Reality does not exist until you measure it, confirms quantum parlor trick |  Science

Reality does not exist until you measure it, confirms quantum parlor trick | Science

The moon is not necessarily there if you do not look at it. So says quantum mechanics, which says that what exists depends on what you measure. Proving that reality is like this usually involves comparing mysterious probabilities, but physicists in China have made the point in a clearer way. They performed a matching game where two players use quantum effects to win each time – something they can not if measurements only reveal reality as it already exists.

“But I know this is the easiest thing [scenario] where this happens, says Adan Cabello, a theoretical physicist at the University of Seville who explained the game in 2001. Such quantum pseudothelopathy depends on correlations between particles that only exist in the quantum realm, says Anne Broadbent, a quantum information researcher at the University of Ottawa. “We observe something that has no classical equivalent.”

A quantum particle can exist under two mutually exclusive conditions simultaneously. For example, a photon can be polarized so that the electric field in it rotates vertically, horizontally or both at the same time – at least until it is measured. The bidirectional state then randomly collapses to either vertical or horizontal. It is crucial, no matter how the two-way state collapses, an observer can not assume that the measurement only reveals how the photon was already polarized. The polarization occurs only with the measurement.

The last bit ranked Albert Einstein, who thought something like the polarization of a photon should have a value regardless of whether it is measured. He suggested that particles can carry “hidden variables” that determine how a two-way state will collapse. However, in 1964, the British theorist John Bell found a way to prove experimentally that such hidden variables could not exist by exploiting a phenomenon known as entanglement.

Two photons can be wrapped so that each is in an uncertain two-way state, but their polarizations are correlated so that if one is horizontal, the other must be vertical and vice versa. It is difficult to investigate complications. To do this, Alice and Bob must each have their own measuring device. These devices can be oriented independently so that Alice can test whether her photon is polarized horizontally or vertically, while Bob can tilt his detector at an angle. The relative orientation of the detectors affects how much their measurements are correlated.

Bell envisioned Alice and Bob orienting their detectors randomly over many measurements and then comparing the results. If hidden variables determine the polarization of a photon, the correlations between Alice’s and Bob’s measurements can only be so strong. But, he argued, quantum theory allows them to be stronger. Many experiments have seen the stronger correlations and ruled out hidden variables, albeit only statistically over many experiments.

Now Xi-Lin Wang and Hui-Tian Wang, physicists at Nanjing University, and colleagues have made the point clearer through the Mermin-Peres game. In each round of the game, Alice and Bob share not one, but two pairs of entangled photons that they can measure at will. Each player also has a three-to-three grid and fills each square in it with a 1 or a -1 depending on the result of these measurements. In each round, a judge 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 on the route, they win the round.

Sounds simple: Alice and Bob put 1 in each square to guarantee a victory. Not so fast. Additional “parity” rules require that all entries across Alice’s row must be multiplied by 1 and those down Bob’s column must be multiplied by -1.

If hidden variables predetermine the results of the measurements, Alice and Bob cannot win every round. Each possible set of values ​​for the hidden variables effectively specifies a grid that is already populated with -1s and 1s. The results of the actual measurements only tell Alice which one to choose. The same goes for Bob. But, as easily shown in pencil and paper, no single grid can satisfy both Alice and Bob’s parity rules. So their grids must disagree on at least one square, and on average they can win a maximum of eight out of nine rounds.

Quantum mechanics lets them win every time. To do so, 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 makes the goals related to the squares in the row specified by the referee, and Bob, those for the squares in the specified column. Entanglement guarantees that they agree on the number in the key field and that their measurements also comply with the parity rules. The whole scheme works because the values ​​appear only as the measurements are made. The rest of the grid is irrelevant, since values ​​do not exist for measurements that Alice and Bob never do.

Generating two pairs of entangled photons at the same time is impractical, says Xi-Lin Wang. So instead, the experimenters used a single pair of photons that are entangled in two ways – through polarization and so-called orbital angular momentum, which determines whether a wave-like photon corkscrew to the right or left. The experiment is not perfect, but Alice and Bob won 93.84% of 1,075,930 rounds, and exceeded the maximum limit of 88.89% with hidden variables, the team reports in a press survey at. Physical review letters.

Others have demonstrated the same physique, says Cabello, but Xi-Lin Wang and colleagues “use the exact language of the game, which is nice.” The demonstration could have practical applications, he says.

Broadbent has a real use in mind: to verify the work of a quantum computer. That task is important, but difficult because a quantum computer should do things a regular computer cannot. However, Broadbent says, if the game was woven into a program, monitoring it could confirm that the quantum computer is manipulating tangled states properly.

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

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