COMPUTING WITH QUANTUMDOT CELLULAR AUTOMATA - Bio-Inspired and Nanoscale Integrated Computing - page 122

cells. I/O for molecular implementations of QCA is significantly more difficult, and
there is currently no established method for I/O in these future molecular scale
devices. In the case of metallic island and semiconductor QCA, the I/O mechanism
is already established using single electron transistors (SETs) for reading the output
and electrodes for setting the input [13, 25, 26]. The SET is a highly sensitive
electrometer capable of detecting a fraction of the elementary charge present at its
gate electrode. SETs operate in the Coulomb blockade regime and generally have to
be cooled to extremely low temperatures. Significant research has been invested in
developing techniques to realize room temperature [58] and high frequency [59]
SETs since their application extends beyond sensitive electrometers.
Although magnetic QCA is still in the early stages of development, it appears
that the I/O mechanism for this implementation is relatively simple to construct.
At the input stage, a single wire element, through which a current is passed,
provides sufficient magnetization to cause its neighboring MQCA cell to relax to
one of the two polarizations [42]. At the output of the circuit a magnetization
sensor, based on the giant magnetoresistance (GMR) effect, or a magnetic tunnel
junction can be used to detect the magnetization of output cells.

4.7. QCA LOGIC

4.7.1. Inverter
The most common inverter design is shown in Figure 4.10. The kink energy
between two cells changes sign when the cells are oriented at 45

with respect to
each other. The output wire and the two legs of the inverter have this sign-change
coupling and the ground state of this device has complementary input and output.

1

4.7.2. Majority Gate
The fundamental logic primitive available with QCA technology is the majority
gate. This gate performs the following Boolean function:

Maj ð A ; B ; C Þ¼ AB þ AC þ BC :

1 0

Figure 4.10. Layout of a fork inverter. The two arms of the fork reinforce the
inverter function by doubling the coupling to the output cells.