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The moon is not necessarily there unless you are looking at it. So says quantum mechanics, which says that what exists depends on what you measure. Proving reality usually involves comparing puzzling probabilities, but physicists in China have made this clearer. They played a match game in which two players use quantum effects to win every time—which they can’t if the measurements just show reality as it already exists.

“As far as I know, this is the easiest [scenario] where it happens,” says Adan Cabello, a theoretical physicist at the University of Seville who developed the game in 2001. Such quantum pseudo-telepathy depends on correlations between particles that exist only in the quantum realm, says Anne Broadbent, a quantum information scientist. at the University of Ottawa. “We are seeing something that has no classical equivalent.”

A quantum particle can exist in two mutually exclusive states at once. For example, a photon can be polarized so that the electric field in it wriggles vertically, horizontally, or in both directions at the same time – at least until it is measured. The two-sided state is then randomly folded to either a vertical or horizontal position. It is important to note that no matter how the two-way state collapses, the observer cannot assume that the measurement simply shows how the photon has already been polarized. Polarization occurs only during measurement.

This last circumstance stung Albert Einstein, who believed that something like the polarization of a photon should matter whether or not it was measured. He suggested that the particles might carry “hidden variables” that determine how the two-sided state would collapse. However, in 1964, British theorist John Bell found a way to experimentally prove that such hidden variables could not exist, using a phenomenon known as entanglement.

Two photons can be entangled so that each is in an indeterminate two-way state, but their polarizations are correlated so that if one photon is horizontal, the other must be vertical, and vice versa. Revealing confusion is not easy. To do this, Alice and Bob must have a measuring device. These devices can be oriented independently, so Alice can check if her photon is polarized horizontally or vertically, while Bob can tilt his detector at an angle. The relative orientation of the detectors affects the degree of correlation between their measurements.

Bell assumed that Alice and Bob arbitrarily orient their detectors across multiple dimensions and then compare the results. If the photon’s polarization is determined by hidden variables, the correlation between Alice’s and Bob’s measurements can be very strong. But, he argued, quantum theory allows them to be stronger. Many experiments have found these stronger correlations and excluded latent variables, albeit only statistically in many trials.

Now Xi-Lin Wang and Hui-Tien Wang, physicists at Nanjing University, and their colleagues have made this point clearer with the Mermin-Perez game. In each round of the game, Alice and Bob share not one, but two pairs of entangled photons, on which they can make any measurements. Each player also has a 3 by 3 grid, and each square in it is filled with the number 1 or -1, depending on the result of these measurements. In each round, the judge randomly selects one of Alice’s rows and one of Bob’s columns that overlap in one square. If Alice and Bob have the same number in this square, they win the round.

Sounds simple: Alice and Bob bet 1 on each square to guarantee a win. Not so fast. Additional “evenness” rules require that all entries in Alice’s row be multiplied by 1, and those lower in Bob’s column must be multiplied by -1.

If the hidden variables determine the results of the measurements, Alice and Bob cannot win every round. Each possible set of values for the latent variables actually defines a grid already filled with -1s and ones. The results of the actual measurements simply tell Alice which one to choose. The same goes for Bob. But, as is easy to show with pencil and paper, no grid can satisfy both Alice’s and Bob’s parity rules. Thus, their grids must diverge in at least one square, and on average they can win no more than eight out of nine rounds.

Quantum mechanics allows them to win every time. To do this, they must use a set of measurements developed in 1990 by David Mermin, a theorist at Cornell University, and Asher Peres, a former theorist at the Israel Institute of Technology. Alice takes the measurements associated with the squares in the row specified by the judge, and Bob measures the squares in the specified column. The entanglement ensures that they agree with the number in the key square and that their measurements also obey the parity rules. The whole circuit works because the values appear only as measurements are taken. The rest of the grid doesn’t matter, since there are no values for the measurements, which Alice and Bob never do.

Generating two pairs of entangled photons at the same time is impractical, says Xi-Ling Wang. So instead, the experimenters used a single pair of photons, which are entangled in two ways – through polarization and the so-called orbital angular momentum, which determines whether the undulating photon twists to the right or to the left. The experiment isn’t perfect, but Alice and Bob won 93.84% of 1,075,930 rounds, surpassing the 88.89% maximum with latent variables, the team reports in a study published online. Physical Review Letters.

Others have demonstrated the same physics, says Cabello, but Xi-Lin Wang and his colleagues “are using the language of the game, which is nice.” The demonstration could have practical applications, he said.

Broadbent is referring to a real-world application: testing the operation of a quantum computer. This task is necessary but difficult because a quantum computer must do things that a conventional computer cannot. However, according to Broadbent, if the game were built into the program, monitoring it could confirm that the quantum computer was manipulating entangled states as it should.

Xi-Lin Wang says the experiment was mainly intended to show the potential of the team’s favorite technology – photons entangle in both polarization and angular momentum. “We want to improve the quality of these hyper-entangled photons.”

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