The Development of Quantum-Dot Cellular Automata - Field-Coupled Nanocomputing: Paradigms, Progress, and Perspectives - page 8

most important quantity in a logic device: power gain. Without power gain the input
signal would degrade in a line, due to the unavoidable energy dissipation at each
stage, and fan-out would be impossible. Clocking in QCA requires a variable barrier
to control the tunneling of electrons between dots. Since the tunnel barrier in metal dot
QCA is a fixed aluminum oxide layer whose barrier height cannot be modulated,
clocking dots are introduced into the QCA cell as intermediate dots. These dots are
coupled to clock electrodes that control the potential of the central dots. A positive
clock voltage pulls the electrons to the central dots to produce the null state. A
negative voltage forces the electrons to leave in a direction that is determined by the
cell's input. In the initial experiment, a differential input voltage is applied to the left
side of the cell and electrometers measure the potential of the top and bottom dots of
the right half of the cell. Measured output waveforms confirmed proper operation of
the cell [ 44 ]. A clocked QCA cell can also be used as a latch, a short-term memory
element, as demonstrated in our experiments [ 45 , 46 ].
As shown by theory, the power gain of a QCA cell is not fixed. If the input is
weak, the cell pulls power from the clock to restore the signal level. Since the signal
energy is fixed for a given cell, the amount of power pulled from the clock will depend
on the weakness of the input. An experimental demonstration of power gain involves a
measurement of the charge on the dots of the QCA cell [ 43 ] as the inputs and clock are
moved through one clock period. In this way the work done by the input on the cell
can be calculated, along with the work done by the cell on the next cell. If the work
done by the cell exceeds the work done on the cell, then the cell has demonstrated
power gain. In our experiment an input with one-half the normal potential swing was
applied to the input. The resulting experiment demonstrated a power gain of 2.07, in
agreement with theory [ 47 ].
Shift Registers
Clocking in QCA enables not only power gain, but also the control of the flow of
information in the computational system, needed for data pipelining. The basic element
in a data flow structure is the shift register. A QCA shift register consists of a row of
cells controlled by different clock phases. In our experiment we fabricated a shift
register of two cells. Although this is a very short shift register, it can be used as a long
register. For our experiment we fabricated the two clocked QCA cells with elec-
trometers coupled to each cell so that we could measure the polarization of each cell
independently. In the experiment a bit of information is latched into the first cell, and
the input removed. The bit is then copied into the second cell, and erased in the first.
Then the bit is copied back into the first cell and erased in the second. In this way the bit
is shifted between cells, just as it would be in a long shift register. The experiment
demonstrated 5 bit transfers, limited only by thermally induced errors [ 48 - 50 ].
Fan-Out
An important element in a general logic system is fan-out, where the output of one
element is sent to the inputs of two or more elements. Since the energy of the output is
split, power gain in the following logic elements is needed to restore the signal level.
To demonstrate fan-out in QCA we fabricated a circuit with three cells. In the