Quantum Radio, Do You Read Me?

Atomic antennae transmit quantum information across a microchip

The Innsbruck research group led by physicist Rainer Blatt suggests a fundamentally new architecture for quantum computation. In an international first, they have experimentally demonstrated quantum antennae, which enable the exchange of quantum information between two separate memory cells located on a computer chip. This offers new opportunities to build practical quantum computers. The researchers have published their work in the scientific journal Nature.

Six years ago scientists at the University of Innsbruck realized the first quantum byte – a quantum computer with eight entangled quantum particles; a record which still stands. “None the less, to make practical use of a quantum computer that performs calculations, we need a lot more quantum bits,” says Prof. Rainer Blatt, who, with his research team at the Institute for Experimental Physics, created this first quantum byte in an electromagnetic ion trap. “In these traps we cannot just string together large numbers of ions and control them simultaneously." To solve this problem, the scientists have started to design a quantum computer based on a system of many small registers which must then be linked together. To achieve this, Innsbruck quantum physicists have now developed a revolutionary approach based on a concept formulated by theoretical physicists Ignacio Cirac and Peter Zoller. In their experiment, the physicists electromagnetically coupled two groups of ions over a distance of about 50 micrometers. Here, the motion of the particles serves as an antenna. “The particles oscillate like electrons in the poles of a TV antenna and thereby generate an electromagnetic field,” explains Blatt. “If the antennae are tuned in to one another, the receiving end picks up the signal of the sender, which results in coupling.” The energy exchange that takes place in this process could be the basis for fundamental computing operations of a quantum computer.

Antennae amplify transmission

“We implemented this new concept in a very simple way,” explains Rainer Blatt. In a miniaturized ion trap a double-well potential was created, in which the calcium ions were trapped. The two wells are separated by 54 micrometers. “By applying a voltage to the electrodes of the ion trap, we were able to match the oscillation frequencies of the ions,” says Blatt. “Coupling and energy exchange is achieved in this process, which can be used to transmit quantum information.” Never before has a direct coupling of two mechanical oscillations at the quantum level been demonstrated. In addition, the scientists show that the coupling is amplified by using more ions in each well. “These additional ions function as antennae and increase the distance and speed of the transmission,” says Rainer Blatt, clearly excited about the new concept. This work constitutes a promising approach for building a fully functioning quantum computer. “The new technology offers the possibility to distribute entanglement. At the same time, we are able to target each memory cell individually,” explains Rainer Blatt. The new quantum computer could be based on a chip with many micro traps, where ions communicate with each other through electromagnetic coupling. This new approach represents an important step towards practical quantum technologies for information processing.

This picture illustrates our vision of a future quantum computer. Strings of ions are held as separate strings above an “ion trap chip”. Through the antennae-effect, quantum information can be exchanged between neighbouring ion strings. Graphics: Harald Ritsch. Copyright statement: The picture may be freely used provided that the source is correctly stated.

The quantum researchers are supported by the Austrian Science Fund FWF, the European Union, the European Research Council and Tyrolean Industry.

Links

• Trapped-ion antennae for the transmission of quantum information
  Maximilian Harlander, Regina Lechner, Michael Brownnutt, Rainer Blatt, Wolfgang Haänsel
  Nature , Advance online publication: 24 February 2011.
• Coupled quantized mechanical oscillators
  K. R. Brown, C. Ospelkaus, Y. Colombe, A. C. Wilson, D. Leibfried & D. J. Wineland
  Nature , Advance online publication: 24 February 2011.