Limits of Adiabatic Clocking in Quantum-dot Cellular Automata

Ultimate bounds on the maximum operating frequency of networks of quantum dot cellular automata devices have yet to be established. We consider the adiabaticity of such networks in the two-state approximation where clocking is achieved via modulation of the inter-dot tunneling barriers. Estimates of the maximum operating frequency that would allow a 99% probability of observing the correct logical output are presented for a subset of the basic components used in QCA network design. Simulations are performed both in the coherent limit and for a simple dissipative model. We approach the problem of tunnel-based clocking from the perspective of quantum annealing, and present an improved clocking schedule allowing for faster operation. Using an analytical solution for driven QCA wires, we show that the maximum operating frequency in the coherent limit falls off with the square of the wire length, potentially limiting the size of clocked regions.