Physicists in Innsbruck and Geneva have developed a new reliable method to verify entanglement in the laboratory, using a minimal number of assumptions about the system and measuring devices.

Many quantum technologies require entanglement, so 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, University Innsbruck, which holds the world record for largest number of entangled particles.

“To verify the entanglement, we had to make some experimentally calibrated assumptions [that] make any subsequently derived statements vulnerable,” explains Monz. So together with Julio Barreiro, who recently moved on the Max Planck Institute of Quantum Optics in Garching, and Jean-Daniel Bancal from the group of Nicolas Gisin at the University of Geneva, now at the Center for Quantum Technologies in Singapore, the physicists derived and implemented a new method to verify entanglement between several objects.

Finding correlations

The new 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 Barreiro.

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.”

Assumptions as Achilles heel

For physicists, such procedures that are based on very few assumptions are highly interesting. By being basically independent of the system, 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 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. The technical demands, however, also increase accordingly.