Reality doesn’t exist until you measure it, quantum parlor trick confirms

If you don't look at the Moon, it might not be there. According to quantum mechanics, what exists depends on what is being measured. Normally, proving that reality is like that entails comparing obscure probability, but Chinese scientists have made the case more succinctly. They performed a matching game in which two players depend on quantum effects to consistently prevail. If measurements just disclose reality as it now is, however, they are unable to do so.

The game was first described in 2001 by Adan Cabello, a theoretical physicist at the University of Seville, who claims that it is the simplest [situation] in which this occurs. According to Anne Broadbent, a quantum information scientist at the University of Ottawa, this quantum pseudotelepathy hinges on correlations between particles that are unique to the quantum domain. We are witnessing something that has no analog in traditional physics.

A quantum particle can coexist in two situations that are mutually incompatible. A photon, for instance, can be polarized to have an electric field that oscillates either vertically, horizontally, or simultaneously in both directions—at least until the electric field is measured. Then, the two-way state randomly collapses to either the vertical or horizontal plane. Importantly, an observer cannot believe the measurement only reflects how the photon was previously polarized, regardless of how the two-way state collapses. Only after measurement does the polarization become apparent.

This final statement infuriated Albert Einstein, who believed that a property like the polarization of a photon should have a value regardless of how it is measured. He proposed that particles could include "hidden variables" that control how a two-way state collapses. But in 1964, British theorist John Bell discovered a means to scientifically demonstrate that such hidden variables are impossible by taking use of a phenomena called entanglement.

It is possible for two photons to get entangled and be in an uncertain both-ways condition, but because of the correlation between their polarizations, if one is horizontal, the other must be vertical, and vice versa. Entanglement is difficult to probe. Alice and Bob will need measurement devices in order to do this. Both of those devices have separate orientation capabilities, allowing Bob to cant his detector by an angle while Alice tests whether her photon is polarized vertically or horizontally. The degree of correlation between the measurements of the detectors depends on their relative orientation.

Bell imagined Alice and Bob randomly orienting their detectors across a number of measurements, then contrasting the outcomes. The correlations between Alice and Bob's measurements can only be so high if hidden factors are what determine a photon's polarization. However, he said, quantum theory enables them to be more powerful. Even though only statistically over many trials, several investigations have detected those greater relationships and ruled out hidden factors.

The Mermin-Peres game has now been used by Nanjing University physicists Xi-Lin Wang and Hui-Tian Wang and associates to clarify their thesis. Each round of the game involves Alice and Bob sharing two pairs of entangled photons, allowing them to conduct any measurements they choose. In accordance with the outcome of those measures, each player likewise has a three-by-three grid and fills each square in it with a 1 or a -1. One of Alice's rows and one of Bob's columns, which overlap in one square, are chosen at random by the referee for each round. The round is won by Alice and Bob if their numbers in that square match.

It sounds simple: Alice and Bob just place a 1 in each square to ensure a victory. Wait a minute. The entries across Alice's row and the ones down Bob's column must both multiply to 1 according to further "parity" requirements, and they must both multiply to -1.

Alice and Bob are unable to prevail in every assessment if hidden factors foretell the outcomes. Each set of hidden variable values basically specifies a grid with -1s and 1s already filled in. The outcomes of the actual measures only indicate to Alice which option to select. Bob is in the same boat. However, no single grid can fulfill both Alice's and Bob's parity standards, which is easily demonstrated using a pencil and paper. They can only win a maximum of eight out of nine rounds if their grids conflict in at least one tile.

Due to quantum mechanics, they are always successful. They must achieve this by employing a set of metrics that were developed in 1990 by David Mermin, a theorist at Cornell University, and Asher Peres, a former theorist at the Israel Institute of Technology. Bob takes the measurements for the squares in the provided column, and Alice takes those for the squares in the supplied row. Entanglement ensures that they share the same key square number and that their measurements adhere to the parity criteria. Because the values only become apparent after the measurements are taken, the entire plan works. Since values don't exist for measurements that Alice and Bob never take, the remainder of the grid is meaningless.

It is not feasible to concurrently produce two pairs of entangled photons, according to Xi-Lin Wang. Instead, they employed a single pair of photons that are entangled in two different ways: through polarization and what is known as orbital angular momentum, which controls whether a wavelike photon corkscrews to the right or to the left. Despite the flaws in the experiment, Alice and Bob prevailed in 93.84% of the 1,075,930 rounds, exceeding the 88.89% maximum with hidden variables, according to the team's paper, which is now under review by Physical Review Letters.

Although other researchers have shown the same physics, Xi-Lin Wang and colleagues "employ exactly the language of the game, which is fantastic," according to Cabello. According to him, the demonstration may have useful applications.

A practical use that Broadbent has in mind is validating a quantum computer's output. This task is crucial yet challenging since a quantum computer is meant to be able to perform tasks that a conventional computer cannot. But, according to Broadbent, if the game were integrated into a software, watching it may prove that the quantum computer is manipulating entangled states as it should.

The experiment, according to Xi-Lin Wang, was primarily intended to demonstrate the potential of the group's preferred technology—photons entangled in both polarization and angular momentum. "We want to make these hyperentangled photons better,"