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Further confirmation of quantum mechanics


Quantum physics is a popular scientific discipline - or so it would seem from the Big Bell Test on 30 November 2016, which involved more than 100,000 people around the world. On the day, 12 scientific institutions, including ETH Zurich, explored a question once argued over by Niels Bohr and Albert Einstein in the 1930s. The researchers have reported in the latest edition of the journal Nature that the experiment - once again - confirms Nils Bohr's point of view.

The strange quantum world

The starting point for the debate between the two famous physicists was the fact that quantum mechanics postulates phenomena that seem totally at odds with our everyday experiences. Whereas Bohr could accept the strange aspects of the quantum world, Einstein struggled with the idea that, for example, the properties of objects change as soon as the objects are observed.

In 1964, the British physicist John Bell succeeded in describing the disputed theorem, which also has philosophical relevance, in formal mathematical terms. This allowed Einstein's position - that the theory of quantum mechanics is not yet complete - to be tested experimentally, and a number of so-called Bell tests have been conducted over the last few decades. All of them came out in favour of Nils Bohr.

Humans instead of machines

However, physicists are stubborn, and they took issue with the fact that, so far, the experiments had failed to address an important loophole. Specifically, the test proposed by Bell requires that the measurements be carried out at random. But what happens if the machine that generates the randomness only appears to be operating at random? The result would then only appear to confirm the theory of quantum mechanics - so Einstein could still have been right.

It is precisely this loophole that the Big Bell Test was now meant to close. The idea was that the randomness would no longer be generated by a machine, but rather by humans. The assumption is that, because humans are possessed of free will, they are able to generate numbers independently of one another - and that, overall, these numbers will be random.

For the Big Bell Test, the researchers developed an online game that was to be played by at least 30,000 people around the world on the day of the test. During the game, the players had to come up with a random sequence of numbers - the more random the sequence, the better their score in the game. The random numbers generated in this way were then used as the starting point for quantum physics experiments in the 12 participating laboratories.

Considerable outreach effect

Ultimately, more than three times the required number of people took part in the experiment - a fact that, Andreas Wallraff, Professor of Solid State Matter Physics, considers a huge success. He is also delighted at the positive secondary benefit of this large-scale experiment: "At the same time, the online game was an opportunity for us to bring people closer to the principles of quantum physics. So we've made an important contribution to the goal of disseminating knowledge."

Wallraff took part in the Big Bell Test together with his postdoc Christian Kraglund Andersen and his doctoral candidate Johannes Heinsoo by conducting an experiment in the field of solid state physics. "For our contribution, we used a superconducting circuit that we had developed entirely at ETH Zurich," Andersen explains. "We used two of the four qubits in this circuit to measure the state of two entangled particles at the same time." In total, the researchers in Zurich performed 8 million measurements. The conclusion is that the particles behave just as quantum mechanics predicts.

More large-scale collaborations

Although the group used an established circuit for the Big Bell Test, performing the experiment was still quite a challenging task. "The experiment ran for 48 hours in total," says Heinsoo. "We had to set up the entire apparatus on a specific day in such a way that the measurements could be carried out fully automatically. This was a new idea for us and ultimately involved more work than we initially thought."

However, the Big Bell Test was also significant for the ETH researchers in another respect: "Until now, experiments in quantum physics tended to be conducted in smaller partnerships," Wallraff explains. "In the future, however, as quantum physicists we will increasingly work together in large-scale collaborations." In this context, the researcher also has the European Union's flagship quantum technology programme in mind. With a billion euros of funding, the programme aims to develop commercially viable technologies based on quantum mechanics within the next ten years.

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Source: ETH Zürich