If you measure one particle in an entangled pair whose partner is miles away, the measurement seems to rip through the intervening space to affect its partner. The main focus of tests of quantum theory has been this silly action at a distance.

Non-locality is amazing. Adn Cabello is a physicist at the University of Sevilla in Spain.

Cabello wants to investigate contextuality, a lesser-known aspect of quantum mechanics. The properties of particles are only found in the context of a measurement. Rather than thinking of particles as having fixed values, think of them as words in language that can change depending on the context. There are fruit flies.

Physicists think contextuality is more of a hallmark feature of quantum systems than nonlocality is. A single particle is a quantum system in which you can't even think about nonlocality. It's important to understand the power of quantum systems and to understand why quantum theory is the way it is.

There are links between contextuality and problems that quantum computers can solve that ordinary computers can't.

With renewed interest comes a renewed attempt to prove that our world is contextual. The first loophole-free experimental test of contextuality was performed by Cabello and Kihwan Kim at Tsinghua University in Beijing, China.

John Stewart Bell is a physicist from Northern Ireland. In 1965, he showed that the high degree of correlations between the particles can't be explained by local variables. The entangled pair needs to be shared between the particles.

Bell showed a similar idea about contextuality. It is impossible for a quantum system to have hidden variables that define the values of all their properties.

Kochen and Specker looked at a single particle with a quantum property called spin, which has both a magnitude and a direction. If you measure the spin's magnitude along any direction, you will get either 1 or 0. The researchers asked if it was possible that the particle knew what the result would be before it was measured. Is it possible to assign a hidden variable to all the possible outcomes of a single measurement at the same time?

Kochen and Specker came up with a contradiction using this rule. They assumed that each particle had a fixed value. They assigned either 0 or 1 to the outcome after conducting a hypothetical spin measurement. They were able to satisfy the 101 rule by either assigning a value to the outcome or deducing what the value must be in order to meet it.

The contradiction came up in the 116th direction. The 101 rule dictated that the spin must be 1. Both 0 and 1 could not be returned from the measurement. The physicists concluded that there is no way a particle can have hidden variables that are unchanging.

The proof showed that quantum theory requires contextuality, but there wasn't a way to show it. Physicists have come up with more practical, experimentally implementable versions of the original Bell-Kochen- Specker Theorem involving multiple entangled particles.

One of the simplified versions of the Bell-Kochen- Specker theorem was shown to be equivalent to a basic quantum computation.

The proof was named Mermin's star after the originator, David Mermin. Measurement-based quantum computing is based on the logic of how earlier measurement shapes the outcome of later measurement. Researchers have been struggling to understand why quantum computers can solve problems faster than classical computers.

A physicist at the University of British Columbia has shown that contextuality is necessary for a quantum computer to beat a classical computer at some tasks. It's probably not the right question to ask if contextuality powers quantum computers. We need to ask the right questions. We get an answer when we ask a question that we know how to ask. The next question is asked by us.

The world is contextual according to Bell, Kochen and Specker. They say that hidden variables have not been ruled out.

Cabello and Kim announced in February that they had closed every possible loophole by doing the Bell-Kochen- Specker experiment.

The choice of measurement on one ion defined the context for the other ion as part of the experiment. Although making a measurement on one ion does not affect the other, it changes the context and thus the outcome of the second ion's measurement, according to physicists.

Skeptics want to know if the context created by the first measurement is what changed the second measurement outcome. Cabello and Kim showed that the outcomes don't change if the context isn't present. They decided that the only reasonable explanation for their results was contextuality.

The level of contextuality and the power of quantum computing could be tested in the future.

Cabello said that if you want to understand how the world works, you need to go into detail.