Information Technology Reference
In-Depth Information
First of all, it was realized that due to their quantum nature small molecules
consisting of a relatively small number of atoms could not be expected to act as
deterministic switches analogous to semiconductor devices.
The theoretical feasibility of information-logic devices at the molecular level is
only helpful in determining general criteria rather than specific rules for the
selection of molecular elements for building devices with desired functions.
In order to explain specific difficulties in solving this problem, let us consider
one possible option when individual molecules are regarded as logic elements, with
control of molecules and transmission of information carried out by monochromatic
light and optical communication.
It is known that an information processing system of arbitrary complexity can be
constructed using logic elements of only three types: NOT, AND, and OR.
At first glance, many atomic or molecular systems with their discrete energy
levels are ideally suited as the basic logic elements. Let us consider, for example,
the processes of selective excitation of electronic or vibrational states of the atom
(ion) or polyatomic molecule by monochromatic radiation. Let us assume that
initially only the ground state is populated in the atom or the molecule and that
the excitation of a particular level by a quantum of radiation occurs with a high
(almost complete) probability. Naturally, we assume that the transitions between
these states are allowed.
Figure
3.27
shows primitive examples of possible implementations of basic
logic functions at the molecular level.
It is easy to see (Fig.
3.27
) that the possibilities of excitation of the CO
2
molecule, which is situated in the ground vibrational state (0, 0, 0), into one of
the excited states (0, 1, 0) or (0, 0, 1) are mutually exclusive over the lifetime of the
excited state. This process can be compared with the NOT operation. Excitation of
the chromium ion in a ruby crystal by radiation with a frequency
ʽ
1
or
ʽ
2
leads to the
ʽ
ˆ
(the OR element). Selective two-step
excitation of the ground singlet electronic state
S
0
of the rhodamine molecule by
joint action of two ultrashort pulses with the frequencies
emission of a photon with the frequency
ʽ
1
and
ʽ
2
simulates the
logical operation AND.
Nevertheless, despite the fact that the possibilities of using individual atoms and
molecules as basic elements in computational devices seem obvious, fairly simple
considerations reveal a number of discrepancies between the characteristics of
atomic and molecular systems and the requirements imposed on the basic elements.
Thus:
- Basic elements should have high reliability (probability of response) upon
control activation.
- The average power of element's reaction transmitted to another element should
not be much smaller than the average power of the input stimulus applied to the
element. In our case this means that the number of quanta per unit of time at the
input of the element should not substantially exceed their number at the output.
Otherwise, the probability of element response after combining them in a chain
will decrease with the distance from the beginning of the chain.
