Quantum computation: Fragile yet error-free

In a close collaborative effort, Spanish and Austrian physicists have experimentally encoded one quantum bit (qubit) in entangled states distributed over several particles and for the first time carried out simple computations on it. The 7-qubit quantum register could be used as the main building block for a quantum computer that corrects any type of error. The researchers' results have now been published in Science.

Even computers are error-prone. The slightest disturbances may alter saved information and falsify the results of calculations. To overcome these problems, computers use specific routines to continuously detect and correct errors. This also holds true for a future quantum computer, which will require procedures for error correction as well.

"Quantum phenomena are extremely fragile and error-prone. Errors can spread rapidly and severely disturb the computer," says Thomas Monz, member of Rainer Blatt's research group at the Institute for Experimental Physics at the University of Innsbruck. Together with Markus Müller and Miguel Angel Martin-Delgado from the Department for Theoretical Physics at the Complutense University in Madrid, the physicists in Innsbruck developed a new quantum error-correcting method and tested it experimentally. "A quantum bit is extremely complex and cannot be simply copied. Moreover, errors in the microscopic quantum world are more manifold and harder to correct than in conventional computers," underlines Monz. "To detect and correct general errors in a quantum computer, we need highly sophisticated so-called quantum error-correcting codes."