Parallel Worlds (27 page)

In the future,
we will reach the limits of etching on silicon wafers. The Age of Silicon will
soon be coming to a close. Perhaps it will usher in the quantum era. Silicon
Valley could become a Rust Belt. One day we may be forced to compute on atoms
themselves, introducing a new architecture for computation. Computers today
are based on the binary system—every number is based on zeros and ones. Atoms,
however, can have their spin pointed up, down, or sideways, simultaneously.
Computer bits (0s and is) could be replaced by "qubits" (anything
between 0 and 1), making quantum computation much more powerful than ordinary
computers.

A quantum
computer, for example, could shake the foundations of international security.
Today, large banks, multinational corporations, and industrial nations code
their secrets with complex computer algorithms. Many secret codes are based on
factorizing huge numbers. It would take centuries, for example, for an ordinary
computer to factorize a number containing a hundred digits. But for a quantum
computer, such calculations may be effortless; they could break the secret
codes of the nations of the world.

To see how a
quantum computer would work, let's say that we align a series of atoms, with
their spins pointing in one direction in a magnetic field. Then we shine a
laser beam on them, so many of the spins flip as the laser beam reflects off
the atoms. By measuring the reflected laser light, we have recorded a complex
mathematical operation, the scattering of light off atoms. If we calculate this
process using the quantum theory, following Feynman, we must add together all
possible positions of the atoms, spinning in all possible directions. Even a
simple quantum calculation, which would take a fraction of a second, would be
almost impossible to perform on a standard computer, no matter how much time is
allotted.

In principle, as
David Deutch of Oxford has stressed, this means that when we use quantum
computers, we would have to sum over all possible parallel universes. Although
we cannot directly make contact with these alternate universes, an atomic
computer could calculate them using the spin states existing in parallel
universes. (While we are no longer coherent with the other universes in our living
room, the atoms in a quantum computer are, by construction, vibrating
coherently in unison.)

Although the
potential of quantum computers is truly staggering, in practice, the problems
are equally enormous. At present, the world record for the number of atoms used
in a quantum computer is seven. At best, we can multiply three by five, to get
fifteen on a quantum computer, hardly impressive. For a quantum computer to be
competitive with even an ordinary laptop, we would need hundreds, perhaps
millions of atoms vibrating coherently. Because even the collision with a
single air molecule could make the atoms decohere, one would have to have
extraordinarily clean conditions to isolate the test atoms from the environment.
(To construct a quantum computer that would exceed the speed of modern
computers would require thousands to millions of atoms, so quantum computing is
still decades away.)

QUANTUM TELEPORTATION

There may
ultimately be another practical application to physicists' seemingly pointless
discussion of parallel quantum universes: quantum teleportation. The
"transporter" used in
Star
Trek
and other science
fiction programs to transport people and equipment through space seems like a
marvelous way to zip across vast distances. But as tantalizing as it seems, the
idea of teleportation has stumped physicists because it seems to violate the
uncertainty principle. By making a measurement on an atom, you disturb the
state of the atom, and hence an exact copy cannot be made.

But scientists
found a loophole in this argument in 1993, through something called quantum
entanglement. This is based on an old experiment proposed in 1935 by Einstein
and his colleagues Boris Podolsky and Nathan Rosen (the so-called EPR paradox)
to show how crazy the quantum theory really is. Let's say that there is an
explosion, and two electrons fly apart in opposite directions, traveling at
near light speed. Since electrons can spin like a top, assume that the spins
are correlated—that is, if one electron has its spin axis pointing up, the
other electron is spinning down (such that the total spin is zero). Before we
make a measurement, however, we do not know which direction each electron is
spinning.

Now wait several
years. By then, the two electrons are many light-years apart. If we now make a
measurement of the spin of one electron and find that its axis of spin points
up, then we instantly know that the other electron is spinning down (and vice
versa). In fact, the fact that the electron is found to be spinning up
forces
the other electron to spin down. This means that we now know
something about an electron many light-years away, instantly. (Information, it
seems, has traveled faster than the speed of light, in apparent violation of
Einstein's special relativity.) By subtle reasoning, Einstein could show that,
by making successive measurements on one pair, one could violate the
uncertainty principle. More important, he showed that quantum mechanics is
more bizarre than anyone had previously thought.

Up to then,
physicists believed the universe was local, that disturbances in one part of
the universe only spread out locally from the source. Einstein showed that
quantum mechanics is essentially
nonlocal
—disturbances
from one source can instantly affect distant parts of the universe. Einstein
called it a "spooky action-at- a-distance," which he thought was
absurd. Thus, thought Einstein, the quantum theory must be wrong.

(The critics of
quantum mechanics could resolve the Einstein- Podolsky-Rosen paradox by
assuming that, if our instruments were only sensitive enough, they could really
determine which way the electrons were spinning. The apparent uncertainty in
the spin and position of an electron was a fiction, due to the fact that our
instruments were too crude. They introduced the concept called hidden
variables—that is, there must be a hidden
sub
quantum
theory, in which there is no uncertainty at all, based on new variables called
hidden variables.)

The stakes were
raised enormously in 1964, when physicist John Bell put the EPR paradox and
hidden variables to the acid test. He showed that if one performed the EPR
experiment, there should be a numerical correlation between the spins of the
two electrons, depending on which theory one used. If the hidden variable
theory was correct, as the skeptics believed, then the spins should be correlated
in one way. If quantum mechanics was correct, the spins should be correlated in
another way. In other words, quantum mechanics (the foundation of all modern
atomic physics) would rise and fall on the basis of a single experiment.

