Scientists have unveiled the world's first universal programmable quantum computer, capable of processing two quantum bits or qubits which store more data than the simple 'on' or 'off' bits of conventional computing. But, the test programme has revealed significant hurdles which the scientists claim must be overcome before it is ready for real work, the New Scientist reported.
A team, led by the National Institute of Standards and Technology in Colorado, has developed the experimental device which uses beryllium ions to store qubits in the way they spin while the laser-pulse quantum gates perform the simple logic operations on the qubits.
According to the scientists, the trick to making a quantum logic gate is in designing a series of laser pulses that manipulate the beryllium ions in a way which processes information. Another laser then reads off the results of the calculations.
"Once we had demonstrated we could successfully combine lots of components in this way, we ask -- what can you do with that?" David Hanneke, who led the team, said.
They found their answer in quantum computational theory. "One of the more interesting results to come out of the early years of quantum information was that you can do any quantum operation on any number of qubits using only single and two-qubit logic gates," said Hanneke.
Although one and two-qubit gates have been built and used to perform specific algorithms, no one built a device capable of all quantum routines till now.
At the heart of the device is a gold-patterned aluminium wafer containing a tiny electromagnetic trap some 200 micrometres across, into which the team placed four ions -- two of magnesium and two of beryllium. The magnesium ions act as 'refrigerants', removing unwanted vibrations from the ion chain and so keeping the device stable.
There are an infinite number of possible two-qubit operations, so the team chose a random selection of 160 to demonstrate the universality of the processor.
The team ran each of the 160 programmes 900 times. By comparing the results with theoretical predictions, they were able to show that the processor had worked as planned.
"But it did so with an accuracy of only 79 per cent. Each gate is more than 90 per cent accurate but when you stack them together the total figure falls to 79 per cent or so for a given operation," Hanekke said.
That's because each of the laser pulses that act as the gates varies slightly in intensity. "They're not 'square' pulses (that switch on and off cleanly) -- they fluctuate," he says. And the beam has to be split, reflected and manipulated in various ways beforehand, which also introduces errors.
Such errors would drown the results of any more extensive computations. The fidelity needs to increase to around 99.99 per cent before it could be a useful component of a quantum computer. That could be done by improving the stability of the laser and reducing the errors from optical hardware, the scientists said.
The findings have been published in the latest edition of the Nature Physics journal.
