by up to 73% and area by 62%, establishing a highly efficient and fault-tolerant methodology for nanoscale QCA systems.