UIUC researchers establish a criterion for non-local quantum behavior in networks

July 14, 2023 — A new theoretical study provides a framework for understanding nonlocality, a feature quantum networks must possess to perform operations inaccessible to standard communication technology. By clarifying the concept, the researchers determined the conditions necessary to create systems with strong quantum correlations.

Entangled quantum objects can be used to network separate systems. The researchers demonstrate what is needed for non-local correlations, a requirement for a useful quantum network. Credit: Uicu.

The study, published in Physical Review Letters, adapts techniques from quantum computation theory to create a new classification scheme for quantum nonlocality. Not only has this allowed the researchers to unify previous studies of the concept into a common framework, but it has facilitated a proof that networked quantum systems can exhibit non-locality only if they possess a particular set of quantum characteristics.

On the surface, quantum computing and nonlocality in quantum networks are different things, but our study shows that, in a sense, they are two sides of the same coin, said Eric Chitambar, professor of electrical and computer engineering at the University of Illinois Urbana. -Champaign (UIUC) and project leader. In particular, they require the same fundamental set of quantum operations to provide effects that cannot be replicated with classical technology.

Nonlocality is a consequence of entanglement, in which quantum objects experience strong connections even when separated over vast physical distances. When entangled objects are used to perform quantum operations, the results show statistical correlations that cannot be explained by non-quantum means. Such correlations are called non-local. A quantum network must possess a degree of nonlocality to ensure that it can perform truly quantum functions, but the phenomenon is still poorly understood.

Professor Eric Chitambar

To facilitate the study of nonlocality, Chitambar and physics graduate student Amanda Gatto Lamas applied the formalism of quantum resource theory. By treating nonlocality as a resource to be managed, the researchers’ framework allowed them to view past studies of nonlocality as separate instances of the same concept, just with different restrictions on resource availability. This facilitated the proof of their main result, that non-locality can only be achieved with a limited set of quantum operations.

Our result is the quantum network analogue to an important result of quantum computing called the Gottesman-Knill theorem, explained Gatto Lamas. While Gottesman-Knill clearly defines what a quantum computer must do to surpass a classical one, we show that a quantum network must be built with a particular set of operations to do things that a standard communication network cannot do.

Chitambar believes that the framework will not only be useful for developing criteria for evaluating the quality of a quantum network based on the degree of nonlocality it possesses, but that it can be used to expand the concept of nonlocality.

Right now, there’s a relatively good understanding of the kind of non-locality that can emerge between two parties, he said. However, one can imagine for a quantum network made up of many connected parts that there might be some kind of global property that cannot be reduced to individual pairs on the network. This property could intimately depend on the overall structure of the network.

Source: Michael O’Boyle, Grainger College of Engineering, UIUC

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