Seeking Quantum Leap
In the Computing World
By JOHN MARKOFF
SANTA BARBARA, California — Modern computers are not unlike the looms of the industrial revolution: They follow programmed instructions to weave intricate patterns. With a loom, you see the result in a cloth or carpet. With a computer, you see it on an electronic display.
Now a group of physicists and computer scientists funded by Microsoft is trying to take the analogy of interwoven threads to what some believe will be the next great leap in computing, so-called quantum computing.
If the scientists are right, their research could lead to the design of computers that are far more powerful than today’s supercomputers and could solve problems in fields as diverse as chemistry, material science, artificial intelligence and code-breaking.
They recently met to explore an approach to quantum computing that is based on “braiding” exotic particles known as anyons — what physicists describe as “quasiparticles” that exist in just two dimensions rather than three — in order to form the building blocks of a supercomputer that exploits the weird physical properties of subatomic particles.
Conventional computing is based on a bit that can be either a 1 or a 0, representing a single value in a computation. But quantum computing is based on qubits, which simultaneously represent both 0 and 1 values. If they are placed in an “entangled” state — physically separated but acting as though they are connected — with many other qubits, they can represent a vast number of values simultaneously.
And the existing limitations of computing power are thrown out the window. In the approach that Microsoft is pursuing, which is described as “topological quantum computing,” precisely controlling the motions of pairs of subatomic particles as they wind around one another would manipulate entangled quantum bits. Although the process of braiding particles takes place at subatomic scales, it is evocative of the motions of a weaver overlapping threads to create a pattern.
The mathematics of their motions would correct errors that have so far proved to be the most daunting challenge facing quantum computer designers.
First proposed by the physicist Richard Feynman in 1982, quantum computing has mostly been of interest to academics, the National Security Agency and the Pentagon’s Defense Advanced Research Projects Agency.
But in recent years, quantum computing has caught the attention of the corporate world. While scientists have created individual qubits, they are extremely fragile, and creating the arrays of hundreds or thousands of circuits necessary to build a useful quantum computer has proved daunting. Microsoft’s topological approach is generally perceived as the most high-risk because the type of anyon particle needed to generate qubits has not been definitively proved to exist.
That may change soon. The company has been exploring a long-hypothesized class of subatomic particles known as Majorana fermions. Proving their existence would mean that it was likely they could be used to form qubits.
Microsoft supported research, led by the physicist Leo Kouwenhoven at the Delft University of Technology in the Netherlands, that in 2012 produced the strongest evidence that the long-predicted particles exist.
“They have really done something very special,” said Charles M. Marcus, a physicist at the University of Copenhagen.
His laboratory is now growing molecular-scale nano-wires that will work like one-dimensional train tracks, making it possible to control the movement of fermions around one another.
Ensembles of these particles can be used to construct qubits in the topological computing model proposed by the mathematician Michael Freedman and the physicists Chetan Nayak and Sankar Das Sarma in 2005.
Microsoft began supporting the effort after Dr. Freedman approached Craig Mundie, one of Microsoft’s top executives, and convinced him that there was a new path to quantum computing based on ideas in topology originally proposed in 1997 by the physicist Alexei Kitaev.
Mr. Mundie said the idea struck him as the kind of gamble the company should be pursuing.
The researchers acknowledge that they have not yet even made a working prototype of the basic element of their system.
Many thought quantum computers would be useful only for factoring huge numbers. But now many scientists believe that quantum computers could tackle new kinds of problems.
When Mr. Mundie asked Dr. Freedman what he might do with a working quantum computer, he said that the first thing he would program it to do would be to model an improved version of itself.
Today, The New York Times International Weekly, Saturday, July 5, 2014, Pg 11