Quantum Computing with Ions [Repost]

TRAPPED-ION COMPUTERS could encode and process data with strings of ions that act somewhat like the suspended metal balls in a Newton's cradle (as seen in this artist's conception). The ions interact through oscillatory motions. Researchers can manipulate the particles by training laser beams on them. Image: David Emmite (computer setup); George Retseck (spheres)

In Brief

• Quantum computers can store and process data using atoms, photons or fabricated microstructures. These machines may someday be able to perform feats of computing once thought to be impossible.
• The manipulation of trapped ions is at the forefront of the quantum computing effort. Researchers can store data on the ions and transfer information from one ion to another.
• Scientists see no fundamental obstacles to the development of trapped-ion computers.

Editor?s note (10/9/2012): We are making the text of this article freely available for 30 days because the article was cited by the Nobel Committee as a further reading in the announcement of the 2012 Nobel Prize in Physics and was also written by one of the prize winners. The full article with images, which appeared in the August 2008 issue, is available for purchase here.

Over the past several decades technological advances have dramatically boosted the speed and reliability of computers. Modern computer chips pack almost a billion transistors in a mere square inch of silicon, and in the future computer elements will shrink even more, approaching the size of individual molecules. At this level and smaller, computers may begin to look fundamentally different because their workings will be governed by quantum mechanics, the physical laws that explain the behavior of atoms and subatomic particles. The great promise of quantum computers is that they may be able to perform certain crucial tasks considerably faster than conventional computers can.

Perhaps the best known of these tasks is factoring a large number that is the product of two primes. Multiplying two primes is a simple job for computers, even if the numbers are hundreds of digits long, but the reverse process?deriving the prime factors?is so extraordinarily difficult that it has become the basis for nearly all forms of data encryption in use today, from Internet commerce to the transmission of state secrets.

In 1994 Peter Shor, then at Bell Laboratories, showed that a quantum computer, in theory, could crack these encryption codes easily because it could factor numbers exponentially faster than any known classical algorithm could. And, in 1997, Lov K. Grover, also at Bell Labs, showed that a quantum computer could significantly increase the speed of searching an unsorted database?say, finding a name in a phone book when you have only the person?s phone number.

Actually building a quantum computer, however, will not be easy. The quantum hardware?the atoms, photons or fabricated microstructures that store the data in quantum bits, or qubits?needs to satisfy conflicting requirements. The qubits must be sufficiently isolated from their surroundings; otherwise stray external interactions will halt their computations. This destructive process, known as decoherence, is the bane of quantum computers. But the qubits also have to interact strongly with one another and must ultimately be measured accurately to display the results of their calculations.

Scientists around the globe are pursuing several approaches to building the first prototype quantum computers. Our own research focuses on processing information with singly charged positive ions, atoms that have been stripped of one electron. We have trapped short strings of ions?confining the particles in a vacuum using electric fields produced by nearby electrodes?so that they can receive input signals from a laser and share data with one another. Our goal is to develop quantum computers that are scalable?that is, systems in which the number of qubits could be increased to the hundreds or thousands. Such systems would fulfill the promise of the technology by accomplishing complex processing tasks that no ordinary computer could match.

Trapping Ions
Quantum mechanics is a theory based on waves. Just as the sound waves from two or more piano strings can merge into a chord, different quantum states can be combined into a superposition. For example, an atom may be simultaneously in two locations or in two different states of excitation. When a quantum particle in a superposition state is measured, the conventional interpretation is that the state collapses to a single result, with the probability of each possible measurement given by the relative proportions of the waves in the superposition. The potential power of a quantum computer derives from these superpositions: unlike a conventional digital bit, which can have a value of either 0 or 1, a qubit can be both 0 and 1 at the same time. A system with two qubits can hold four values simultaneously?00, 01, 10 and 11. In general, a quantum computer with N qubits can simultaneously manipulate 2^N numbers; a collection of only 300 atoms, each storing a quantum bit, could hold more values than the number of particles in the universe!

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