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,"
