Adán Cabello: A team of researchers claims to have demonstrated “quantum pseudotelepathy” from a game | Technology

One of the most fascinating properties of quantum physics is entanglement. This allows an effect, such as a rotation, on one particle (often referred to as Alice) to affect another (Bob) instantly, even though they are far apart. This ability is key in one of the most potential applications of this science: computing. But its foundations are still under study. A group of Chinese researchers believes they have demonstrated “quantum pseudotelepathy” in an experiment based on a game. The work is based, as indicated Science, in a proposal by Adán Cabello, professor at the University of Seville, published in 2001 in Physical Review Letters.

Jia-Min Xu of the University of Science and Technology of China leads the latest research, titled Experimental demonstration of quantum pseudotelepathy and also published in Physical Review Letters. The test has been carried out from magic squares devised in the 1990s by David Mermin of Cornell University (New York) and Asher Peres of the Israel Institute of Technology.

“It’s not telepathy or magic, it’s physical,” explains Cabello. “This is a game that is impossible to win every time unless a special form of quantum non-locality is used. We call it non-locality all or nothing. It has since been called pseudo-quantum telepathy, and that name has attracted a lot of attention. The experiment uses a set of measures discovered by Peres and Mermin, which can be placed in a 3 x 3 table and is sometimes called the magic square”, adds the Sevillian professor.

Based on this proposal, Jia-Min Xu’s team believes they have found one of the keys to this physics: “We have achieved a faithful experimental demonstration of quantum pseudo-telepathy through the non-local version of the Mermin magic squares game -Peres, where Alice and Bob cooperatively fill in a magic square of three by three grids. We adopt the hyperentanglement scheme and prepare two pairs of entangled photons both in polarization freedom and in orbital angular momentum. Our results show that quantum players can simultaneously win all games.”

“We are looking at something that has no classical equivalent,” says University of Ottawa quantum information scientist Anne Broadbent in Science about pseudotelepathy.

“It’s not a name that I like, but it is true that it looks like telepathy,” says the researcher from Seville. And he explains: “Imagine that Alicia [Alice en los experimentos] and Benedict [Bob] they are isolated in different places and have no way of communicating. In each round of the game, a referee asks them to fill in, by putting a 1 or a -1 in each box, three boxes of a table that has 3 rows and 3 columns. Alicia has to fill in the three boxes of the row that the referee indicates and the product of her three numbers must always be 1. For example, it is okay to put 1, -1 and -1, but it is not okay to put -1, 1 and 1. Benito has to fill in the three boxes in the column indicated by the referee and the product of his three numbers must always be -1. Neither Alicia nor Benito know which boxes the other has to fill in. They win if they write the same number in the box common to Alice’s row and Benito’s column, and they lose if they write different numbers. Without quantum physics, at best, they could win 8 out of 9 moves. But quantum physics allows them to always win. Without knowing quantum physics, one could conclude that Alicia and Benito are communicating telepathically. However, the only thing they are doing is measuring systems that they prepared before starting the game.”

Cabello considers the experiment different from other non-localities. “Normal experiments allow games to be won with greater probability than is possible without quantum physics. However, this experiment points to a situation in which the players always win. This makes it more interesting. [Albert] Einstein, who was a declared antagonist of quantum physics, would have been in for a surprise”, affirms the professor. “Also”, he adds, “it is a more difficult experiment to do because it is not enough to use a pair of cubits [bits cuánticos] intertwined in each move; you have to use two pairs of cubits hyper-intertwined”.

Based on the approaches of the physicist from the University of Seville, Jia-Min Xu has reproduced the game and obtained a correct result in 93.84% of the 1,075,930 games played. The victory can be considered total if the limitations of the experiment are taken into account. Xi-Lin Wang of Nanjing University and co-author of the research believes they can “improve the quality of hyperentangled photons.”

Anne Broadbent explains in Science that “if the game were incorporated into a program, it would allow us to check if a quantum computer is manipulating the entangled states as it should”.

Three quantum cryptography experiments

The experiment was published at the end of July after months of reviews, coinciding with a scientific boil in the quantum field. In three articles published simultaneously (two in Nature and the third in Physical Review Letters), Both groups of researchers have shown the feasibility of the so-called “device-independent quantum cryptography”, which is the way of using quantum physics that best guarantees the secrecy of communications because, as explained in Physical Review Letters Antonio Acín, from the Barcelona Institute of Photonic Sciences, users “do not need to model the devices; they can be treated as black boxes.”

The experiments have been carried out in the UK, Germany and China and use, respectively, entangled ions, atoms and photons. Adán Cabello highlights the progress made with this research: “The Oxford one is the best of the three experiments because it is the only one that actually manages to generate a secure key. The one in Munich is very spectacular because it involves greater distance. China’s is the most interesting for the future because it’s the only one that uses photons, which is what we’re really going to use for real communications.”

The Sevillian physicist admits the difficulties of bringing these advances to the real world. “But it’s fantastic,” he says, “that there are experiments that have managed to get to this level of finesse in reproducing quantum mechanics.” “These demonstrations are a breakthrough for cybersecurity,” according to Charles Lim of the National University of Singapore, who participated in the Munich experiment, and provide security “against an adversary with arbitrary processing power or even a quantum computer.” ”, adds Jean-Daniel Bancal, from the French National Center for Scientific Research (CNRS), which has participated in the Oxford experiment.

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