Results show that discord can deliver a quantum advantage

"We've shown that quantum discord is a resource that we can tap with the right quantum tools" — Mile Gu, Centre for Quantum Technologies. Read Mile's popular take on the work on the FQXi blog. This is an artist's illustration of discord being harnessed to enhance the extraction of information from a system. Credit: Timothy Yeo / CQT, National University of Singapore.

New research lends hope that a phenomenon called quantum discord could be harnessed to bring quantum technologies within easier reach than expected. A pair of papers published 5 August in Nature Physics reports experiments showing that discord is enough to deliver a quantum advantage. Theoretical scientists from the Centre for Quantum Technologies at the National University of Singapore contributed to the two results.

Until a few years ago, researchers thought that realising quantum technologies would mean harnessing the most difficult-to-tame properties of the quantum world. For example, entanglement, the phenomenon referred to by Einstein as spooky-action-at-a-distance, was thought to be a resource required to run a quantum computer. This presents a challenge. In a laboratory setting, entanglement can be protected with near ideal conditions. Outside the lab, however, entanglement is fragile and transient.

But now researchers realise that entanglement may not always be necessary. In the past few years, scientists have discovered examples of technologies that seem to gain a quantum advantage without entanglement. Researchers are left with the question, where does the quantum power come from?

The two papers in Nature Physics make the case that discord can be a quantum power source, exploring its role in two tasks. One paper shows that accessing discord can enhance the extraction of information encoded in a pair of laser beams, the other paper shows that discord is the resource required for 'remote state preparation', which is a variant of quantum teleportation.

"We've shown that quantum discord is a resource that we can tap with the right quantum tools," said Mile Gu, a Research Fellow at CQT and co-author on the first paper, "Observing the operational significance of discord consumption". Two researchers jointly affiliated with CQT and the University of Oxford, Kavan Modi and Vlatko Vedral, are also co-authors. The three theorists collaborated with experimental physicists at The Australian National University (ANU) and the University of Queensland to measure discord's value.

Researcher Helen Chrzanowski operates an experiment at the Australian National University that unlocks quantum discord in pairs of laser beams. Credit: The Australian National University

The ANU team encoded information onto laser light, across a pair of beams. They show that they can retrieve more information by using quantum discord than if the discord is not accessed.

Ping Koy Lam, Professor at ANU, said "The experiment is analogous to decoding music from a AM/FM radio simulcast that is badly affected by static." They found that discord is similar to shared quantum static and that more 'music' can be extracted from this simulcast with the right quantum tools. It isn't considered a quantum computation, but it shows that discord has potential that can be unlocked for quantum technologies.

For the second paper, "Quantum discord as resource for remote state preparation", Vlatko and CQT researcher Tomasz Paterek collaborated with scientists from a cluster of quantum research centres in Vienna, Austria. The team demonstrated that photons with discord but no entanglement could be used for remote state preparation, which is the control of the state of one photon by manipulating the state of second, remote photon by making certain measurements. Furthermore, they showed that the quality of the state preparation depended on the discord present.

Quantum discord has been shown to be present in many systems, and might previously have been characterised as unwanted noise. This has made some scientists sceptical that it could be useful. The new results suggest otherwise.

Researchers are now looking for other tasks that may be enhanced by quantum discord. The hope is that discord could prove an easier path to future quantum technologies than entanglement. With a scientist's caution, Lam said "our work hints towards the possibility that the requirements on certain quantum technologies could be relaxed."

For details of the work, see M. Gu et al, "Observing the operational significance of discord consumption", Nature Physics, doi:10.1038/nphys2376. A preprint is available at arXiv:1203.0011 and B. Dakic et al "Quantum discord as resource for remote state preparation", Nature Physics, doi:10.1038/nphys2377 (2012).

Selected media coverage

• FQXi Blog: Quantum Discord & The Power of Knowing Quantum Mechanically (by CQT author Mile Gu)
• Science Daily: Information advantage gained from surprising quantum source
• PhysOrg: Discord strikes the right quantum note
• R&D Mag: Quantum insight: Non-entangled states can outperform entangled counterparts
• ECN Mag: Quantum physics: New insights into the remote control of quantum systems