But experiments
have conclusively proved Einstein wrong. In the early 1980s, Alan Aspect and
colleagues in France performed the EPR experiment with two detectors 13 meters
apart, which measured the spins of photons emitted from calcium atoms. In 1997,
the EPR experiment was performed with detectors separated by 11 kilometers.
Each time the quantum theory won. A certain form of knowledge
does
travel faster than light. (Although Einstein was wrong on
the EPR experiment, he was right on the larger question of faster-than- light
communication. The EPR experiment, although it does allow you to know something
instantly about the other side of the galaxy, does not allow you to send a
message in this way. You cannot, for example, send Morse code. In fact, an
"EPR transmitter" would send only random signals, since the spins you
measure are random each time you measure them. The EPR experiment allows you to
acquire information about the other side of the galaxy, but it does not allow
you to transmit information that is useful—that is, not random.)

Bell liked to
describe the effect by using the example of a mathematician called Bertelsman.
He had the strange habit of every day wearing a green sock on one foot and a
blue sock on the other, in random order. If one day you notice that he is
wearing a blue sock on his left foot, you now know, faster than light, that his
other sock is green. But knowing that does not allow you to communicate information
in this fashion. Revealing information is different from sending it. The EPR
experiment does not mean that we can communicate information through
telepathy, faster-than-light travel, or time travel. But it does mean that it
is impossible to completely separate ourselves from the oneness of the
universe.

It forces us to
hold a different picture of our universe. There is a cosmic
"entanglement" between every atom of our body and atoms that are
light-years distant. Since all matter came from a single explosion, the big
bang, in some sense the atoms of our body are linked with some atoms on the
other side of the universe in some kind of cosmic quantum web. Entangled
particles are somewhat like twins still joined by an umbilical cord (their wave
function) which can be light-years across. What happens to one member
automatically affects the other, and hence knowledge concerning one particle
can instantly reveal knowledge about its pair. Entangled pairs act as if they
were a single object, although they may be separated by a large distance.
(More precisely, since the wave functions of the particles in the big bang were
once connected and coherent, their wave functions might still be partially
connected billions of years after the big bang, so that disturbances in one
part of the wave function can influence another distant part of the wave
function.)

In 1993,
scientists proposed using the concept of EPR entanglement to provide a
mechanism for quantum teleportation. In 1997 and 1998, the scientists at Cal
Tech, Aarhus University in Denmark, and the University of Wales made the first
experimental demonstration of quantum teleportation when a single photon was
teleported across a tabletop. Samuel Braunstein of the University of Wales, who
was part of this team, has compared entangled pairs to lovers "who know
each other so well that they could answer for their lover even if separated by
long distances."

(Quantum
teleportation experiments require three objects, called A, B, and C. Let B and
C be two twins that are entangled. Although B and C may be separated by a large
distance, they are still entangled with each other. Now let B come in contact
with A, which is the object to be teleported. B "scans" A, so the
information contained in A is transferred to B. This information is then
transferred automatically to the twin C. Thus, C becomes an exact replica of
A.)

Progress in
quantum teleportation is moving rapidly. In 2003, scientists at the University
of Geneva in Switzerland were able to tele- port photons a distance of 1.2
miles through fiber optic cable. Photons of light (at 1.3-mm wavelength) in one
laboratory were tele- ported into photons of light of a different wavelength
(1.55 mm) in another laboratory connected by this long cable. Nicolas Gisin, a
physicist on this project, has said, "Possibly, larger objects like a
molecule will be teleported in my lifetime, but really large objects are not
teleportable using foreseeable technologies."

Another
significant breakthrough was made in 2004, when scientists at the National
Institute of Standards and Technology (NIST) teleported not just a quantum of
light but an entire atom. They successfully entangled three beryllium atoms
and were able to transfer the characteristics of one atom into another, a major
accomplishment.

The practical
applications of quantum teleportation are potentially enormous. However, one
should point out that there are several practical problems to quantum
teleportation. First, the original object is destroyed in the process, so that
you cannot make carbon copies of the object being teleported. Only one copy is
possible. Second, you cannot teleport an object faster than light. Relativity
still holds, even for quantum teleportation. (To teleport object A into object
C, you still need an intermediate object B connecting the two that travels
slower than the speed of light.) Third, perhaps the most important limitation
on quantum teleportation is the same one facing quantum computing: the objects
in question must be coherent. The slightest contamination with the environment
will destroy quantum teleportation. But it is conceivable that within the
twenty- first century the first virus may be teleported.

Teleporting a
human being may pose other problems. Braunstein observes, "The key thing
for now is the sheer amount of information involved. Even with the best
communication channels we could conceive of at the moment, transferring all
that info would take the age of the universe."

WAVE FUNCTION OF THE UNIVERSE

But perhaps the
ultimate realization of the quantum theory may come when we apply quantum
mechanics not just to individual photons but to the entire universe. Stephen
Hawking has quipped that whenever he hears the cat problem, he reaches for his
gun. He has proposed his own solution to the problem—to have a wave function of
the entire universe. If the entire universe is part of the wave function, then
there is no necessity for an observer (who must exist outside the universe).

In the quantum
theory, every particle is associated with a wave. The wave, in turn, tells you
the probability of finding the particle at any point. However, the universe,
when it was very young, was smaller than a subatomic particle. Therefore,
perhaps the universe itself has a wave function. Since the electron can exist
in many states at the same time, and since the universe was smaller than an
electron, perhaps the universe also existed simultaneously in many states,
described by a super wave function.