Quantum Computers

John K Clark (johnkc@well.com)
Tue, 28 Jan 1997 10:19:41 -0800 (PST)


Very recently there have been two new developments in the world of Quantum
Computers. In the January 17 1997 issue of Science is a long article called
"Bulk Spin-Resonance Quantum Computation" by Neil A Gershenfield and Issac
L Chuang, there is also an editorial about the article in the same issue.
It talks about hardware methods of isolating a Quantum Computer from the
environment so you don't get Quantum decoherience and suggests using Nuclear
Magnetic Resonance (NMR). The nucleus of atoms are like spinning bar magnets,
and they can be in 2 states, spin up or spin down. If you send in a radio
pulse of the exact frequency you can change the spin state of the atom from
one to another. This frequency can change if an atom is coupled to another
atom, so if you pick the correct frequency you could flip the spin of a
carbon atom if and only if the Hydrogen atom it is coupled with is spinning
up. This could make a controlled logic NOT gate.

This would also be a Quantum logic gate, because a radio pulse can also put
an atom in a superposition of states, both spin up and spin down at the same
time, and anything coupled to that state would also be in a superposition of
states. There are 4 advantages to using NMR to make a Quantum computer rather
than other methods:

1) The quantum state of interest is in the nucleus and is protected from
contamination from the outside world by the electron cloud so the quantum
state lasts a long time. In most other quantum systems decoherence happens
in pico or femtoseconds but with NMR it can last for thousands of seconds.

2) You don't have to worry about every single atom, if only 50.000001% point
up, that's good enough. I quote Tim Havel, who independently discovered
most of the thing in the article at the about the same time, "It turns out
having a well-defined statistical excess of a single quantum state is
enough to do quantum computing. You don't need to have every molecule
doing exactly the same thing".

3) By using a large number of atoms to store a qbit it further protects it
against unwanted external interactions.

4) You don't need expensive exotic gadgets, cheap off the shelf equipment
will do for NMR.

The authors actually hope to have a small (10 qbit) quantum computer in
operation in a few years, possibly as soon as next summer. With such a
machine you could factor (drum roll please) the number 15. Hey don't laugh,
it would be a proof of concept and could serve as a test bed to develop
better quantum algorithms, and remember, each time you add a qbit to your
computer you double its power.

Speaking of better quantum algorithms, we'll need some to go much higher than
30 qbits using NMR, and to quote Gershenfield "Somewhere between 50 and 100
qbits is where calculations start to get interesting". Still, people are
optimistic, to quote Gershenfield again "If researchers can figure out how to
extend the technology to larger qbit numbers the future is unlimited.

In a separate development, on January 27 researchers at Xerox announced that
they have made a small molecule of manganese, oxygen, carbon and hydrogen
that acts like a powerful magnet. They said it might be possible to use this
material to make a disk drive that could store information at the molecular
level and hold millions of times as much data as we can today, but that's not
what they emphasized. I think it's interesting that at the announcement they
spent more time talking about how it could be used in a Quantum computer
rather than a conventional one. The material has a property called Quantum
Magnetic Hysteresis, it can be in a superposition of many different magnetic
states at the same time, and that's just what a Quantum Computer is all about.

For me It's still hard for me for it to sink in that an otherworldly object
like a Quantum computer could actually exist in our worldly universe, but
it's beginning to look like one can really be built. If so the world will
never be the same.

John K Clark johnkc@well.com

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