/FONT#0=BASL30/FONT#1=BASI30/FONT#2=BASB30/FONT#3=SUP
␈↓␈↓↓␈↓α␈↓β␈↓
␈↓ ↓H
MOLECULAR MEMORIES
␈↓ ↓H

␈↓ ↓H

␈↓ ↓H
␈↓ α_The␈α⊃ultimate␈α⊃in␈α⊃memory␈α⊃is␈α⊃to␈α⊃store␈α⊃information␈α⊃in␈α⊃single␈α⊂molecules.␈α⊂Then␈α⊂a␈α⊂mole␈α⊂of␈α⊂the
␈↓ ↓H
␈↓ ↓Hsubstance␈αused␈αcould␈αstore␈α6␈αx␈α10␈↓β23␈↓␈αunits␈αof␈αinformation,␈αwhere␈αthe␈αunit␈αmight␈αbe␈αa␈αbit␈αor␈αmight␈αbe
␈↓ ↓H
␈↓ ↓Hmore.␈α
My␈α
interest␈α
in␈α
the␈α
matter␈α
came␈α
from␈α
trying␈α
to␈α
set␈α
an␈α
upper␈α
limit␈α
on␈α
information␈α
density␈α
and␈α
so
␈↓ ↓H
␈↓ ↓Htrying␈α∞to␈α∞prove␈α∞that␈α∞single␈α∞molecules␈α∞could␈α∞not␈α∞be␈α
used,␈α
and␈α
finally␈α
coming␈α
to␈α
the␈α
conclusion␈α
that
␈↓ ↓H
␈↓ ↓Hthey␈α∀could.␈α∀Whether␈α∀such␈α∀a␈α∀memory␈α∀could␈α∀have␈α∀practical␈α∀speed␈α∀is␈α∀still␈α∀questionable␈α∀but␈α∪an
␈↓ ↓H
␈↓ ↓Hinteresting␈α
question.
␈↓ ↓H

␈↓ ↓H
␈↓ α_One␈αapproach␈αto␈αa␈αmolecular␈αmemory␈αis␈αto␈αuse␈αa␈αregular␈αsolid,␈αsay␈αa␈αcrystal␈αof␈αsome␈αsort,␈αand
␈↓ ↓H
␈↓ ↓Hhave␈α⊂a␈α⊂state␈α∂change␈α∂in␈α∂each␈α∂element␈α∂of␈α∂the␈α∂crystal␈α∂used␈α∂to␈α∂store␈α∂information.␈α∂I␈α∂can't␈α∂Addressing
␈↓ ↓H
␈↓ ↓Hwould␈α∪involve␈α∪locating␈α∪the␈α∪specific␈α∪molecule␈α∪and␈α∪reading␈α∪or␈α∪changing␈α∩it␈α∩without␈α∩affecting␈α∩its
␈↓ ↓H
␈↓ ↓Hneighbors.␈α
prove␈α
this␈α
to␈α
be␈α
impossible,␈α
but␈α
I␈α
haven't␈α
been␈α
able␈α
to␈α
think␈α
of␈α
a␈α
way␈α
of␈α
doing␈α
it.
␈↓ ↓H

␈↓ ↓H
␈↓ α_I␈α
have␈α
thought␈α
of␈α
one␈α
bit-per-molecule␈α
memory,␈α
and␈α
I␈α
want␈α
to␈α
describe␈α
enough␈αof␈αit␈αto␈αgive
␈↓ ↓H
␈↓ ↓Hthe␈αgeneral␈αidea.␈α
However,␈α
it␈α
is␈α
not␈α
in␈α
a␈α
state␈α
to␈α
be␈α
regarded␈α
as␈α
a␈α
rival␈α
for␈α
development␈α
memory,␈α
so␈α
I
␈↓ ↓H
␈↓ ↓Hwould␈α∞like␈α∞it␈α∞to␈α
be␈α
considered␈α
as␈α
an␈α
argument␈α
that␈α
a␈α
long␈α
range␈α
research␈α
program␈α
aimed␈α
at␈α
large
␈↓ ↓H
␈↓ ↓Hmemories␈α⊂should␈α⊂encourage␈α⊂and␈α⊂organize␈α⊂some␈α∂brain-storming␈α∂on␈α∂molecule-per-bit␈α∂memories␈α∂and
␈↓ ↓H
␈↓ ↓Hshould␈α∞fund␈α∞any␈α∞plausible␈α∞proof-of-concept␈α∞projects␈α
if␈α
they␈α
aren't␈α
too␈α
expensive.␈α
Let␈α
me␈α
point␈α
out
␈↓ ↓H
␈↓ ↓Hthat␈α
any␈αdevice␈αthat␈αcan␈αdeal␈αwith␈αsingle␈αmolecules␈αin␈αwhole␈αmoles␈αwill␈αstretch␈αthe␈αideas␈αof␈αwhat␈αis
␈↓ ↓H
␈↓ ↓Hscientifically␈α∂possible,␈α∂even␈α∂if␈α∂the␈α∂device␈α∂itself␈α∂is␈α∂not␈α∂of␈α∂immediate␈α∂practical␈α∂value.␈α∂This,␈α∂in␈α∂turn,
␈↓ ↓H
␈↓ ↓Hwould␈α
start␈α
people␈α
thinking␈α
about␈α
other␈α
ways␈α
of␈α
dealing␈α
with␈α
single␈α
molecules␈α
in␈α
this␈α
or␈α
other␈α
fields.
␈↓ ↓H

