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Table 3.1 State of the logic
scheme with several inputs
and outputs controlled by
soliton switching
Channel
1
2
3
4
Chain 1
1
0
1
1
Chain 2
1
1
0
1
Chain 3
1
1
1
0
presented in Fig.
3.25
which shows two conjugated chains and two different
chromophores. The configuration of the chain 1 is such that the first chromophore
does not absorb light, while the second one may be photoexcited. Passage of a
soliton along the chain 1 will switch on the first chromophore and switch off the
second one. A soliton traveling along the chain 2 will switch off both
chromophores.
More complex switching systems with a sufficiently large number of inputs and
outputs can be built based on the same principles. Figure
3.25
shows an example of
such a system built from several chains
A
D
joined into a common
system by strings of conjugated bonds. Here
A
,
C
, and
D
are the electron acceptor,
the intermediate group, and the electron donor, respectively. In this system, as in the
case of the NAND element considered above, Carter proposed to use the tunneling
transition of the electron. Soliton channel switching (I-IV) can be carried out
through three channels of polyene chains (horizontal directions in the figure).
Note that each group (
A
,
C
,
D
) separated from each other by a dotted line can be
attributed to different states, depending on the distribution of single and double
bonds between them. Variants of the system states are shown in Table
3.1
. Here
unity denotes the presence of a double carbon-carbon bond between
A
and
C
,
C
and
C
, and
C
and
D
in the corresponding channel. It is easy to see that the passage of the
electron along the vertical chain
A
C
C
...
...
...
D
in Fig.
3.25
is only possible with all unities
in the corresponding table columns. For example, the fragment
C
is disconnected
from in the channel II by the chain 1, while the fragment
C
is disconnected from the
second fragment
C
in the channel III by the second chain. The system state changes
significantly during the passage of a soliton along the corresponding chain.
The soliton switching mechanism can serve as the basis for implementing more
complex logic functions. Thus, the systems shown in Fig.
3.26
can perform quite
complex operations. The structure depicted in this figure consists of a carbon atom
linking three semi-infinite trans-polyacetylene chains. Assuming that the presence
of two successive single bonds blocks further propagation of the soliton, one can
see that the soliton propagation along one of the chains will shift and redistribute
the single and double bonds, as if rotating the system around the central carbon
atom. In such a case, each soliton propagation corresponds to the group operation—
rotation by 120
—blocking the propagation of the soliton to the other chain (figure,
soliton propagation between the left and the bottom chain is impossible). A cyclic
system combining three fragments of type
a
(Fig.
3.26
) ensures the execution of
group operations corresponding to the relevant point group
D
2
. Similar operations
are performed by the system shown in Fig.
3.26c
.
While the simple system (a) has three different states, the cyclic configuration of
three such systems (b, c) has four different states, and the configuration shown in
...
