The Quantum Future of Computing

"With quantum computing you are able to attack some problems on the time scales of seconds, which might take an almost infinite amount of time with classical computers."

Professor David Awschalom of the University of California, Santa Barbara.

Quantum computers can efficiently render every physically possible quantum environment, even when vast numbers of universes are interacting. Quantum computers can also efficiently solve certain mathematical problems, such as factorization, which are classically intractable, and can implement types of cryptography which are classically impossible.

"Quantum computation," summarzies Oxford physicist David Deutsch, "is a qualitatively new way of harnessing nature."

Quantum computing sounds like science fiction -as satellites, moon shots, and the original microprocessor once were.  But the age of computing in not even at the end of the beginning.

Traditional computing, with its ever more microscopic circuitry etched in silicon, will soon reach a final barrier: Moore's law, which dictates that the amount of computing power you can squeeze into the same space will double every 18 months, is on course to run smack into a silicon wall by 2015, due to  overheating, caused by electrical charges running through ever more tightly packed circuits.

To leapfrog the silicon wall, we have to figure out how to manipulate the brain-bending rules of the quantum realm - an Alice in Wonderland world of subatomic particles that can be in two places at once. 

Where a classical computer obeys the well understood laws of classical physics, a quantum computer is a device that harnesses physical phenomenon unique to quantum mechanics (especially quantum interference) to realize a fundamentally new mode of information processing.

The fundamental unit of information in quantum computing (called a quantum bit or qubit), is not binary but rather more quaternary in nature, which differs radically from the laws of classical physics.

A qubit can exist not only in a state corresponding to the logical state 0 or 1 as in a classical bit, but also in states corresponding to a blend or superposition of these classical states. In other words, a qubit can exist as a zero, a one, or simultaneously as both 0 and 1, with a numerical coefficient representing the probability for each state.  This may seem counterintuitive because everyday phenomenon are governed by classical Newtonian physics, not quantum mechanics -- which takes over at the atomic level. 

The reason this is exciting is that it's derived from the massive quantum parallelism achieved through superposition, is the equivalent of performing the same operation on a classical super computer with ~10150 separate processors, which is impossible.

The idea of a computational device based on quantum mechanics was first explored in the 1970's and early 1980's by physicists and computer scientists such as Charles H. Bennett of the IBM Thomas J. Watson Research Center,  Paul A. Benioff of Argonne National Laboratory in Illinois, David Deutsch of Oxford, and the late Richard P. Feynman, Nobel laureate of the California Institute of Technology were pondering the fundamental limits of computation.

They understood that if technology continued to abide by Moore's Law, then the continually shrinking size of circuitry packed onto silicon chips would eventually reach a point where individual elements would be no larger than a few atoms.  Here a problem arose because at the atomic scale the physical laws that govern the behavior and properties of the circuit are inherently quantum mechanical in nature, not classical. 

This then raised the question of whether a new kind of computer could be devised based on the principles of quantum physics.

Feynman was among the first to attempt to provide an answer to this question by producing an abstract model in 1982 that showed how a quantum system could be used to do computations.  He also explained how such a machine would be able to act as a simulator for quantum physics.  In other words, a physicist would have the ability to carry out experiments in quantum physics inside a quantum mechanical computer.

In 1985, Deutsch realized that Feynman's assertion could eventually lead to a general purpose quantum computer and published a crucial theoretical paper showing that any physical process, in principle, could be modeled perfectly by a quantum computer.  Thus, a quantum computer would have capabilities far beyond those of any traditional classical computer.  After Deutsch published this paper, the search began to find interesting applications for such a machine.

The breakthrough occurred in 1994 when Shor circulated a preprint of a paper in which he set out a method for using quantum computers to crack an important problem in number theory, namely factorization.  He showed how an ensemble of mathematical operations, designed specifically for a quantum computer, could be organized to enable a such a machine to factor huge numbers extremely rapidly, much faster than is possible on conventional computers. 

With Shor's breakthrough, quantum computing transformed from a mere academic curiosity directly into a national and world interest.

Quantum hardware, on the other hand, remains an emerging field, but the work done thus far suggests that it will only be a matter time before we have devices large enough to test Shor's and other quantum algorithms. 

Beyond the actual creation of a quantum computer, our chief limitations are the imaginations of software engineers. This will be the major challenge of the Google whiz kids of tomorrow: to take computing and networking power that is effectively infinite and create interfaces that are simple enough for mere humans to understand.

Recent breakthroughs pioneered by Stuart Wolff of the University of Virginia  allow us to take electricity out of the equation, and get rid of the overheating problem that is undercutting Moore's law. Single electrons have been made to adjust their spin. Subatomic circuitry is within our grasp.

Posted by Casey Kazan

Links:

http://news.bbc.co.uk/2/hi/technology/7085019.stm