␈↓ ↓H
␈↓ α_At␈α∩this␈α∩hour,␈α∩I␈α∩have␈α∩time␈α∩to␈α∩deal␈α⊃only␈α⊃with␈α⊃reading,␈α⊃but␈α⊃writing␈α⊃involves␈α⊃similar␈α⊃ideas.
␈↓ ↓H
␈↓ ↓HInformation␈αis␈αrepresented␈αin␈αa␈αchain␈αmolecule,␈αand␈αeach␈αmolecule␈αcontains␈αits␈αaddress␈αas␈α
a␈α
string␈α
of
␈↓ ↓H
␈↓ ↓H0s␈αand␈α1s,␈αand␈α
each␈α
bit␈α
is␈α
physically␈α
represented␈α
by␈α
one␈α
of␈α
two␈α
radicals␈α
at␈α
the␈α
corresponding␈α
point␈α
in
␈↓ ↓H
␈↓ ↓Hthe␈α⊃chain.␈α⊃In␈α⊃this␈α⊃simplest␈α⊂form,␈α⊂there␈α⊂is␈α⊂only␈α⊂one␈α⊂bit␈α⊂of␈α⊂information␈α⊂in␈α⊂the␈α⊂molecule␈α⊂which␈α⊂is
␈↓ ↓H
␈↓ ↓Hrepresented␈α⊂by␈α⊂one␈α⊂of␈α⊂two␈α∂radicals␈α∂(not␈α∂necessarily␈α∂from␈α∂the␈α∂same␈α∂two)␈α∂at␈α∂the␈α∂end␈α∂of␈α∂the␈α∂chain.
␈↓ ↓H
␈↓ ↓HBetween␈αeach␈αtwo␈αinformation␈αradicals␈αis␈αa␈αpunctuation␈αradical,␈αalways␈αthe␈αsame␈αbut␈αdifferent␈αfrom
␈↓ ↓H
␈↓ ↓Hthe␈α∪information␈α∪radicals.␈α∪(You␈α∪might␈α∪imagine␈α∪that␈α∪the␈α∪molecules␈α∪are␈α∩DNA␈α∩or␈α∩proteins,␈α∩but␈α∩I
␈↓ ↓H
␈↓ ↓Hcontemplate␈α
a␈α
method␈αof␈αreading␈αand␈αwriting␈αthat␈αdoesn't␈αdepend␈αon␈αusing␈αenzymes,␈αi.e.␈αa␈αcomplex
␈↓ ↓H
␈↓ ↓Hmolecule␈α∞that␈α∞moves␈α∞along␈α∞the␈α∞chain.␈α∞[Hmm.␈α∞A␈α∞simple␈α
radical␈α
that␈α
was␈α
attached␈α
to␈α
each␈α
molecule
␈↓ ↓H
␈↓ ↓Hand␈α∞moved␈α∞along␈α∞the␈α∞chain␈α∞might␈α∞do␈α∞what␈α∞I␈α∞want,␈α∞but␈α
that␈α
is␈α
a␈α
different,␈α
though␈α
possibly␈α
better
␈↓ ↓H
␈↓ ↓Hidea])
␈↓ ↓H

