Scientists use 'voting' and 'penalties' to overcome errors in quantum optimization

Feb 06, 2014

Seeking a solution to decoherence—the "noise" that prevents quantum processors from functioning properly—scientists at USC have developed a strategy of linking quantum bits together into voting blocks, a strategy that significantly boosts their accuracy.

In a paper published today in Nature Communications, the team found that their method results in at least a five-fold increase in the probability of reaching the correct answer when the processor solves the largest problems tested by the researcher, involving hundreds of qubits.

The team, led by Daniel Lidar—director of the USC-Lockheed Martin Quantum Computing Center at the USC Viterbi School of Engineering—ran their tests on the 512-quantum-bit D-Wave Two processor. The D-Wave Two is among the first commercially available quantum processors, a device so advanced that there are only two in use outside the Canadian company where they were built: The first one went to USC and Lockheed Martin, and the second to NASA and Google.

"We have demonstrated that our quantum annealing correction strategy significantly improves the success probability of the D-Wave Two processor on the benchmark problem of antiferromagnetic chains, and are planning to next use it on computationally hard problems," Lidar said. His team includes graduate student Kristen Pudenz and postdoctoral fellow Tameem Albash.

Lidar added that all quantum are expected to be highly susceptible to decoherence, so that error correction is viewed as an essential and inescapable part of .

Quantum processors encode data in qubits, which have the capability of representing the two digits of one and zero at the same time – as opposed to traditional bits, which can encode distinctly either a one or a zero. This property, called superposition, along with the ability of quantum states to "interfere" (cancel or reinforce each other like waves in a pond) and "tunnel" through energy barriers, is what may one day allow quantum processors ultimately perform optimization calculations much faster than traditional processors.

Decoherence knocks qubits out of superposition, forcing them to behave as traditional bits, and robbing them of their edge over traditional processors.

Pudenz, Albash and Lidar developed and tested a strategy of grouping three qubits together into larger blocks of encoded qubits that can be decoded by a "majority vote." This way, if decoherence affects one of the qubits and causes it to "flip" to the incorrect value, the other two qubits in the block ensure that the data is still correctly encoded and can be correctly decoded by out-voting the errant .

These voting blocks of qubits are then magnetically tied to a fourth qubit in such a way that if any one "flips" then all four must flip. In effect, it makes the whole block of four so massive that it's difficult for one lonely qubit acting under the influence of decoherence to throw a wrench in the works.

Explore further: Quantum holograms as atomic scale memory keepsake

More information: Paper: www.nature.com/ncomms/2014/140… full/ncomms4243.html

Related Stories

Large-scale quantum chip validated

Jun 28, 2013

A team of scientists at USC has verified that quantum effects are indeed at play in the first commercial quantum optimization processor.

Quantum computer built inside a diamond

Apr 04, 2012

Diamonds are forever – or, at least, the effects of this diamond on quantum computing may be. A team that includes scientists from USC has built a quantum computer in a diamond, the first of its kind to include protection ...

Advancing quantum computing

May 30, 2012

European researchers have made important advances in understanding the major stumbling block to realisation of quantum computers, a phenomenon known as decoherence.

D-Wave sells first commercial quantum computer

Jun 01, 2011

(PhysOrg.com) -- Last week, Burnaby, British Columbia-based company D-Wave Systems, Inc., announced that it sold its first commercial quantum computer. Global security company Lockheed Martin, based in Bethesda, ...

Recommended for you

Cooling with molecules

10 hours ago

An international team of scientists have become the first ever researchers to successfully reach temperatures below minus 272.15 degrees Celsius – only just above absolute zero – using magnetic molecules. ...

A 'Star Wars' laser bullet

11 hours ago

Action-packed science-fiction movies often feature colourful laser bolts. But what would a real laser missile look like during flight, if we could only make it out? How would it illuminate its surroundings? ...

User comments : 0