To prove quantum entanglement
To prove quantum entanglement
Some experiments confirm the "spooky action at a distance" rejected by Einstein.
12 billion years ago, fast particles of light came out of a very luminous object called quasar and began their journey towards a planet that did not exist yet. More than 4 billion years later, other photons left another quasar for a similar trip. While the Earth and the Solar System were forming, the particles continued their journey. Finally, they arrived at a pair of telescopes on the Canary Island of La Palma, set up to perform an experiment that seeks to know the true nature of reality.
The experiment was designed to study quantum entanglement, a phenomenon that connects quantum systems in ways that are impossible in our classic macro world. When two particles, like a pair of electrons, are intertwined, it is impossible to measure one without knowing something of the other. Its properties, like the moment and the position, are united.
The entanglement arose as a mental experiment devised by Einstein (and that forms the Paradox Einstein-Podolksy-Rosen or EPR). In a 1935 paper, Einstein and his two colleagues showed that if quantum mechanics fully describes reality, then making a measurement in one part of an interlaced system would instantly affect our knowledge of future measurements in the other part, which would mean that It would be sending information between particles faster than the speed of light. The German physicist called that "phantasmagoric action at a distance" and since that is supposed to be impossible, in the framework of relativity, it would imply that there would be something wrong or unknown in quantum physics.
Decades later, quantum entanglement has been confirmed experimentally several times. While physicists learned to control and study the effect, there is still a mechanism to explain or reach a consensus on what it means.
THE BELL THEOREM
The world of quantum mechanics - the physics that governs the behavior of the universe at very small scales - is really strange. According to its laws, the building blocks of Nature are both particles and waves, without local definition in space. We need another system that observes or measures to push these wave-particles to "choose" a definitive state. And the intertwined particles seem to affect the "decision" of the other instantaneously, at any distance.
For Einstein this could not be real, but an illusion product of our ignorance of what he called "hidden variables". In the 1960s, physicist John Bell thought of a test or test for models with hidden variables called "Bell's inequalities" or Bell's theorem.
From the Bell Theorem and the EPR Paradox these assumptions arise:
-Objects have properties that are maintained whether observed or not (realism)
-Nothing can influence anything far enough so that a signal between them needs to travel faster than light (locality)
-The physicists can make measurements freely and without influences of hidden variables (freedom of decision)
Testing quantum entanglement is the key to testing these assumptions. If the experiments show that nature obeys these ideas, then we live in a world that we can understand in classical form and the hidden variables only create the illusion of entanglement. If the experiments show that the world does not follow these ideas, then the entanglement is real, even if it is rare or difficult to understand.
Statistical correlations
Physicists can measure properties of particles, such as spin, momentum or polarization. The experiments showed that when the particles are intertwined, the result of those measurements is more statistically correlated than expected in a classical system, violating Bell's inequalities.
There are different forms of Bell's experiments. In one, scientists send two intertwined photons to separate detectors. If the photons reach the detectors it depends on their polarization: if they are perfectly aligned, they will pass through the detectors; otherwise, there is a probability that they will be blocked, depending on the angle of alignment. Physicists seek to know if interlaced particles end up with the same polarization more frequently than expected in classical statistics. If they do, at least one of Bell's ideas can not be true. If the world does not obey realism, the properties of the particles are not well defined before the measurements. If particles can be influenced instantaneously, then they communicate faster than light, violating locality and special relativity.
Scientists have speculated that previous experimental results could be explained if the world does not obey one or both of Bell's two ideas, realism or locality. But recent work showed that perhaps the culprit would be the third argument, the freedom of decision. Perhaps scientists when deciding on the angle by which the photons pass through the polarizer is not as free and random a decision as they thought.
The quasars experiment was done to test the idea of freedom of decision. The scientists determined the angle that would allow to enter to the photons to the detectors based on the wavelength of the light of two distant quasars, something determined 7.8 and 12.2 billions of years back.
In this way, physicists say, those quasars took the place of a random number generator and thus eliminate the experiment of human influences.
Finally, the team found many more correlations between entangled photons than expected in a classical world. This means that if there were classic hidden variables determining the results of the experiment, in the most extreme case, the decision of the measurement should have been fixed long before human existence, which would suppose a highly predetermined universe.
The quantum entanglement has not yet been explained, just as it is not known how, when measuring an interlaced system, it becomes classic or if the particles communicate with each other. Paradoxes usually cease to be paradoxes when new knowledge is acquired, that is, they seem inexplicable only because something is missing. Hence, it is logical to think about the hidden variables. If it is impossible to transmit information faster than the speed of light, then how to explain that two "twin" particles, which are born together and interlaced, when changing a characteristic of a particle (such as polarization), then that same characteristic also changes in the other particle, although, meanwhile, both went away at speeds close to the speed of light and are very far from each other.