␈↓ ↓H
␈↓ α_The␈αmemory␈αconsists␈αof␈αa␈αlarge␈αnumber␈αof␈αmolecules␈α-␈α
up␈α
to␈α
some␈α
fraction␈α
of␈α
a␈α
mole␈α
-␈α
in␈α
a␈α
jar
␈↓ ↓H
␈↓ ↓Hof␈αwater.␈αNo␈αattempt␈αis␈αmade␈αto␈αkeep␈αtrack␈αof␈αthe␈αlocation␈αof␈αa␈α
particular␈α
molecule.␈α
There␈α
should␈α
be
␈↓ ↓H
␈↓ ↓Honly␈α
one␈α
molecule␈α
with␈α
a␈α
given␈α
address.␈α
At␈α
least,␈α
if␈α
there␈α
are␈α
several,␈α
they␈α
must␈α
all␈αhave␈αthe␈αsame
␈↓ ↓H
␈↓ ↓Hdata␈α
radical␈α
on␈α
the␈α
end.
␈↓ ↓H

␈↓ ↓H
␈↓ α_Addressing␈α∞is␈α∞carried␈α∞out␈α∞by␈α∞sequencing␈α∞the␈α∞system␈α∞among␈α
three␈α
macroscopic␈α
physical␈α
states
␈↓ ↓H
␈↓ ↓Hcalled␈α∂␈↓↓zero,␈α∂one,␈α∂and␈α∂comma␈↓.␈α∂Between␈α∂each␈α∂two␈α∂address␈α∂bits␈α∂(␈↓↓zero␈↓␈α∂or␈α∞␈↓↓one␈↓)␈α∞is␈α∞a␈α∞␈↓↓comma␈↓,␈α∞so␈α∞a␈α∞typical
␈↓ ↓H
␈↓ ↓Hsequence␈αis␈α"0,1,0,0,0,1,1"␈αetc.␈αImagine␈αeach␈αphysical␈αstate␈αto␈αconsist␈αof␈αirradiating␈αthe␈αsolution␈αwith␈αa
␈↓ ↓H
␈↓ ↓Hmicrowave␈α∂frequency␈α∂(from␈α∂a␈α∂maser)␈α∞that␈α∞will␈α∞stimulate␈α∞a␈α∞particular␈α∞reaction,␈α∞but␈α∞there␈α∞are␈α∞other
␈↓ ↓H
␈↓ ↓Hpossibilities.
␈↓ ↓H

␈↓ ↓H
␈↓ α_Besides␈α∞the␈α∞information␈α∞carrying␈α∞molecules␈α∞in␈α∞the␈α∞solution,␈α∞there␈α∞is␈α
a␈α
␈↓↓substrate␈↓␈α
consisting␈α
of
␈↓ ↓H
␈↓ ↓Hmolecules␈αthat␈αcan␈αattach␈αthemselves␈αto␈αthe␈αaddress␈αradicals␈αto␈α?orm␈αa␈αside␈αchain.␈αThe␈αcondition␈αfor
␈↓ ↓H
␈↓ ↓Hattachment␈αof␈αa␈αsubstrate␈αmolecule␈αto␈αa␈α0␈αaddress␈αbit␈α
is␈α
that␈α
the␈α
physical␈α
state␈α
be␈α
␈↓↓zero␈↓␈α
and␈α
there␈α
be␈α
a
␈↓ ↓H
␈↓ ↓Hsubstrate␈αmolecule␈αattached␈αto␈αthe␈αpunctuation␈αradical␈αon␈αthe␈αleft.␈α
The␈α
condition␈α
for␈α
attachment␈α
of␈α
a
␈↓ ↓H
␈↓ ↓Hsubstrate␈α⊂molecule␈α⊂to␈α⊂a␈α⊂1␈α⊂address␈α⊂bit␈α⊂is␈α⊂that␈α⊂the␈α⊂physical␈α⊂state␈α⊂be␈α⊂␈↓↓one␈↓␈α⊂and␈α⊂there␈α∂be␈α∂a␈α∂substrate
␈↓ ↓H
␈↓ ↓Hmolecule␈α⊂attached␈α⊂to␈α⊂the␈α⊂punctuation␈α⊂radical␈α⊂on␈α⊂the␈α⊂left.␈α∂The␈α∂condition␈α∂for␈α∂the␈α∂attachment␈α∂of␈α∂a
␈↓ ↓H
␈↓ ↓Hsubstrate␈α⊃molecule␈α⊃to␈α⊃a␈α⊃punctuation␈α⊃radical␈α⊃is␈α⊂that␈α⊂the␈α⊂physical␈α⊂state␈α⊂be␈α⊂␈↓↓comma␈↓␈α⊂and␈α⊂there␈α⊂be␈α⊂a
␈↓ ↓H
␈↓ ↓Hsubstrate␈α⊃molecule␈α⊃attached␈α⊃to␈α⊃the␈α⊃information␈α⊃radical␈α⊃to␈α⊃the␈α⊃left.␈α⊃The␈α⊃effect␈α⊃of␈α⊂the␈α⊂addressing
␈↓ ↓H
␈↓ ↓Hsequence␈α
will␈α
be␈α
to␈α
attach␈α
a␈α
side␈α
chain␈αof␈αsubstrate␈αmolecules␈αto␈αthe␈αaddress␈αchains␈αthat␈αmatch␈αthe
␈↓ ↓H
␈↓ ↓Hphysical␈α
states␈α
as␈α
they␈α
occur.␈α
Only␈α
the␈α
word␈α
addressed␈α
will␈α
have␈α
a␈α
full␈α
chain␈α
attached␈α
to␈α
it.
␈↓ ↓H

