According to researcher Pedram Roushan, there had already been efforts to build and study individual parts of a quantum processor but this particular project involves putting them all together in a basic building block that can be fully controlled and potentially scaled up into a functional quantum computer.
"You're dealing with particles – qubits in this case – that are interacting with one another, and they're interacting with external fields," Roushan said.
To help solve this problem, the group’s quantum processing system had to be built from a single qubit up, in order to better understand the states, behaviours and interactions that can occur.
By engineering the pulse sequences used to manipulate the spins of the photons in their system, the researchers created an artificial magnetic field affecting their closed loop of three qubits, causing the photons to interact strongly with not only each other, but also with the pseudo-magnetic field.
"The more control we have over a quantum system, the more complex algorithms we would be able to run," said scientist Anthony Megrant. "However, every time we add a control line, we're also introducing a new source of de-coherence." At the level of a single qubit, a margin of error may be tolerated, the researchers explained, but even with a relatively small increase in the number of qubits, the potential for error multiplies exponentially.
To combat the potential for error while increasing their level of control, the team had to reconsider both the architecture of their circuit and the material that was being used in it. Instead of their traditionally single-level, planar layout, the researchers redesigned the circuit to allow control lines to ‘cross over’ others via a self-supporting metallic ‘bridge’. The dielectric – the insulating material between the conducting control wires – was itself found to be a major source of errors so a more precisely fabricated and less defective substrate was made to minimise the likelihood of de-coherence.
"If we can control these systems very precisely – maybe at the level of 30 qubits or so – we can get to the level of doing computations that no conventional computer can do," Roushan concluded.
