Most people have never heard of quantum computers. If they
have, their knowledge of what one is, and can do, likely derives from science
fiction novels or breathless accounts in the popular press.
We're in the latter category, but the subject seemed
sufficiently intriguing that we wanted to take a stab at separating fact from
fiction. Is it really possible that something you can't even see could have
more computing power than all the desktops in the world, put together?
As with so many things in the very peculiar world of
atomic physics, the answer is both yes and no.
In order to get the lowdown on quantum computers, it made sense to us to
visit a physicist--Professor Olivier Pfister of the University of Virginia--who
is actually building one.
Pfister's first words are: "The thing about quantum computers
is that they don't exist or, rather, they exist in a very primitive state.
They're really stupid. There's one that can factor 15, which means reduce it to
the product of two prime numbers, 3 and 5. That's about it."
Even envisioning what to do with a better quantum
computer--if we could make one, which we can't--is daunting. There are only two
or three algorithms that we know of, he says, where quantum computing is faster
than classical computing. Factoring is one of them.
All public key encryption systems are based on the product
of two extremely large prime numbers, and depend for their security on the
difficulty of factoring their keys. They're hard to crack because a classical
computer has to keep dividing by the next highest prime until it finds the
right one, a process that can take months or years. A quantum computer could do
it in a couple of minutes, making present encryptions useless.
So, are we spending countless millions of dollars of
research money on a super code breaker that could bring down our entire
financial system?
Pfister laughs. Yes, that's where we are at the moment, he
admits. But what keeps him in the field, as an experimenter, is the prospect of
a computer that could "solve physical problems that we can't solve right now."
Okay, we say, taking a deep breath, can you describe for a
lay audience what a quantum computer is, and how it works? Well, he's game if
we are.
The extraordinary potential of a quantum computer lies in
its ability to perform exponentially faster than an ordinary one. Take
factoring. We can factor 15 in our heads because it's a small number. But as
the number you want to factor gets larger, it begins to take an exponentially
longer time to solve the problem.
Classical computers are limited by the number of
transistors you can cram into a given space. Those transistors switch between
on and off, or 0 and 1, thus giving us the "bits" which lie at the heart of our
present-day binary computing system. All calculations have to be carried out
step by step, one at a time.
In addition, as transistors get smaller and smaller, they
will eventually approach the mysterious line that divides the macroscopic world
from the microscopic world of quantum mechanics. If transistors reach the size
of a few atoms, Pfister says, "they're not going to work like transistors
anymore. They'd be subject to quantum mechanics, and a lot of strange things
would start happening."
A quantum computer, conversely, already inhabits the
quantum world, and because it does, it could perform many, many computations
simultaneously. Its primary constituent is called a "qubit," which is a single
atom. (Photons might be used instead of atoms--in fact, that's the area where
Pfister is working--but describing photon-based computers is well beyond our abilities.)
Qubits are significantly different from transistor-based
bits, because each can exist in two states at once. So, as you add qubits to
your array, your computing power doubles with each new one (i.e., becomes 2^{n},
where n = number of qubits). Thus, with 10 classical computing bits,
you do 10^{1} calculations simultaneously = 10; with 10 qubits, you
could do 2^{10}, or 1,024 calculations. Make it 100 qubits, and you
could do 100 billion billion billion.
This extraordinary compounding effect is due to two
things. The first is superposition, a concept that was one of the core
discoveries of quantum mechanics. In the most simplified description,
superposition is the ability of an atom to be in two separate states
simultaneously. (We can't see this happening, but we know it does because we
can and do measure its effects.)
By state, the physicist means something rather too
complicated to go into here. But, again, we can simplify matters by saying that
an atom can be either in an excited state or a de-excited (ground) state. A
single atom in ground state becomes excited when it is hit by a photon and
absorbs that photon's energy. Somewhat counterintuitively, it emits a photon
and de-excites when struck by a second photon. And so on, indefinitely.
That's "the quantum gate," Pfister says. "You send a
photon each time you want to change the state."
A change of state takes place in about a nanosecond (10^{-9}
seconds). Having a switch that changes every nanosecond would be nice all by
itself, but superposition makes it twice as nice. It allows you to compound the
effect exponentially.
In order to take advantage of this, you have to tap into
the second core quantum principle, entanglement. First described by
Einstein in the 1930s, entanglement occurs when two or more closely associated
atoms are held in the same state of superposition at the same time. It turns
out that when one entangled atom changes state, the other does, too,
simultaneously. There is a predictable, measurable correlation, and it always
happens. Theoretically, you could entangle two, ten, a thousand or more atoms,
and the more you have, the more the power of your computer doubles and
re-doubles.
Bizarrely, once entangled, atoms stay that way, even when they are moved far
from their original proximity. A group in Switzerland, Pfister says, has
separated entangled particles by 60 kilometers, and the correlation still holds.
Imagine the possibilities if your quantum computer were
entangled with a thousand others scattered across the globe. Of course, if we
could do that, the world would be a very different place. We can't--yet--because
it's hard.
Merely holding one atom in superposition is a
daunting task. It must be totally isolated from its environment; otherwise, decoherence
occurs, whereby the atom reverts to its normal condition, which includes the
spontaneous emission of absorbed energy. Unless spontaneous emission is ruled
out, all measurements become meaningless. One stray photon, and you've got a
quantum computer crash, so to speak.
Such a high degree of control is incredibly difficult to
achieve, Pfister says, but "you can do it if you place a single atom between
two mirrors that are very, very close, 10 micrometers apart. Only a few groups
in the world today have that ability."
Once you get there, though, the atom responds to the laser
beam striking it, and to nothing else, and you can measure its state by
tracking the photon output, which can change every nanosecond. Presto, you have
a computer. Now, all we have to do is await the day when someone discovers how
to string a bunch of such atoms together...
Before taking our leave, we want to touch briefly on the
subject of parallel universes, which have received imaginative treatments by
science fiction writers, and occupied the thoughts of more than a few
mainstream physicists, as well.
In the theory of parallel universes, superposition is
explained, not by something being in two different states at the same time, but
by an interaction between universes.
Could be, Pfister says, but we can't know. "The parallel
universe stuff is just a different way of understanding quantum mechanics. It
gives exactly the same experimental predictions as any other quantum theory. So
as an experimenter, I don't care. It doesn't matter."
What, then, does lie out there, at the limits of
our ability to conceive? There's always life itself.
If we had a quantum computer made of several hundred or a
thousand qubits, "it could help us describe small systems," Pfister says. "Not
the universe, but maybe a bacteria. It would be fun to try, to see what a
quantum description of life would be."
As for somehow defining consciousness, "I don't like to
think about ways of measuring it," Pfister admits, "since the results are not
reproducible. But some people do. And if you like to think about that, well
then, build a big quantum computer, turn it on, and see if it comes alive."
Could it?
Professor Pfister shrugs. "Who knows?" he says.
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Posted
02-20-2007 4:34 PM
by
DougHornig