Experiment records extreme quantum weirdness

Precision measurement of entangled photon pairs yields correlation measure approaching the Tsirelson bound
09 November 2015

Approaching Tsirelson's bound in a photon pair experiment

This experiment at the Centre for Quantum Technologies in Singapore has made a record measurement of entanglement – approaching the quantum limit with extreme precision. Photo Credit: Alessandro Cerè / CQT, National University of Singapore


An experiment at the Centre for Quantum Technologies (CQT) has pushed quantum weirdness close to its absolute limit.

Researchers from CQT and the University of Seville in Spain have reported the most extreme 'entanglement' between pairs of photons ever seen in the lab. The result was published 30 October in Physical Review Letters, where it is highlighted as an Editors' Suggestion.

The achievement is evidence for the validity of quantum physics and will bolster confidence in schemes for quantum cryptography and quantum computing designed to exploit this phenomenon.

"For some quantum technologies to work as we intend, we need to be confident that quantum physics is complete," says Poh Hou Shun, who carried out the experiment. "Our new result increases that confidence," he says.

Local realism

Entanglement says that two particles, such as photons, can be married into a joint state. Once in such a state, either particle observed on its own appears to behave randomly. But if you measure both particles at once, you notice they are perfectly synchronized.

Albert Einstein was famously troubled by this prediction of quantum physics. He didn't like the randomness that came with just one particle. He said "God does not play dice". He didn't like the correlations that came with two particles, either. He referred to this as "spooky action at a distance".

Experiments since the 1970s have been collecting evidence that quantum predictions are correct. Recently an experiment in the Netherlands became the first to do away with all assumptions in the data-gathering.

Technically known as a 'loophole-free Bell test', the experiment leaves no wiggle room in meaning: entangled particles do behave randomly, and they synchronize without exchanging signals. (The results appeared in Nature on 21 October, doi:10.1038/nature15759).

Entangled to the max

Making quantum entangled photons