Reliable Quantum Computing Superconducting Qubit Array

By James Anderson •  Updated: 04/24/14 •  3 min read

A new level of reliability in a five-qubit array has been achieved by a team of physicists at UC Santa Barbara. This moves us a step closer to making a quantum computer a reality.

A functional quantum computer is a dream of many physicists. Contrasted with regular computers, the quantum computer would use quantum bits, or qubits, which make use of the multiple states of quantum phenomena.

When built, a quantum computer would have millions of times more power at certain computations than today’s supercomputers.

Quantum computing relies on complex facets of quantum mechanics such as superposition. This idea holds that any physical object, such as an atom or electron, what quantum computers use to store information, may exist in all of its theoretical states simultaneously. This could raise parallel computing to new levels.

Qubit Error Correction

“Quantum hardware is very, very unreliable compared to classical hardware,”

said UCSB staff scientist Austin Fowler.

“Even the best state-of-the-art hardware is unreliable. Our paper shows that for the first time reliability has been reached. Qubits are faulty, so error correction is necessary,”

said co-lead author Julian Kelly.

Although the team has shown logic operations at the threshold, the array must operate below the threshold to provide an acceptable margin of error.

“We need to improve and we would like to scale up to larger systems,” said lead author Rami Barends. “The intrinsic physics of control and coupling won’t have to change but the engineering around it is going to be a big challenge.”

Xmon Power

The novel configuration of the group’s array stems from the flexibility of geometry at the superconductive level, which allowed the scientists to create cross-shaped qubits they named Xmons.

“Superconductivity comes when certain materials are cooled to a critical level that eliminates electrical resistance and eliminates magnetic fields. The team chose to place five Xmons in a single row, with each qubit talking to its nearest neighbor, a simple but effective arrangement. Motivated by theoretical work, we started really thinking seriously about what we had to do to move forward. It took us a while to figure out how simple it was, and simple, in the end, was really the best,”

said physics professor John Martinis.

“If you want to build a quantum computer, you need a two-dimensional array of such qubits, and the error rate should be below 1 percent,” said Fowler. “If we can get one order of magnitude lower — in the area of 10-3 or 1 in 1,000 for all our gates — our qubits could become commercially viable. But there are more issues that need to be solved. There are more frequencies to worry about and it’s certainly true that it’s more complex. However, the physics is no different.”

Reference: R. Barends, J. Kelly, A. Megrant, A. Veitia, D. Sank, E. Jeffrey, T. C. White, J. Mutus, A. G. Fowler, B. Campbell, Y. Chen, Z. Chen, B. Chiaro, A. Dunsworth, C. Neill, P. O’Malley, P. Roushan, A. Vainsencher, J. Wenner, A. N. Korotkov, A. N. Cleland, John M. Martinis. Superconducting quantum circuits at the surface code threshold for fault tolerance. Nature, 2014; 508 (7497): 500 DOI:10.1038/nature13171

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