␈↓ ↓H
␈↓ α_Once␈α
addressing␈α
is␈α
complete␈α
there␈α
are␈α
several␈α
ways␈α
of␈α
proceeding.␈α
One␈α
is␈α
to␈α
have␈α
two␈α
more
␈↓ ↓H
␈↓ ↓Hsubstrate␈αtypes␈αof␈αmolecules,␈αone␈αof␈αwhich␈αstarts␈αa␈αchain␈αreaction␈α
from␈α
the␈α
end␈α
of␈α
a␈α
side␈α
chain␈α
and␈α
a
␈↓ ↓H
␈↓ ↓H0␈αinformation␈αradical␈αand␈αthe␈αother␈αstarts␈αa␈αchain␈αreaction␈αfrom␈αa␈α1␈αinformation␈αradical.␈αThe␈αchain
␈↓ ↓H
␈↓ ↓Hreactions␈α⊂are␈α⊂allowed␈α⊂to␈α⊂proceed␈α∂until␈α∂a␈α∂macroscopically␈α∂detectable␈α∂effect␈α∂occurs␈α∂-␈α∂for␈α∂example,␈α∂a
␈↓ ↓H
␈↓ ↓Hmolecular␈α
resonance.
␈↓ ↓H

␈↓ ↓H
␈↓ ↓HOne␈α
of␈α
two␈α
resonances␈α
occurs␈α
according␈α
to␈α
whether␈α
the␈α
information␈α
bit␈α
is␈α
a␈α
0␈α
or␈α
1.
␈↓ ↓H

␈↓ ↓H
␈↓ α_Finally,␈α⊂as␈α∂a␈α∂cleanup,␈α∂the␈α∂system␈α∂is␈α∂put␈α∂into␈α∂a␈α∂physical␈α∂state␈α∂that␈α∂causes␈α∂all␈α∂the␈α∂side␈α∂chain
␈↓ ↓H
␈↓ ↓Hbonds␈α⊂to␈α⊂break␈α⊂and␈α∂the␈α∂product␈α∂of␈α∂the␈α∂chain␈α∂reaction␈α∂to␈α∂decompose␈α∂so␈α∂that␈α∂we␈α∂are␈α∂back␈α∂to␈α∂the
␈↓ ↓H
␈↓ ↓Horiginal␈α
state.
␈↓ ↓H

␈↓ ↓H
␈↓ α_Well,␈α∂this␈α∂complicated␈α∂mechanism␈α∂may␈α∂seem␈α∂unlikely␈α∂to␈α∂work␈α∂exactly␈α∂as␈α∂described,␈α∂but␈α∂the
␈↓ ↓H
␈↓ ↓Hreader␈αwho␈αinclines␈αto␈αthe␈αidea␈αthat␈αit␈αis␈α
impossible␈α
to␈α
make␈α
a␈α
molecule-per-bit␈α
memory␈α
should␈α
try␈α
to
␈↓ ↓H
␈↓ ↓Hprove␈αit␈αimpossible.␈αIn␈αthe␈αcourse␈αof␈αtrying␈αto␈αdebug␈αhis␈αproof,␈αhe␈αmay␈αinvent␈αa␈αbetter␈αscheme␈αthan
␈↓ ↓H
␈↓ ↓Hthis␈α
one.
␈↓ ↓H

␈↓ ↓H
␈↓ α_[Returning␈α
to␈α
the␈α
bracketed␈α
afterthought,␈α
it␈α
may␈αbe␈αeasier␈αto␈αmake␈αa␈αradical␈αmove␈αdown␈αthe
␈↓ ↓H
␈↓ ↓Hchain␈α
in␈α
accordance␈α
with␈α
the␈α
addressing␈α
sequence␈α
of␈α
physical␈α
states␈α
than␈α
to␈α
build␈α
a␈α
side␈α
chain.]
␈↓ ↓H