A TECHNIQUE that does away with the most difficult part of quantum computing has been made to work for the first time. It may open the way for practical quantum computers.
Such a computer uses quantum particles to store information in a set of quantum bits or 鈥渜ubits鈥�. It carries out its logical operations by changing the quantum states of the particles to compute an output. Because quantum particles can be in more than one state at a time, a quantum computer can explore many lines of computation all at once 鈥� and this is what gives it an advantage over conventional computers.
But a practical, functioning quantum computer still exists only in concept, because it depends on operations that are exceptionally delicate. One of the biggest hurdles has been producing and manipulating a quantum property known as entanglement. This refers to the way two or more quantum particles can be intimately linked, so that doing something to one immediately influences the other. 鈥淐reating and controlling entanglement is very difficult,鈥� says Philip Walther of the University of Vienna, Austria. He points out that physicists using this technique have only managed to perform a few of the hundreds of different operations that a real quantum computer would need.
Advertisement
Now there might be a simpler way. It is based on a technique suggested four years ago by Robert Raussendorf and Hans Briegel of the Ludwig Maximilian University, Munich, Germany. They showed that, in theory, a quantum computer might be built by packing all the difficult aspects of entanglement into one initial step. The idea is that a number of quantum particles are set up in a special, highly entangled group called a cluster state, and this state would provide all the entanglement that is needed for computing. Any subsequent computation could be carried out by making a sequence of simple measurements that would only involve reading out the state of single particles. There would be no further need to create and control entangled particles (Physical Review Letters, vol 86, p 5188). 鈥淲hat makes this important is that single-particle measurements are very easy to do,鈥� says Walther. 鈥淭hey are much simpler than complicated entangling operations.鈥�
鈥淭he idea works in practice. Their rudimentary quantum computer was able to run a powerful algorithm鈥�
Walther and his colleagues have now demonstrated that the idea works in practice. They created a cluster of four entangled photons that stored quantum information on the photons鈥� state of polarisation, which can be either vertical or horizontal. Then they showed how a sequence of polarisation measurements on the photons could be used to carry out basic quantum computations. Their rudimentary quantum computer was able to run 鈥淕rover鈥檚 algorithm鈥� 鈥� a powerful quantum algorithm for searching a list for specific items (Nature, vol 434, p 169).
Physicist Daniel Loss of the University of Basle in Switzerland is excited by the development, though he points out that it is relatively easy to create a cluster of entangled photons. It may not be so with other types of quantum particles.
