Physicists have crafted a new, reliable method to verify entanglement in the laboratory using a minimal number of assumptions about the system and measuring devices - it witnesses the presence of useful entanglement, a ‘verification without knowledge’.

Quantum computation, quantum communication and quantum cryptography often require entanglement. For many of these upcoming quantum technologies, entanglement – this hard to grasp, counter-intuitive aspect in the quantum world – is a key ingredient. Therefore, experimental physicists often need to verify entanglement in their systems.

“Two years ago, we managed to verify entanglement between up to 14 ions,” explains Thomas Monz, who works in the group of Rainer Blatt at the Institute for Experimental Physics at the University of Innsbruck - that is the record for the largest number of entangled particles. “In order to verify the entanglement, we had to make some, experimentally calibrated, assumptions. However, assumptions, for instance about the number of dimensions of the system or a decent calibration, make any subsequently derived statements vulnerable.”

*Certified entangled! Credit: Uni Innsbruck/Ritsch*

The presented device-independent method is based on a single assumption: “We only have to make sure that we always apply the same set of operations on the quantum objects, and that the operations are independent of each other,” explains lead author Julio Barreiro. “However, which operations we apply in detail – this is something we do not need to know.”

This 'Device Independent' approach allows them to get around several potential sources of error, and subsequently wrong interpretations of the results.

“In the end, we investigate the correlations between the settings and the obtained results. Once the correlations exceed a certain threshold, we know that the objects are entangled,” said Barreiro.

For the experimentally hardly avoidable crosstalk of operations applied to levitating calcium ions in the vacuum chamber in Innsbruck, co-author Jean-Daniel Bancal adapted the threshold according to a worst-case scenario. “When this higher threshold is breached, we can claim entanglement in the system with high confidence.”

**Assumptions as Achilles heel**

For physicists, limiting assumptions is as much art as science because they provide high confidence and strengthen the results of experimentalists. “Assumptions are always the Achilles heel – be that for lab data or theory work,” stresses co-author Thomas Monz. “We managed to reduce the number of assumption to verify entanglement to a minimum. Our method thus allows for reliable statements about the entanglement in a system.”

In the actual implementation, the physicists in Innsbruck could verify entanglement of up to 6 ions. This new method can also be applied for larger systems but the technical demands also increase accordingly.

Citation: Julio T. Barreiro, Jean-Daniel Bancal, Philipp Schindler, Daniel Nigg, Markus Hennrich, Thomas Monz, Nicolas Gisin, and Rainer Blatt, 'Demonstration of genuine multipartite entanglement with device-independent witnesses', Nature Physics DOI: 10.1038/NPHYS2705

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