IEEE Spectrum September, 2007 - 31

The second lesson was that the way we used spin posed some
big challenges. The core of our quantum computer consisted of
a custom-synthesized organic molecule in a solution. It had five
fluorine and two carbon nuclei whose spins we used to store
seven units of information, called quantum bits, or qubits. We
blasted the molecule with radio-frequency pulses to alter the
spins according to the computational steps of the factoring
algorithm. To read out the qubits, we used nuclear magnetic
resonance, or NMR, to generate a frequency spectrum of each
spin. It worked beautifully for seven qubits, and in fact that system remains the only one to have factored a number to this day.
But designing molecules suitable for more complex calculations
became just too hard.
If we wanted a quantum computer that we could scale up, we
needed a system that would let us precisely manipulate tiny bits
of energy, that could be effectively shielded from external interference, and-most important-that could be
built by replicating tiny identical building blocks
within a small area. We needed something less
like a test tube-and more like a microchip.

and the external field are pointing in the same direction, or they
are pointing in opposite directions. These two possibilities are
also referred to as spin up and spin down, respectively.
More interesting-and bizarre-is that spin can also exist in
a combined state of up and down. This superposition state is one
of the things that set quantum computers apart from classical
ones. A three-bit conventional memory, for example, can hold any
combination of three bits at a time: 000, 001, 010, 011, 100, 110, 101,
or 111. But using qubits, and representing spin up as 0 and spin
down as 1, you can do much better: a three-qubit memory can
hold all those eight states simultaneously. As a result, if you perform a calculation using those three qubits, you in effect perform
a calculation on all eight states at once. As you add more qubits,
this quantum parallel processing increases exponentially.
To perform quantum computations, however, you need to link
the qubits somehow. The way researchers do that is by using the
quantum phenomenon of entanglement. Two
entangled spins can exist in a superposition
of, say, up-down and down-up. You don't
know which electron has which spin until you
measure it. But as soon as you measure one
spin, that means the other spin must have the
opposite value. How do they "know" which
way to point? Scientists devised ingenious
experiments to test entanglement and concluded that entangled particles don't carry a
"preprogrammed" behavior. Instead, according
to quantum mechanics, the pair of electrons
forms a single entity. Each electron's spin by
itself has no definite orientation until one of
them is measured, no matter how far apart they
are. Einstein rejected this notion and famously
called it "spooky action at a distance."
Spooky indeed. But those are the rules of
quantum mechanics, and we might as well use them to our
advantage. Quantum researchers not only accept spin's weirdness, but they also embrace it. They think of spin as a vector in a mathematical domain called a Hilbert space. Basically,
this vector describes the probabilities of obtaining spin up or
down when a particle's spin is measured. The researchers perform a host of mathematical transformations to those vectors
to concoct quantum computing algorithms. But as physicist
Asher Peres has put it, "Quantum phenomena do not occur
in a Hilbert space, they occur in a laboratory." And it's in the
lab that our group and many others set out to build a practical
quantum computer.

TO sCaLe Up
OUr QUanTUM
COMpUTer,
We needed
sOMeTHinG
Less LiKe a
TesT TUBe and
MOre LiKe a
MiCrOCHip

all artwork: Bryan christie design

a SeMiCOndUCTOr quantum computer is now
the goal of dozens of research groups worldwide.
In the last few years, these groups, including my
own at Delft University of Technology, in the
Netherlands, have made rapid progress in creating qubits based on materials and processes similar to those used in the microelectronics industry
to manufacture standard processors and memory
chips. [See "The Trap Technique," IEEE Spectrum,
August, for the first part of this report.]
The advantage of a solid-state design over
the NMR approach is the ability to fabricate
large arrays of miniature electronic devices that
can be individually addressed and interconnected-just as we do
with transistors in an integrated circuit. One promising approach
to such a solid-state system was put forward by Daniel Loss of
the University of Basel, in Switzerland, and David DiVincenzo
of the IBM T.J. Watson Research Center, in Yorktown Heights,
N.Y. In their January 1998 paper, "Quantum Computation with
Quantum Dots," in Physical Review A, they proposed trapping
individual electrons in semiconductor structures called quantum dots and then using the electrons' spins as qubits.
With typical dimensions from a few nanometers to a few
micrometers-about the size of a virus-a quantum dot is a tiny
area in a semiconductor that can hold anything from a single
electron to several thousand. To make a quantum dot that's suitable for a quantum computer, you start with a half-millimeter- OUr STarTinG POinT was Loss and DiVincenzo's proposal and
thick wafer of gallium arsenide and cover it with an even thinner, related concepts. Clearly, it would be too difficult to build a
100-nm-thick layer of silicon-doped aluminum-gallium-arsenide. whole computer at once. So the idea has been to develop a set of
Free electrons will concentrate at the interface between the two basic functions that any working system would need. These are
materials, forming a thin electron sheet. Next, you attach a set an initialization mechanism to set all of the qubits to a known
of gold electrodes to the top layer and apply negative voltages to state before computations begin, a readout scheme to measure
them. The electrodes will repel electrons in the sheet underneath the individual spins, and a set of spin-manipulation techniques
and create small islands of electrons isolated from the rest.
capable of carrying out any possible quantum computations.
Creating such electron puddles is relatively straightforward,
Here's the basic design: the core of the machine will consist
but manipulating electron spin is a different matter. Like charge of a single chip, which will sit inside an ultracold receptacle
and mass, spin is considered an intrinsic property of electrons, called a dilution refrigerator, which in turn will be encircled by
and yet it remains somewhat mysterious. We can measure spin a powerful superconducting magnet. (As DiVincenzo once said,
because it interacts with an external magnetic field, much as "This is not going to be a laptop computer!")
an ultrasmall magnet rotating about its own axis would. But
Whereas a conventional microchip is packed with transistors,
unlike with a real magnet, when we measure an electron's spin the quantum-computing chip will be packed with quantum
orientation, there will be only two possible outcomes: the spin dots. The dots-dozens, hundreds, or perhaps thousands-will

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Table of Contents for the Digital Edition of IEEE Spectrum September, 2007