Ordinary light could drive quantum computers

by Eric Smalley, Technology Research News

One reason quantum computers are not likely to show up in your neighborhood electronics store any time soon is the laboratory equipment needed to build today's prototypes is hard to come by and difficult to use.

With some improvements to a couple of key devices, though, that could change. Thanks to a scheme concocted by researchers at the Los Alamos National Laboratory, researchers should be able to build quantum computers using common linear optics equipment.

Practical quantum computers could be developed sooner with the means for building prototypes within reach of a greater number of researchers. Quantum computers are expected to solve certain problems like cracking codes and searching large databases much faster than any other conceivable computer.

To achieve quantum computing, researchers manipulate the quantum states of photons or atoms to perform logic operations. Photon manipulation traditionally requires nonlinear optics methods, which use powerful lasers to coax photons from special materials.

The effect the lasers have on the atoms of these materials increases faster than the increase in intensity of the light. Ordinarily, the effect is proportional. This nonlinearity produces strange phenomena, like entangled pairs of photons, that are useful for quantum computing.

"We show that nonlinear optical elements can be simulated using linear optics and photo-detectors, a very surprising result," said Emanuel Knill, a mathematician at Los Alamos National Laboratory. "It opens up an entirely new path toward realizing quantum computers."

Quantum computers based on the Los Alamos linear optics scheme would create quantum bits, or qubits, by using two opposite conditions of individual photons to represent the 0 and 1 values used in binary computing.

There are two sets of opposite conditions. The first is the two possible paths a photon can take when it encounters a beam splitter. The second is either of two pairs of polarizations. Photons are polarized, or oriented, in one of four directions: vertical, horizontal, and two diagonals. Each polarization is paired with its opposite: vertical with horizontal and diagonal with diagonal.

Multiple bits can be used to represent larger numbers. Four bits can represent 24 or 16 numbers and 24 bits can represent 224 or more than 16 million numbers. Ordinary computers process these numbers one at a time. So, for example, in order to find one number out of 16 million an ordinary computer will have to look through an average of eight million numbers.

What makes a qubit different from an ordinary bit is that it can be in a third state, the quantum mechanical condition of superposition, which is essentially a mix of both 0 and 1. This means it's possible to perform a series of quantum mechanical operations on a series of qubits all at once. For some applications, the number of quantum mechanical operations is exponentially smaller than the number of steps required for a classical computer.

The quantum mechanical operations are sequenced to make up logic gates, which perform the basic mathematics of computing. Most quantum logic gate schemes require particles in more complicated quantum arrangements like entanglement. According to Knill, however, it is possible to create logic gates by manipulating the photons that are in the superpositions created by the linear optics.

Quantum computers based on photons rather than atoms will be easier to network because there will be no need to transfer quantum information between atoms and photons. "The only realistic proposals for long distance quantum communication are based on photons," Knill said.

Before the scheme can be implemented, however, researchers will need to improve both the light source and the photon detector. Two recently developed single-photon emitters hold out the promise that the necessary equipment could be available to researchers within a few years, said Knill.

"I think it's a neat idea," said John Preskill, professor of theoretical physics and director of the Institute for Quantum Information at the California Institute of Technology. "Any theoretical ideas that help make realizations of quantum logic technically less demanding might turn out to be important ideas."

Preskill led a research team that proposed a different scheme for quantum computing using linear optics, though that scheme requires its initial state to be prepared using nonlinear optics.

"There have been a lot of previous discussions of using information encoded in photons to [make] universal quantum gates, but always involving some kind of nonlinear coupling between photons, and those are hard to manage," said Preskill. "The stuff that Knill et al are talking about in principle is much easier. It uses tools that are available in lots of laboratories," he said.

Despite the potential for linear optics to speed things up, it would be a significant achievement if in 25 years a quantum computer can solve problems that are beyond the reach of classical computers, said Knill.

"Quantum computation by any means is a long way off," he said. "Our proposal adds to the tool box of possible experimental realizations, which may help speed things up. The fact is, the necessary experiments are extremely demanding."

Knill's research colleagues were Raymond Laflamme of Los Alamos National Laboratory and Gerard J. Milburn of the University of Queensland in Australia. They published the research in the January 4, 2001 issue of Nature. The research was funded by the Department of Energy and the National Security Agency.

Preskill's research colleagues were Daniel Gottesman of the University of California at Berkeley and Alexei Kitaev of Microsoft Research. Their work is scheduled the published in the journal Physical Review A. The research was funded by the Department of Energy and the Defense Advanced Research Projects Agency.

Timeline:   25 years
Funding:   Government
TRN Categories:   Quantum Computing
Story Type:   News
Related Elements:  Technical paper, "A scheme for efficient quantum computation with linear optics," Nature, January 4, 2001; Technical paper, "Encoding a qudit in an oscillator," http://arXiv.org/abs/quant-ph/?0008040


January 31, 2001

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Ordinary light could drive quantum computers

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