The
massive amount of processing power generated by computer
manufacturers has not yet been able to quench our thirst for
speed and computing capacity. In 1947, American computer
engineer **Howard Aiken** said that just six electronic
digital computers
would satisfy the computing needs of the United States. Others
have made similar errant predictions about the amount of
computing power that would support our growing technological
needs. Of course, Aiken didn't count on the large amounts of
data generated by scientific research, the proliferation of personal
computers or the emergence of the Internet,
which have only fueled our need for more, more and more
computing power.
Will we ever have the amount of computing power we need, or
want? If, as **Moore's Law** states, the number of
transistors on a microprocessor
continues to double every 18 months, the year 2020 or 2030
will find the circuits on a microprocessor measured on an
atomic scale. And the logical next step will be to create
**quantum computers**, which will harness the power of
atoms and molecules to perform memory
and processing tasks. Quantum computers have the potential to
perform certain calculations billions of times faster than any
silicon-based computer.

Scientists have already built basic quantum computers that
can perform certain calculations; but a practical quantum
computer is still years away. In this edition of **How
Stuff Will Work**, you'll learn what a quantum computer
is and just what it'll be used for in the next era of
computing.

Defining the Quantum Computer

You don't have
to go back too far to find the origins of quantum computing.
While computers have been around for the majority of the 20th
century, quantum computing was first theorized just 20 years
ago, by a physicist at the Argonne
National Laboratory. **Paul Benioff** is credited with
first applying quantum theory to computers in 1981. Benioff
theorized about creating a quantum Turing machine. Most
digital computers, like the one you are using to read this
article, are based on the **Turing Theory**.

The Turing machine, developed by **Alan Turing** in the
1930s, consists of tape of unlimited length that is divided
into little squares. Each square can either hold a symbol (1
or 0) or be left blank. A read-write device reads these
symbols and blanks, which gives the machine its instructions
to perform a certain program. Does this sound familiar? Well,
in a quantum Turing machine, the difference is that the tape
exists in a quantum state, as does the read-write head. This
means that the symbols on the tape can be either 0 or 1 or a
**superposition** of 0 and 1. While a normal Turing machine
can only perform one calculation at a time, a quantum Turing
machine can perform many calculations at once.

Today's computers, like a Turing machine, work by
manipulating bits that exist in one of two states: a 0 or a 1.
Quantum computers aren't limited to two states; they encode
information as quantum bits, or **qubits**. A qubit can be
a 1 or a 0, or it can exist in a superposition that is
simultaneously both 1 and 0 or somewhere in between. Qubits
represent atoms that
are working together to act as computer
memory and a processor.
Because a quantum computer can contain these multiple states
simultaneously, it has the potential to be millions of times
more powerful than today's most powerful supercomputers.

This superposition of qubits is what gives quantum
computers their inherent **parallelism**. According to
physicist **David Deutsch**, this parallelism allows a
quantum computer to work on a million computations at once,
while your desktop PC works on one. A 30-qubit quantum
computer would equal the processing power of a conventional
computer that could run at 10 **teraflops** (trillions of
floating-point operations per second). The fastest
supercomputers have achieved speeds of about 2 teraops
(trillions of fixed-point operations per second).

Quantum computers also utilize another aspect of quantum
mechanics known as **entanglement**. One problem with the
idea of quantum computers is that if you try to look at the
subatomic particles, you could bump them, and thereby change
their value. But in quantum physics, if you apply an outside
force to two atoms, it can cause them to become entangled, and
the second atom can take on the properties of the first atom.
So if left alone, an atom will spin in all directions; but the
instant it is disturbed it chooses one spin, or one value; and
at the same time, the second entangled atom will choose an
opposite spin, or value. This allows scientists to know the
value of the qubits without actually looking at them, which
would collapse them back into 1's or 0's.

Today's Quantum Computers

Quantum computers
could one day replace silicon chips, just like the transistor
once replaced the vacuum tube. But for now, the technology
required to develop such a quantum computer is beyond our
reach. Most research in quantum computing is still very
theoretical.

The most advanced quantum computers have not gone beyond
manipulating more than 7 qubits, meaning that they are still
at the "1 + 1" stage. However, the potential remains that
quantum computers one day could perform, quickly and easily,
calculations that are incredibly time-consuming on
conventional computers. Several key advancements have been
made in quantum computing in the last few years. Here's a look
at a few of the quantum computers that have been developed:

- In August 2000, researchers at IBM-Almaden
Research Center developed what they claimed was the most
advanced quantum computer developed to date. The 5-qubit
quantum computer was designed to allow the nuclei of five
fluorine atoms to interact with each other as qubits, be
programmed by radio
frequency pulses and be detected by nuclear magnetic
resonance (NMR) instruments similar to those used in
hospitals (see How Magnetic
Resonance Imaging Works for details). Led by Dr. Isaac
Chuang, the IBM team was able to solve in one step a
mathematical problem that would take conventional computers
repeated cycles. The problem, called
**order-finding**,
involves finding the period of a particular function, a
typical aspect of many mathematical problems involved in
cryptography.
- In March 2000, scientists at Los
Alamos National Laboratory announced the development of
a 7-qubit quantum computer within a single drop of liquid.
The quantum computer uses NMR to manipulate particles in the
atomic nuclei of molecules of trans-crotonic acid, a simple
fluid consisting of molecules made up of six hydrogen and
four carbon atoms. The NMR is used to apply electromagnetic
pulses, which force the particles to line up. These
particles in positions parallel or counter to the magnetic
field allow the quantum computer to mimic the
information-encoding of bits in
digital computers.
- In 1998, Los Alamos and MIT researchers managed to
spread a single qubit across three nuclear spins in each
molecule of a liquid solution of alanine or
trichloroethylene molecules. Spreading out the qubit made it
harder to corrupt, allowing researchers to use entanglement
to study interactions between states as an indirect method
for analyzing the quantum information.

If functional
quantum computers can be built, they will be valuable in
factoring large numbers, and therefore extremely useful for
decoding and encoding secret information. If one were to be
built today, no information on the Internet would be safe. Our
current methods of encryption
are simple compared to the complicated methods possible in
quantum computers. Quantum computers could also be used to
search large databases in a fraction of the time that it would
take a conventional computer.
But quantum computing is still in its early stages of
development, and the technology needed to create a practical
quantum computer is years away. Quantum computers must have at
least several dozen qubits to be able to solve real-world
problems, and thus serve as a viable computing method.

For more information on quantum computers and related
topics, check out the links on the next page!

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