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TABLE 4 . 1 . Kink Energy Between Adjacent Cells
Cell Type
Cell Size
Kink Energy
Molecular QCA (
e
r
=1)
o
2nm
W
0.3 eV
Self-assembled (
e
r
=12.9)
5 nm
9.13meV
Lithographically defined (
e
r
=12.9)
10 nm
4.56meV
Lithographically defined (
e
r
=12.9)
20 nm
2.28meV
4.3.2. Cell Switching
Cell switching involves the change of electronic configuration (polarization) of the
cell, as illustrated in Figure 4.5. Switching is driven by the clock and the final state
is determined by the neighboring cells or possibly unwanted space charges [48].
Depending on the level of kink energy, a cell can be excited into one of the other
low lying states by unwanted thermal fluctuations [49, 50]. For this possibility to
be avoided, the kink energy should be much larger than the thermal ambient
energy, k
B
T.
The attainable switching speed of a QCA circuit depends primarily on the
particular choice of implementation. If the system is strongly coupled to the sub-
strate, inelastic processes will dominate and the switching speed will be determined
by the strength of the coupling between the cell and the substrate. However, if
elastic processes dominate the switching, the switching speed can be determined by
modeling the system using the Schro¨ dinger equation. Tougaw et al. showed that
the standard cell has a switching time as low as 2 ps [51] when operating in this
coherent regime. Table 4.2 lists the order of the theoretical cell switching
frequencies for different cell implementations reported in the literature.
4.4. GROUND STATE COMPUTING
QCA computing has often been referred to as ground state computation since the
design of QCA circuits involves finding a layout of cells where the ground state of
the layout for a particular set of boundary conditions provided by the inputs is the
solution to the designed logical function. When a suitable environment is
P=1
P=0.5
P=0
P=
−
0.5
P=
−
1
Time
Figure
4.5.
Cell switching from P=1 to P=1.
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