In devices known as quantum-dot cellular automata (QCA), the logic levels are represented by the spatial configuration of electrons, and coding of information in the device is controlled and manipulated by the position of a single electron. A clocked cell modulates the barrier between two quantum dots and allows the charge of the electron to be stored, or latched, into position, and thus provides a signal for the next cycle.
Using a device consisting of three quantum dots connected in series by tunnel junctions, Orlov et al. demonstrate a clocked QCA device in which the middle dot acts as a “latch” by forming a tunable barrier between the input and output dots controlled by clock pulses. The characteristics of the device are described by phase diagrams that illustrate the parameter space for device operation. Starting off in the null state (point A), when the clock signal applied to the middle dot is “low,” the device remains in the null (neutral) state, irrespective of the input signal (within operational limits). With a “high” clock signal, the device becomes active and switches to the logic 1 (point C) or logic 0 state (point B), and remains in this state when the input signal is set to zero or even reversed. With appropriately applied clock and input signals, the device can be programmed through a whole cycle of null-active-locked-active-null states. — ISO
Appl. Phys. Lett.78, 1625 (2001).