Entanglement with multiple parties: when everything is connected

Entanglement is a ubiquitous concept in modern physics research: it occurs in topics ranging from quantum gravity to quantum computers. In a publication that appeared in Physical Review Letters Last week UvA-IoP physicist Michael Walter and his collaborator Sepehr Nezami shed new light on the properties of quantum entanglement – especially when many particles are involved.

In the quantum world, physical phenomena occur that we never observe in our large-scale everyday world. One of these phenomena is quantum entanglement, where two or more quantum systems share certain properties in a way that affects the measurements on the systems. The famous example is that of two electrons that can be entangled in such a way that – even if they are taken very far apart – they can be observed spinning in the same direction, say clockwise or counterclockwise, despite the fact that the direction of rotation of neither of the individual electrons can be predicted in advance.

Entanglement with multiple parties

This example is somewhat limited: entanglement does not necessarily have to be between two quantum systems. Multi-part systems can also be intertwined, even in such an extreme way that if a certain property is perceived for one of them (think of ‘turning clockwise again’), the same property will also be perceived for all other systems. This multiparty entanglement is known as aGHZ state, after physicists Daniel Greenberger, Michael Horne and Anton Zeilinger.

In general, multi-party entanglement is poorly understood and physicists do not have much systematic understanding of how it works. In a new article published in Physical Review Letters This week, UvA physicist Michael Walter and his Caltech collaborator Sepehr Nezami begin to close this gap by theoretically examining a rich class of many-body states and their entanglement properties. To do this, they use a mathematical technique known as a ‘tensor network’. The researchers show that the geometrical properties of this network provide a lot of useful information about the entanglement properties of the states studied.

The more detailed understanding of quantum entanglement that the authors obtain could have many future applications. The research was originally prompted by questions in the quest for a better understanding of the quantum properties of gravity, but the technical tools that have been developed are also very useful in the theory of quantum information used to develop quantum computers and quantum software.