Robust Electric-Field Input Circuits for Clocked Molecular Quantum-Dot Cellular Automata

Quantum-dot cellular automata (QCA) is a paradigm for low-power, general-purpose, classical computing designed to overcome the challenges facing CMOS in the extreme limits of scaling. A molecular implementation of QCA offers nanometer-scale devices with device densities and operating speeds which may surpass CMOS device densities and speeds by several orders of magnitude, all at room temperature. Here, a proposal for electric field bit write-in to molecular QCA circuits is extended to QCA circuits clocked using the $z$ component of an applied electric field. Input electrodes, which may be much larger than the cells themselves, apply an input $y$ field component. The input field selects an input bit on a field-sensitive portion of the circuit, and a shift register transmits the input bit to downstream logic. The circuit is sensitive to the input field, since even a very weak input field selects a bit and drives the shift register as desired. The circuit also is robust: the coupling between the field-sensitive segment and the shift register fails at input field strengths much stronger than those needed to write a bit. A simple rotation of the the shift register cells eliminates this circuit failure, even under arbitrarily strong input fields. The modified circuit also tolerates a significant unwanted $x$ field component, which is used neither for clocking nor for input. The write-in of classical bits to molecular QCA circuits is one road-block that must be cleared in order to realize energy-efficient molecular computation using QCA. Write-in may be achieved simply by using relatively large electrodes and clocked molecular circuits that respond to a weak input field and tolerate very strong applied fields without failing.