June 15, 2007 The already breathtakingly rapid evolution of the computer is moving towards a completely new level with Researchers at Delft University of Technology successfully carrying out calculations with two quantum bits, the building blocks of a possible future quantum computer. A concept that has been investigated on a theoretical level since the early 70s, Quantum computers promise problem solving capabilities that far outstrip those of current transistor based machines. The Delft researchers are publishing an article about this important step towards a workable quantum computer in this week’s issue of Nature.

A quantum computer is based on the amazing properties of quantum systems. Rather than measure data in terms of “bits” like a conventional computer, quantum systems utilize a quantum bit (known as a qubit), a particle that exists in two states at the same time. Information from two qubits is entangled in a way that has no equivalent whatsoever in the normal world, giving rise to the complex problem of how to influence these interactions in order to realize a calculation.


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It is highly likely that workable quantum computers will need to be produced using existing manufacturing techniques from the chip industry. Working on this basis, scientists at Delft University of Technology are currently studying two types of qubits: one type makes use of tiny superconducting rings, and the other makes use of ‘quantum dots’.

Now for the first time a ‘controlled-NOT’ calculation with two qubits has been realised with the use of superconducting rings. This is important because it allows any given quantum calculation to be realized. The result was achieved by the PhD student Jelle Plantenberg in a team led by Kees Harmans and Hans Mooij. The research took place within the FOM (Dutch Foundation for Fundamental Research on Matter) concentration group for Solid State Quantum Information Processing.