Physics: Pioneering work honored

Almost 20 years ago, physicist Hans Briegel and his doctoral student Robert Raussendorf presented the idea for a measurement-based quantum computer. Physical Review A has now selected the development of this concept from the year 2003 as one of the 26 most important contributions in the fifty-year history of the journal.
One-way quantum computer
Image: The entanglement structure of a large quantum processor of light (Credit: Shota Yokoyama)

The quantum computer model jointly developed by Hans Briegel and Robert Raussendorf differs from the classical circuit model in that it first generates an entangled quantum state independent of the research question. The actual calculation is then performed by specific measurements on this state, with the individual measurements being determined by the results of previous measurements. Hans Briegel and Robert Raussendorf first presented the theoretical design for this type of quantum computer in 2001 in Physical Review Letters. Back then, still in Munich, the two quantum physicists did not only have a blueprint for a new quantum computer in mind, but rather the foundations for a better understanding of what a quantum computer actually is. "We have left the notion of the computer as a network of logical gates in our concept. Our quantum computer is based solely on measurements on entangled particles," explains Briegel. "It is possible to create entangled states in nature that enable us to calculate with quanta if we just measure them cleverly." The researchers elaborated this concept in detail in a paper published in the journal Physical Review A in 2003. In 2005, researchers led by Anton Zeilinger in Vienna were able to put the principle into practice experimentally for the first time with clusters on four photons. The way in which the states of the particles are measured allows complex mathematical operations to be performed. The measurement thereby destroys the entanglement of the particles, which is why this new model of a quantum computer is also called a one-way quantum computer.

Hans Briegel became Professor at the University of Innsbruck in 2003, Robert Raussendorf is now Associate Professor at the University of British Columbia in Vancouver, Canada. According to Web of Science, the paper in Physical Review Letters has been cited more than 2,400 times, and the work in Physical Review A, which has been honored as one of the most important papers of the past fifty years by PRA, has been cited more than 800 times.

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