###### August 5, 2013

# New, Reliable Method To Verify Entanglement

**AlphaGalileo Foundation**

For quantum physicists, entangling quantum systems is one of their every day tools. Entanglement is a key resource for upcoming quantum computers and simulators. Now, physicists in Innsbruck/Austria and Geneva/Switzerland realized a new, reliable method to verify entanglement in the laboratory using a minimal number of assumptions about the system and measuring devices. Hence, this method witnesses the presence of useful entanglement. Their findings on this 'verification without knowledge' has been published in *Nature Physics*.

**Finding correlations**

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 Julio Barreiro. "However, which operations we apply in detail -- this is something we do not need to know." This approach - called Device Independent - 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." For the experimentally hardly avoidable crosstalk of operations applied to levitating calcium ions in the vacuum chamber in Innsbruck, the Swiss theorist Jean-Daniel Bancal managed to adapt the threshold according to a worst-case scenario. "When this higher threshold is breached, we can claim entanglement in the system with high confidence," states Bancal.

**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 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. The technical demands, however, also increase accordingly.

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