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

The moon isn’t necessarily there if you’re not looking at it. That’s what quantum mechanics says, which says that what exists depends on what you measure. Proving that reality is so usually involves comparing arcane probabilities, but physicists in China have made the point more clearly. They ran a matching game in which two players use quantum effects to win every time – which they can’t do when measurements merely show reality as it already exists.

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

A quantum particle can exist in two mutually exclusive states at the same time. For example, a photon can be polarized in such a way that the electric field inside it winds vertically, horizontally, or in both directions at once—at least until it is measured. The two-way state then randomly collapses either vertically or horizontally. Crucially, no matter how the two-way state collapses, an observer cannot assume that the measurement merely shows how the photon was already polarized. The polarization only occurs during the measurement.

That last part irked Albert Einstein, who thought that something like the polarization of a photon should have a value independent of whether it’s being measured. He suggested that particles might carry “hidden variables” that determine how a two-way state will collapse. However, in 1964 the British theorist John Bell found a way to experimentally prove that such hidden variables cannot exist by exploiting a phenomenon known as entanglement.

Two photons can become so entangled that each is in an unsafe bilateral state, but their polarizations are so correlated that if one is horizontal the other must be vertical and vice versa. Investigating entanglement is difficult. To do this, Alice and Bob must each have a measuring device. These devices can be oriented independently, allowing Alice to test whether her photon is horizontally or vertically polarized, while Bob can tilt his detector at an angle. The relative orientation of the detectors affects how closely their measurements correlate.

Bell imagined Alice and Bob randomly aligning their detectors over many measurements and then comparing the results. When 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 these stronger correlations and ruled out hidden variables, albeit only statistically across many trials.

Now, Xi-Lin Wang and Hui-Tian Wang, physicists at Nanjing University, and colleagues have made the point more clear through the Mermin-Peres game. In each round of the game, Alice and Bob share not just one, but two pairs of entangled photons on which they can make arbitrary measurements. Each player also has a three-by-three grid and fills each square in it with a 1 or a -1, depending on the outcome of those measurements. In each round, an arbiter randomly selects one of Alice’s rows and one of Bob’s columns to overlap in a square. If Alice and Bob have the same number in this field, they win the round.

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

If hidden variables dictate 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 already filled with -1s and 1s. The results of the actual measurements only tell Alice which one to choose. Same goes for Bob. But, as can easily be shown with pencil and paper, no single lattice can satisfy both Alice’s and Bob’s parity rules. So their grids must be different in at least one square, and on average they can win at most eight rounds out of nine.

Quantum mechanics lets them 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 referee and Bob those for the squares in the column specified. The entanglement guarantees that they agree on the number in the key square and that their measurements also obey parity rules. The whole scheme works because the values ​​only arise when the measurements are made. The rest of the grid is irrelevant since there are no values ​​for measurements that Alice and Bob never make.

Simultaneously generating two pairs of entangled photons is impractical, says Xi-Lin Wang. Instead, the experimenters used a single pair of photons that are entangled in two ways — through polarization and what’s known as orbital angular momentum, which determines whether a wavy photon corks to the right or left. The experiment isn’t perfect, but Alice and Bob won 93.84% of 1,075,930 rounds. Exceeding the maximum of 88.89% with hidden variablesthe team reports in a study in press below Physical Verification Letters.

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

Broadbent has a real-world use in mind: verifying the workings of a quantum computer. This task is important but difficult because a quantum computer is supposed to do things that an ordinary computer cannot. However, Broadbent says if the game were woven into a program, monitoring could confirm that the quantum computer is manipulating entangled states as it should.

According to Xi-Lin Wang, the experiment was mainly intended to show the potential of the team’s own 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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