First experiment in seamless quantum contextuality

O contextuality it is essential to explain the power of quantum computing and the security of its communications. It is a feature of quantum mechanics this is why measurements – on, for example, atoms – cannot be considered to reveal pre-existing values ​​in any way.

Now, a team of researchers from the Tsinghua University in Beijing (China) was able to observe quantum contextuality at work in the laboratory, eliminating variables that could interfere. The results, published in the journal advances in science“pave the way to certify quantum systems,” according to the authors.

Chinese researchers locked ytterbium and barium ions in a cavity and ‘interrogated’ them with lasers to experimentally verify contextuality, which “opens a way to certify quantum systems”

The experiment was proposed with two ions, one of ytterbium and another one from barium, which were encased in a cavity in a certain state. later, through lasers, they were excited to see whether or not the ion emitted a photon after a certain time. It’s a kind of interrogation.

“To ask for ytterbium we use a laser on that ion and for barium we use a different laser. Sometimes we order one first and then the other, and sometimes both at the same time. In addition, we repeated the questions”, explains the teacher. adam’s hair from the University of Seville and one of the authors of the study.

“So we observe that each ion always gives the same answer to each question – continues-. This persistence in responses would lead us to think that each response is somehow stored in each ion. But when we compare all the tests, we observe that this explanation is not possible, as the data violate an inequality that any model in which the responses are predefined must satisfy.”

Hands that open and close to understand

“To understand why it’s interesting,” says Cabello, “let’s take the following game. Sergio and Ana show us their hands, with clenched fists. We ask you to open, for a moment, one hand each. just one, and we check if it contains something or not. In each round of the game, we can ask to open the same hand as many times as we want.

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“After playing many rounds, we realized that Sergio’s open hand always has something or is always empty. And the same happens with Ana. If we assume that, in each round, Sergio and Ana have or don’t have something in each of their hands, it can be shown that the sum of certain chances have a limit. If we call this sum S, S cannot be greater than 2. However, in our experiment, S is 2.5. How is it possible?”.

This unique experiment allows us to verify that everything happens exactly as quantum physics predicts. The fact that we have such precise control over such delicate systems shows how far we’ve come.

Adan Cabello (Unit of Seville)

“It is possible –answers the professor–, because Sergio is a ytterbium ion and Ana is a barium ion. An ion is an electrically charged atom. In the experiment, the two ions are trapped in a trap and different lasers are used to take the measurements: to make them open their hands. And in quantum physics we must remember that systems do not have properties when they are not measured: properties are relative to measurements”.

Cabello highlights the importance of this experience: “It is the first time that, on the one hand, questions to Sergio do not change Ana’s answers and vice versa; and on the other hand, when using ions, Sergio and Ana always respond (there is no escape). In other experiments, for example with photons, they sometimes do not respond. Also, we can repeat measurements in any order we want.

“It is a unique experiment which allows us to verify that everything happens exactly as predicted by quantum physics –concludes–. The fact that we have such precise control over such delicate systems shows how far we’ve come.”

Reference:

Pengfei Wang, Kihwan Kim et al. “Significant gapless test of Kochen-Specker contextuality using two species of atomic ions”. advances in science2022

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