
THE COMPUTATIONAL UNIVERSE SETH LLOYD:
I'm a professor
of mechanical engineering at MIT. I build quantum computers that store
information on individual atoms and then massage the normal interactions
between the atoms to make them compute. Rather than having the atoms
do what they normally do, you make them do elementary logical operations
like bit flips, not operations, andgates, and orgates. This allows
you to process information not only on a small scale, but in ways that
are not possible using ordinary computers. In order to figure out how
to make atoms compute, you have to learn how to speak their language
and to understand how they process information under normal circumstances.
A few
years ago I wrote a paper in Nature called "Fundamental Physical
Limits to Computation," in which I showed that you could rate the information
processing power of physical systems. Say that you're building a computer
out of some collection of atoms. How many logical operations per second
could you perform? Also, how much information could these systems register?
Using relatively straightforward techniques you can show, for instance,
that the number of elementary logical operations per second that you
can perform with that amount of energy, E, is just E/H  well, it's
2E divided by pi times hbar. [hbar is essentially 10[34] (10 to the
34) Jouleseconds, meaning that you can perform 10[50] (10 to the
50) ops per second.)]If you have a kilogram of matter, which has mc2
— or around 10[17] Joules (10 to the 17) Joules — worth of
energy and you ask how many ops per second it could perform, it could
perform 10[17] (ten to the 17) Joules / hbar. It would be really spanking
if you could have a kilogram of matter — about what a laptop computer
weighs — that could process at this rate. Using all the conventional
techniques that were developed by Maxwell, Boltzmann, and Gibbs, and
then developed by von Neumann and others back at the early part of the
20th century for counting numbers of states, you can count how many
bits it could register. What you find is that if you were to turn the
thing into a nuclear fireball — which is essentially turning it
all into radiation, probably the best way of having as many bits as
possible — then you could register about 10[30] (10 to the 30)
bits. Actually that's many more bits than you could register if you
just stored a bit on every atom, because Avogadro's number of atoms
store about 10[24] (10 to the 24) bits. 