An idea, no less rare, is this: perhaps space-time is a kind of field, like an electric or magnetic field. So when both particles arise, that field is one way. By changing something in particle A, the associated field of that particle is also modified but also of particle B and the latter changes to particle B. In this scenario, what would be transmitted faster than the speed of light is information of spacetime in itself, which does not contradict special relativity.
THE EXPERIMENT
The experiment was led by Anton Zeilinger with the collaboration of the University of Vienna, who used the Galileo Nazionale Telescope (TNG) along with the William Herschel Telescope (WHT) at Roque de los Muchachos. The results were published in Physical Review Letters.
In the experiment, the telescopes were searched for two different locations in the sky. Each telescope was equipped with a two-channel photometer and collected light from a quasar. An optical filter in the photometer divided the light of the quasar between redder and bluer photons and these, when falling in one of the channels of the photometers, were used as random measurement decisions. That decision was used to control the reference system in which to measure the polarization state in the pair of interlaced photons.
The pair of photons were generated from a mobile laboratory installed on the Nordic Optical Telescope and one was sent to a TNG reception station and the other was sent to a WHT station. There, the individual polarization of each photon was measured in relation to the fluctuations of the light of each quasar.
According to Einstein's "spooky action at a distance", the measurement of a photon in an interlaced system instantaneously influences the measurement of the other. The question is: how do you decide what measurements to make in the two photons? It is evident that it would be desirable to have independent measurements so that they do not have an influence of a common cause.
In this experiment, two quasars were used for the first time as random number generators. The fluctuations of the light of those quasars that lie at 12 and 8 billion light years in opposite places of the sky, decided in each photon what polarization is measured. As the light of the quasars comes to us shortly after the Big Bang, any possible influence on both quasars should have occurred in only 4% of the known universe.
"The crucial challenge in the experiment was to make sure that the decision to measure the polarization in each interlaced photon was made completely independent of us and any environment", said Dominik Rauch, first author of the paper.
Bell inequalities
Why do you say "inequalities"?
What Bell showed can be written like this:
Number (A, not B) + Number B (not C)> = Number A (not C)
Suppose that A is sex, male-female; B is height (low, high); C is eye color (blue, green)
Then we can say that: "The number of low men plus the number of tall people (men or women) with green eyes must be equal to or greater than the number of men with green eyes. This certainly can not be otherwise. "
It is not very difficult to realize, algebraically, that this has to be true, at least for "classic", macroscopic objects with fixed properties. People do not dye their hair in the middle of the experiment, do not put on tacos or wear contact lenses.
But in the quantum world, things change, by Heinsenberg's Uncertainty Principle. What is measured, the polarization (which is also the spin) can not be measured in two different directions (90º, up or 45º) at the same time. Then, we can find the values of both properties if we have interlaced photons. If the spin above is A, and spin 45 is B, then when measuring A in a particle, then we can prove property B in the other. But since you can not measure the three properties of each particle, but only two, you can not tell whether Bell's inequality was broken or not. The best thing to do is run the test on many thousands of particles and consider the result statistics. The first published experiment was done by Clauser, Horne, Shimony and Holt in 1969 (CHSH).
Sources: Andrew Thomas.
https://web.archive.org/web/20081121055459/http://www.ipod.org.uk/reality/reality_entangled.asp
http://www.faculty.umb.edu/gary_zabel/Courses/Parallel%20Universes/Texts/Quantum%20Entanglement.htm
David M. Harrison Bell's Theorem
https://faraday.physics.utoronto.ca/GeneralInterest/Harrison/BellsTheorem/BellsTheorem.html
Sources and related links
"Cosmic Bell test using random measurement settings from high-redshift quasars", Dominik Rauch, Johannes Handsteiner, Armin Hochrainer, Jason Gallicchio, Andrew S. Friedman, Calvin Leung, Bo Liu, Lukas Bulla, Sebastian Ecker, Fabian Steinlechner, Rupert Ursin, Beili Hu, David Leon, Chris Benn, Adriano Ghedina, Massimo Cecconi, Alan H. Guth, David I. Kaiser, Thomas Scheidl, and Anton Zeilinger, Physical Review Letters, 2018.
DOI: https://doi.org/10.1103/PhysRevLett.121.080403
The quest to test quantum entanglement
https://www.symmetrymagazine.org/article/the-quest-to-test-quantum-entanglement
The TNG and the confirmation of quantum entanglement using photons from distant astronomical objects
http://www.tng.iac.es/news/2018/08/23/cosmic/
Bell's Theorem in Wikipedia
https://es.wikipedia.org/wiki/Bore Theorem
About the images
Initial photo: One of the mobile receptors for interlaced photons operated by the Austrian Academy of Sciences in La Palma. Credit: Dominik Rauch / OeAW.
Illustration of a possible Bell experiment. CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=641329
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