Light from old quasars helps to confirm quantum entanglement

The quasar goes back to less than a billion years after the Big Bang. Credit: NASA / ESA / G.Bacon, STScI

Last year, physicists from MIT, the University of Vienna and elsewhere provided strong support for quantum entanglement, the seemingly distant idea that two particles, no matter how far apart in space and time, can be inextricably linked, in a way that rules of classical physics defy.

Take, for example, two particles that are on the opposite edges of the universe. If they are really entangled, their physical properties must be related to each other according to the theory of quantum mechanics in such a way that any measurement made on a particle must immediately convey information about any future measurement results of the other particle correlations that Einstein skeptical saw as "spooky action remotely & # 39;

In the 1960s, the physicist John Bell calculated a theoretical limit above which such correlations should have a quantum and not a classical explanation.

But what if such correlations were not the result of quantum entanglement, but of another hidden classical explanation? Such & # 39; what-ifs & # 39; physicists are known as loopholes in Bell's inequality tests, the most stubborn of which is the freedom-of-choice & # 39; -maas in the law: the possibility that some hidden, classic variable is the measurement can influence what a researcher chooses to perform on a confused particle, so that the result appears to be quantum correlated, while in fact this is not the case.

Last February, the MIT team and their peers restricted the loopholes in freedom of choice by using 600-year-old starlight to determine which properties of two tangled photons were measurable. Their experiment proved that if a classical mechanism caused the correlations found, it would have been set in motion more than 600 years ago before the starlight was first broadcast and long before the actual experiment was conceived.

Now, in a newspaper that is published today in Physical Review LettersThe same team has greatly expanded the issue of quantum confusion and further limited the options for the freedom-of-choice gap. The researchers used distant quasars, of which 7,8 billion years ago light emitted and the other 12.2 billion years ago, to determine the measurements that had to be performed on pairs of entangled photons. They found correlations between more than 30,000 pairs of photons, to an extent exceeding the limit that Bell had originally calculated for a classically based mechanism.

"If there is a conspiracy to simulate quantum mechanics with a mechanism that is actually classic, then that mechanism should have started working – somehow knowing exactly when, where and how this experiment would be done – at least 7.8 billion years That seems incredibly unbelievable, so we have very strong evidence that quantum mechanics is the right explanation, "said co-author Alan Guth, the Victor F. Weisskopf professor of physics at MIT.

"The earth is about 4.5 billion years old, so any alternative mechanism – different from quantum mechanics – that might have caused our results by exploiting this mesh in the law had to exist long before even a planet Earth was, let alone a MIT, "adds David Kaiser, the Germeshausen Professor of History of Science and Professor of Physics at MIT. "So we have reduced all alternative explanations to very early in cosmic history."

Guth and Kaiser's co-authors include Anton Zeilinger and members of his group at the Austrian Academy of Sciences and the University of Vienna, as well as physicists at Harvey Mudd College and the University of California at San Diego.

A decision taken billions of years ago

In 2014, Kaiser and two members of the current team, Jason Gallicchio and Andrew Friedman, proposed an experiment to produce interlaced photons on Earth – a process that is fairly standard in quantum mechanics studies. They planned to photograph each member of the entangled couple in the opposite direction, in the direction of light detectors that would also measure each photon with the aid of a polarizer. Researchers would measure the polarization or orientation of each electric field of each incoming photon by setting and observing the polarizer at different angles or bypassing the photons – a result for each photon that researchers could compare to determine if the particles represent the displayed marker correlations showed through quantum mechanics.

The team added a unique step to the proposed experiment, which consisted of the use of light from old, distant astronomical sources, such as stars and quasars, to determine the angle at which each respective polarizer had to be set. Because each interlaced photon was in flight and with the speed of light reaching towards the detector, the researchers used a telescope at each detector site to measure the wavelength of the incident light of a quasar. If that light was more red than a certain reference wavelength, the polarizer tipped at a certain angle to make a specific measurement of the incoming entangled photon – a measurement choice determined by the quasar. If the quasi-light was bluer than the reference wavelength, the polarizer would tilt in a different angle, with another measurement of the entangled photon being made.

Credit: Massachusetts Institute of Technology

In their previous experiment, the team used small backyard telescopes to measure the light of stars at a distance of 600 light years. In their new research, the researchers used much larger, more powerful telescopes to capture the incoming light from even more ancient, distant astrophysical sources: quasars whose light has traveled to Earth for at least 7.8 billion years – objects that are incredibly far away. and yet they are so luminous that their light can be perceived from the earth.

Difficult timing

On January 11, 2018 "the clock was tapped just past midnight local time," Kaiser recalls when a dozen members of the team gathered on a mountain top in the Canary Islands and began collecting data from two large, four-meter long -large telescopes: the William Herschel telescope and the Telescopio Nazionale Galileo, both located on the same mountain and separated by about one kilometer.

One telescope focused on a particular quasar, while the other telescope looked at another quasar in another piece of the night sky. Meanwhile, researchers at a station that was between the two telescopes created pairs of entangled photons and collected particles from each pair in opposite directions to each telescope.

In the fraction of a second before each entangled photon reached its detector, the instrumentation determined whether a single photon emerging from the quasar was more red or blue, a measurement that then automatically adjusted the angle of a polarizer that eventually entwined and received the incoming tangled photon.

"The timing is very difficult," says Kaiser. "Everything has to happen in very tight windows, update every microsecond or so."

Mute a mirage

The researchers performed their experiments twice, each for about 15 minutes and with two different pairs of quasars. For each run they measured 17,663 and 12,420 pairs of interlaced photons respectively. Within hours of closing the telescope domes and searching preliminary data, the team could see that there were strong correlations between the photon pairs, beyond the limit that Bell calculated, indicating that the photons were correlated in a quantum mechanical way.

Guth led a more detailed analysis to calculate the probability, however small, that a classic mechanism might have produced the correlations that the team observed.

He calculated that, for the best of the two runs, the probability that a mechanism based on classical physics could have reached the observed correlation was about 10 to the minus 20 – that's about one part in a hundred billion billion, "insanely small. "Says Guth. By comparison, researchers estimate the likelihood that the discovery of the Higgs particle was only a fluke by about one in a billion.

"We have certainly made incredibly unbelievable that a local realistic theory could be the basis of the physics of the universe," Guth says.

And yet there is still a small opening for the mesh in the freedom of choice. To further limit it, the team offers ideas to look even further back in time, to use sources such as cosmic microwave background photons that were emitted as residual radiation immediately after the big bang, although such experiments present a host of new technical challenges.

"It is nice to think about new types of experiments that we can design in the future, but for now we are very happy that we have tackled this specific mesh in the air so dramatically." Our experiment with quasars imposes extremely severe limitations on various alternatives to quantum mechanics. As strange as quantum mechanics seems to be, it remains any experimental test that we can think of, "says Kaiser.

Explore further:
Tracing the mystery of tangled particles of light

More information:
Cosmic Bell test using random measurement settings of high-redshift quasars, Physical Review Letters (2018). … 1060c3c4cc4263aa2858

Reference of the magazine:
Physical Review Letters

Supplied by:
Massachusetts Institute of Technology

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