  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 synchronous QCA circuits clocked using an applied
electric field, \(\vec{E}\). Input electrodes, which may be much larger than
the cells themselves, immerse an input circuit in an input field \(E_y
\hat{y}\), in addition to the applied clocking field \(E_z \hat{z}\). The input
field selects the input bit on a field-sensitive portion of the circuit.
Another portion of the circuit with reduced \(E_y\)-sensitivity functions as a
shift register, transmitting the input bit to downstream QCA logic for
processing. It is shown that a simple rotation of the molecules comprising the
shift register makes them immune to unwanted effects from the input field or
fringing fields in the direction of the input field. Furthermore, the circuits
also tolerate a significant unwanted field component \(E_x \hat{x}\) in the
third direction, which is neither the clocking nor input direction. The
write-in of classical bits to molecular QCA circuits is a road-block that must
be cleared in order to realize energy-efficient molecular computation using
QCA. The results presented here show that interconnecting shift registers may
be designed to function in the presence of significant unwanted fringing fields
from large input electrodes. Furthermore, the techniques devloped here may also
enable molecular QCA logic to tolerate these same unwanted fringing fields.
