Reality Doesn't Exist Until You Measure It, Quantum Parlor Trick Confirms

sciencehabit shares a report from Science Magazine: The Moon isn't necessarily there if you don't look at it. So says quantum mechanics, which states that what exists depends on what you measure. Proving reality like that usually involves the comparison of arcane probabilities, but physicists in China have made the point in a clearer way. They performed a matching game [Mermin-Peres game] in which two players leverage quantum effects to win every time -- which they can't if measurements merely reveal reality as it already exists. [...] In each round of the game, Alice and Bob share not one, but two pairs of entangled photons on which to make any measurements they like. Each player also has a three-by-three grid and fills each square in it with a 1 or a -1 depending on the result of those measurements. In each round, a referee randomly selects one of Alice's rows and one of Bob's columns, which overlap in one square. If Alice and Bob have the same number in that square, they win the round. Sounds easy: Alice and Bob put 1 in every square to guarantee a win. Not so fast. Additional "parity" rules require that all the entries across Alice's row must multiply to 1 and those down Bob's column must multiply to -1. If hidden variables predetermine the results of the measurements, Alice and Bob can't win every round. Each possible set of values for the hidden variables effectively specifies a grid already filled out with -1s and 1s. The results of the actual measurements just tell Alice which one to pick. The same goes for Bob. But, as is easily shown with pencil and paper, no single grid can satisfy both Alice's and Bob's parity rules. So, their grids must disagree in at least one square, and on average, they can win at most eight out of nine rounds. Quantum mechanics lets them win every time. To do that, they must use a set of measurements devised in 1990 by David Mermin, a theorist at Cornell University, and Asher Peres, a onetime theorist at the Israel Institute of Technology. Alice makes the measurements associated with the squares in the row specified by the referee, and Bob, those for the squares in the specified column. Entanglement guarantees they agree on the number in the key square and that their measurements also obey the parity rules. The whole scheme works because the values emerge only as the measurements are made. The rest of the grid is irrelevant, as values don't exist for measurements that Alice and Bob never make. Generating two pairs of entangled photons simultaneously is impractical, Xi-Lin Wang says. So instead, the experimenters used a single pair of photons that are entangled two ways -- through polarization and so-called orbital angular momentum, which determines whether a wavelike photon corkscrews to the right or to the left. The experiment isn't perfect, but Alice and Bob won 93.84% of 1,075,930 rounds, exceeding the 88.89% maximum with hidden variables, the team reports in a study in press at Physical Review Letters. The researchers have a real-world use in mind for the demonstration: verifying the work of a quantum computer. "That task is essential but difficult because a quantum computer is supposed to do things an ordinary computer cannot," reports Science Magazine. "[I]f the game were woven into a program, monitoring it could confirm that the quantum computer is manipulating entangled states as it should."

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