D-Wave reveals use of ‘quantum annealing’ to make eight qubits

May 16, 2011 | Source: Physics World

D-Wave Systems' circuit boards use "flux qubits," micrometer sized loops of superconducting metal (credit: D-Wave Systems)

Quantum annealing can be used to make eight coupled quantum bits (qubits) find their ground state, a step forward in building a commercially viable quantum computer,  researchers at D-Wave Systems in Vancouver, B.C. have demonstrated.

Quantum annealing is a method for finding solutions to combinatorial optimization problems. The researchers used it with eight flux qubits (micrometer sized loops of superconducting metal) within one of D-Wave System’s integrated circuits. These contained 128 flux qubits arranged into 16 units of eight.

The researchers adjusted the interactions between the qubits to simulate a 1-D chain of magnets in which each qubit wants to point in the same direction as its two neighbors. The qubit at the right-hand end of the chain was set in the up direction and the qubit at the left-hand end in the down direction. The six qubits in the middle were then allowed to orient their spins according to that of their neighbors. The result was a “frustrated” ferromagnetic arrangement in which two neighbors must have opposing spins.

The qubits were all tilted in the same direction while the barrier was raised. The researchers found that about 45 mK in temperature, the system found its way to the ground state in a manner consistent with the spins flipping because of quantum-mechanical tunneling rather than thermal activation, confirming what quantum mechanics predicts, the researchers said.

Ref.: M. W. Johnson et al., Quantum annealing with manufactured spins,Nature 473,194–198(12 May 2011) doi:10.1038/nature10012