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.cb Chemical Memories Storing a Bit in One or a few Molecules

	Avogadro's number, the number of molecules in a mole, is
6.023%8x%110%523%1.  If we could store a bit in a molecule, the
memory problem would be solved, so it seems worthwhile to ask
whether any fundamental laws of physics or chemistry prevent it.
The answer seems to be that nothing prevents such memory densities,
so it is just a question of trying to invent a suitable scheme.  Here
is one idea, but there are lots more.

	A memory register is a chain molecule.  These molecules are
in solution or in a solid, but they are not in any fixed array,
so that it is not necessary to keep track of their locations unlike
the registers in present memories.  They are addressed by broadcasting
light or microwave signals throughout the container.  They answer by
emitting a photon which is picked up by a photomultiplier.

	Each radical on the chain represents a bit of the address
of the register.  Each has several bits of state some of which are
represented geometrically, i.e. the different states are stereo
isomers of each other, and some temporary information is represented
by electronic excitation.  Transitions are brought about by signals
of particular wave lengths and are influenced by the state of the
radical and by the state of its neighbor to the "left".

	Thus each register is a kind of cellular automaton of a new
sort.  The novelty is that a cell is affect by both its neigbors
and by global signals.  Let us discuss first what kind of cellular
automaton we want before discussing how it might be realized chemically.

Reading

	The register is a linear chain of cells, each storing one bit.  The
cells are identical except that the two end cells are special.  The
global signal has three values - 0, 1, and space.  A cell may %2contain%1
a 0 or a 1, it may be excited or not, and it may be ready or
not, i.e. it has eight states.  A space signal
makes a cell ready and any other signal makes it unready.  If a cell
is unready, neither a 0 nor a 1 has any effect.  If a cell is ready
and unexcited and its left neighbor is excited and if the signal received
(0 or 1) agrees with its contents (0 or 1), then it becomes excited and
unready.  Otherwise, it becomes unexcited and unready.

	A cell at the left end of a chain becomes excited if an only if
it receives a special "start" signal.  It is then unready.  A cell at the
right end of a chain becomes excited when its left neighbor is excited
and emits an output signal that depends on whether it "contains" 0 or 1.

	Now suppose that a signal consisting of "start" followed by
a sequence of 0's and 1's separated by spaces is given to a collection
of registers.  As long as the input sequence of 0's and 1's agrees
with the sequence of bits contained in the cells of the register, the
signal propagates along the register, but as soon as there is a discrepancy,
the signal dies.  If it reaches the end, the output signal corresponding
to the contents of the register is emitted.

	It is possible to store many bits in a register by providing
an output sequence of cells along which the signal can further propagate
and each of which emits a signal that depends on whether it contains
a 0 or 1.  The signals that cause propagation can be of one kind only,
e.g. all 0's, but they must again be punctuated by spaces.

	Now consider the chemistry of possible realizations of this
scheme.  We tentatively suppose that whether a cell %2contains%1 0 or 1
is represented by an isomeric state of the cell.  Readiness and
excitedness are higher quantum states of parts of the cell.  The
output signal is the emission of a photon of an energy depending
on the bit stored.  The photons are detected by photomultipliers.
Since the detection is not perfectly efficient, we must either
excite the end cells repeatedly or have many registers with the same
information.  The former seems preferable.

	The big chemical problem is designing the cells so that their
excitability by the photons representing 0 and 1 depends appropriately
on their readiness, their contents and on the excitation of their
left neighbors.  It should not depend on their right neighbors.

Writing

	With so much memory, it seems as though we should settle for
a "write once" memory.  Both the address and the contents are written.
Again the writing should require the left neighbor to be in an excited
state, so that the writing will propagate along the register.  Again
there should be a "start writing" signal, but it should be weak enough so
that it excites the left cells of only about the desired number of
unwritten registers.  A register that has previously been written should
have its start cell in a state that is immune to further "start writing"
signals.  It would be best if the writing could be done in a memory
box that contained written registers, but if necessary, we could imagine
that registers are written separately and then added to the memory box.
Since it would involve physical motion, it should be avoided if possible.

	We want the 0's and 1's in the memory to be very stable
so that they don't make spontaneous transitions, and this suggests
stereo isomers.  The unwritten state should be a third state of the
cell that can make a transition to a 0 or 1 state, but requires for
this transition that the left neighbor be already written.

	Except for the final fluorescence which takes a few nanoseconds,
each of the other steps can apparently be accomplished in a few
picoseconds if it is desired to read and write that fast.

	I don't understand the chemical problems of realizing this
kind of memory cell, but if this doesn't work, other schemes can
be devised.

.nofill
.turn on "∂"

∂(40)John McCarthy

This draft of CHEMEM[W79,JMC] pubbed at {time} on {date}.