The world's first universal programmable quantum computer has been put through its paces but is still not ready for real work, according to a November 15 article by Colin Barras for NewScientist. Earlier in the year, a team at the National Institute of Standards and Technology (NIST; Boulder, CO) built a quantum computer capable of processing two quantum bits, or qubits, but applied some additional theory to make the breakthrough (see also "Photons promise an exciting route to quantum computing").
Unlike the physical logic gates of a classical computer, the quantum logic gates used in the team's device are each encoded into a laser pulse. The experimental device uses beryllium ions to store qubits in the way they spin while the laser-pulse quantum gates perform simple logic operations on the qubits. The trick to making a quantum logic gate is in designing a series of laser pulses that manipulate the beryllium ions in a way that processes information. Another laser then reads off the results of the calculations.
The researchers used a quantum computational theory that says you can do any quantum operation on any number of qubits using only single and two-qubit logic gates. Although one and two-qubit gates have already been built and used to perform specific algorithms, no one had yet built a device capable of all possible quantum routines. Until now.
At the heart of the device is a gold-patterned aluminum wafer containing a tiny electromagnetic trap some 200 micrometers 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. Each operation involves hitting the two qubits with 31 distinct quantum gates encoded into the laser pulses. The majority were single-qubit gates, and so the pulse needed to interact with just one ion, but a small number were two-qubit gates requiring the pulse to "talk" to both ions. By controlling the voltage on the gold electrodes surrounding the trap, the team can uncouple the ions when single-qubit gates are needed and couple them again for two-qubit operations.
The team ran each of the 160 programs 900 times. By comparing the results with theoretical predictions, they were able to show that the processor had worked as planned, but with an accuracy of only 79%. That's because each of the laser pulses that act as the gates varies slightly in intensity. But the team says that the fidelity needs to increase to around 99.9% before it could be used in a quantum computer, which could be done by improving the stability of the laser and reducing the errors from optical hardware.
See the full article at www.newscientist.com/article/dn18154-first-universal-programmable-quantum-computer-unveiled.html.
