# Quantum Parlor Trick confirms, reality doesn’t exist until you measure it

The Sun is not there if you don’t look at it. So says quantum mechanics, which states that what exists depends on what you measure. Now, physicists through Quantum Parlor Trick confirm that reality doesn’t exist until you measure it.

#### What is quantum mechanics?

Quantum mechanics is the science that deals with the behavior of matter and light on the atomic and subatomic scale. It attempts to describe and account for the properties of molecules and atoms and their constituents – electrons, protons, neutrons, and other more esoteric particles such as quarks and gluons.

The further quantum physicists peer into the nature of reality, the more evidence they are finding that everything is energy at the most fundamental levels. Reality is merely an illusion, although a very persistent one.

Quantum physicists have already discovered that a photon is both a particle and a wave until you choose how to measure it. Physicists have long known that a quantum of light, or photon, will behave like a particle or a wave depending on how they measure it. Now, Quantum Parlor Trick has confirmed the same, that reality doesn’t exist until you measure it.

## Proving reality, quantum effects and measurement

Proving reality is 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 in which two players leverage quantum effects to win every time – which they can’t if measurements merely reveal reality as it already exists.

“To my knowledge this is the simplest scenario in which this happens,” says Adan Cabello, a theoretical physicist at the University of Seville who spelled out the game in 2001.

According to Anne Broadbent, a quantum information scientist at the University of Ottawa, such quantum pseudotelepathy depends on correlations among particles that only exist in the quantum realm. “We’re observing something that has no classical equivalent”, added Broadbent.

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 assume the measurement only reflects how the photon was already polarized, regardless of how the two-way state collapses. Only after measurement does the polarization become apparent.

Albert Einstein opposed with this last remark since he thought a quality like polarization of a photon should have a value independent of how it is measured. In his hypothesis, particles might include “hidden variables” that control how a two-way state collapses. However, British theorist John Bell developed a technique to scientifically demonstrate that such hidden variables cannot exist by making use of a phenomena called entanglement in 1964.

#### Mermin-Peres game experiment

It is difficult to probe entanglements. To do this, Alice and Bob will need some measuring equipment. Due to the fact that both of those devices have independent orientation capabilities, Bob can tilt his detector while Alice checks to see if her photon is polarized vertically or horizontally. The detectors’ respective orientation affects how closely their measurements correlate.

Bell imagined Alice and Bob randomly orienting their detectors over a number of measurements, then contrasting the outcomes. The correlations between Alice and Bob’s measurements can only be so high if hidden variables are what determine a photon’s polarization. But, he argued, quantum theory allows them to be stronger. Many experiments have seen those stronger correlations and ruled out hidden variables, albeit only statistically over many trials.

Now, physicists Xi-Lin Wang and Hui-Tian Wang of Nanjing University and colleagues have made the point more clearly using the Mermin-Peres game. In each round of the game, Alice and Bob share not one, but two pairs of entangled photons with which they can take any measurements they want. Each player also has a three-by-three grid and assigns a 1 or a -1 to each square based on the results of the measurements. A referee chooses one of Alice’s rows and one of Bob’s columns at random in each round, which overlap in one square. Alice and Bob win the round if they have the same number in that square.

What makes this special? Perhaps Alice and Bob placed a 1 in each square to secure 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” criteria, and they must both multiply to -1.

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Alice and Bob are unable to succeed in every matchup if hidden variables are responsible for the measurements’ outcomes. Every possible set of values for a hidden variable describes in detail a grid that is already full of -1s and 1. Only the actual measurements indicate which one Alice should pick. Bob is also on board. However, no single grid can meet both Alice’s and Bob’s parity standards, as is easily shown using a pencil and paper. As a result, their grids must disagree in at least one square, and they can only win eight out of nine rounds on average.

Due to quantum mechanics, they are always victorious. They must use a set of measurements created in 1990 by David Mermin, a theorist from Cornell University, and Asher Peres, a former theorist from the Israel Institute of Technology. Bob measures the squares in the column the referee specifies, while Alice measures the squares in the row the referee specifies. Entanglement ensures that their measurements adhere to the parity rules and that they agree on the important square number. Because the values only become obvious after the measurements are taken, the entire method is effective. Due to Alice and Bob never taking measurements, the remaining grid values are worthless.

It is not practical to produce two pairs of entangled photons simultaneously, according to Xi-Lin Wang. Instead, they used a single pair of photons that are entangled in two different ways: orbital angular momentum, which determines whether a wave-like photon corkscrews to the left or right, and polarization. The experiment is not flawless, but Alice and Bob won 93.84 percent of the 1,075,930 rounds, exceeding the 88.89 percent maximum with hidden variables, according to the team’s report, which is currently under review for publication in Physical Review Letters.

Cabello claims that other people have shown the same physics, but Xi-Lin Wang and colleagues “use exactly the game language, which is nice.” He thinks the demonstration might be useful in real life.

One use that Broadbent can think of is to verify a quantum computer’s output. This is a crucial task, but it is challenging since a quantum computer is expected to be able to perform tasks that a conventional computer cannot. Broadbent claims that 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 designed primarily to demonstrate the potential of the group’s go-to technology – photons entangled in both polarization and angular momentum. “We’d like to boost the quality of these hyperentangled photons” said Wang.

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