Until recently scientists thought that quantum entanglement, when particles such as photons and electrons interact physically and then become separated, was required to run a quantum computer. But although entanglement, a phenomenon famously derided by Einstein as ‘spooky action at a distance’, can be facilitated in the laboratory in near ideal conditions, outside the laboratory the process is fragile and transient, and therefore not ideal.
Now, researchers have realised that entanglement may not always be necessary, and new examples of technologies that can gain a quantum advantage without entanglement have been discovered over the past few years.
A new study, published in the journal Nature Physics, from researchers in Australia, Singapore and the United Kingdom, has focused on a technology called quantum discord. This phenomenon, far more robust and easily accessible than entanglement, can also deliver a quantum advantage: it could be harnessed to bring quantum technologies within easier reach than expected.
By encoding information onto laser light to demonstrate the unlocking of this quantum resource, they showed that it is possible to retrieve more information by using quantum discord than by not accessing the discord.
Another study author, Ping Koy Lam from the Australian National University (ANU), likened their experiment 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.
Quantum discord has been shown to be present in many systems, and might previously have been characterised as unwanted noise, making some scientists sceptical about its potential usefulness, but these new findings suggest otherwise. The experiment carried out isn’t considered a quantum computation, but it shows that discord has potential that can be unlocked for quantum technologies.
Researchers are now looking for other tasks that could be enhanced by quantum discord. The hope is that discord could prove an easier path to future quantum technologies than entanglement.
Their study ‘hints towards the possibility that the requirements on certain quantum technologies could be relaxed’.
Nature Physics – Coherent interactions that generate negligible entanglement can still exhibit unique quantum behaviour. This observation has motivated a search beyond entanglement for a complete description of all quantum correlations. Quantum discord is a promising candidate. Here, we demonstrate that under certain measurement constraints, discord between bipartite systems can be consumed to encode information that can only be accessed by coherent quantum interactions. The inability to access this information by any other means allows us to use discord to directly quantify this ‘quantum advantage’. We experimentally encode information within the discordant correlations of two separable Gaussian states. The amount of extra information recovered by coherent interaction is quantified and directly linked with the discord consumed during encoding. No entanglement exists at any point of this experiment. Thus we introduce and demonstrate an operational method to use discord as a physical resource.
Quantum entanglement has the capacity to enable disruptive technologies that solve outstanding issues in:
Trust, privacy protection, and security in two- and multi-party transactions;
– Novel or enhanced modes of operation of ICT devices;
– Reference standards, sensing, and metrology.
The development of entanglement-based strategies addresses these challenges and provides the foundations for quantum technologies of the 21st century. The practical exploitation of entanglement requires groundbreaking levels of robustness and flexibility for deployment in real-world environments.
This ambitious goal can be reached only through radically new designs of protocols, architectures, interfaces, and components. Q-ESSENCE will achieve this by a concerted application-driven effort covering relevant experimental, phenomenological, and fundamental aspects. Our consortium will target three main outcomes:
– Development of entanglement-enabled and entanglement-enhanced ICT devices: atomic clocks, quantum sensors, and quantum random-number generators;
– Novel physical-layer architectures for long-distance quantum communication that surpass current distance limitations through the deployment of next-generation components;
– Distributed quantum information protocols that provide disruptive solutions to multiuser trust, privacy-protection, and security scenarios based on multipartite entanglement.
These outcomes will be reached through the underpinning science and enabling technologies of: light-matter interfaces providing faithful interconversion between different physical realizations of qubits; entanglement engineering at new scales and distances; robust architectures protecting quantum information from decoherence; quantum information concepts that solve problems of limited trust and privacy intrusion. The project builds on the outstanding expertise of the consortium demonstrated by pioneering works over the past decades, enhanced by a strong industrial perspective.