Los Alamos Scientists Shed New Light On Quantum Computation

A functional quantum computer could solve certain large mathematical problems and crack secret codes at speeds faster than today's fastest supercomputers. If quantum computers can be built, they can factor large numbers, making them extremely useful for decoding information encrypted by means of currently standard methods.

Los Alamos researchers propose to use quanta of light or photons, the smallest unit of electromagnetic energy, as the basic elements for quantum information processing. Previous proposals based on photons required a crucial ingredient: non-linear optical elements that allow photons to interact with each other. While such elements have been used for proof-of-principle demonstrations, they suffer from a fatal flaw: they are much too weak to be combined usefully for quantum computation.

Up to now researchers believed that the only feasible option for a photon-based quantum computer was to make the non-linear elements stronger by several orders of magnitude, which seemed a difficult task. Now Emanuel Knill and Raymond Laflamme of Los Alamos and Gerard Milburn of the University of Queensland have proposed a different approach.

Presented in the Jan. 4 issue of Nature magazine, their idea is to use the high sensitivity of single photon detection and exploit the detection results to simulate the effects of non-linear elements. Although this process results in apparently irreversible loss of the "quantumness" of the system, the researchers have demonstrated that this loss can be prevented by using quantum error correction.

The proposed device has several advantages over its rivals. One advantage is that it can work at room temperature, which potentially makes these devices as accessible as personal computers. Also, it is based on existing technology: beam splitters, phase shifters, single photon sources and detectors. These, however, need to operate at higher precision than currently available.

"It was widely believed that optics without non-linear elements is no more powerful than currently available, classical computers," said Knill. "Although the measurements in our scheme irreversibly alter the system, one can still usefully quantum compute. The unwanted effect of measurements can be considered as an error on the system, and as long as both the location and the type of error are known, the system is surprisingly resilient. This discovery is surprising and unexpected, and we think that it provides a useful blueprint for quantum computers. The challenge will be to put our idea into practice."

Los Alamos National Laboratory has been a leader in theoretical and experimental quantum computation since quantum computers were first proposed in the 1990s as a way to factor large numbers. A three-qubit quantum computer was demonstrated by Knill, Laflamme and their collaborators at Los Alamos in 1998 using nuclear magnetic resonance with trichloroethylene molecules; they built the first seven qubit device in 2000.

Los Alamos National Laboratory is operated by the University of California for the Department of Energy.