The team, consisting of researchers from MIT, the Austrian Academy of Sciences and the University of Vienna, as well as physicists from Harvey Mudd College and the University of California, San Diego, used remote quasars to determine the measurements on pairs of entangled ones photons. One quasar emitted its light 7.8 billion years ago and the other 12.2 billion years ago. The team found correlations of more than 30,000 photon pairs to an extent that far exceeded the bound that Bell originally calculated for a classically based mechanism.
The quasar comes from less than a billion years after the Big Bang. Courtesy of NASA / ESA / G.Bacon, STScI.
"The Earth is about 4.5 billion years old, so any alternative mechanism – unlike quantum mechanics – would have had to produce our results by exploiting this loophole long before there was even a planet Earth, let alone an MIT," said MIT professor David Kaiser. "So we brought any alternative explanation back to cosmic history."
In January 2018, the team began collecting data from two large, 4-meter-wide telescopes: the William Herschel Telescope and the Telescopio Nazionale Galileo, both located on the same mountain in the Canary Islands and about one kilometer apart.
Each telescope focused on another quasar that was in different areas of the night sky. In the meantime, researchers at one station formed pairs of entangled photons between the two telescopes, radiating particles from each pair in opposite directions to each telescope.
In a fraction of a second, before each entangled photon reached its detector, the instrumentation determined whether a single photon coming from the quasar was more red or blue, a measure that then automatically matched the angle of a polarizer that finally received and detected became the incoming entangled photon.
The researchers performed their experiment twice for about 15 minutes each and with two different pairs of quasars. They measured 17,663 and 12,420 pairs of entangled photons for each run. Within a few hours of closing the telescoping domes and looking at preliminary data, the team found that there were strong correlations between the photon pairs, beyond the limit calculated by Bell, suggesting that the photons were quantum mechanically correlated.
MIT Professor Alan Guth conducted a more detailed analysis to calculate the – albeit small – chance that a classical mechanism could have produced the correlations observed by the team.
He calculated that for the best of the two runs, the likelihood that a classical physics-based mechanism could have achieved the observed correlation was about 10 to minus 20 – about one part to one hundred billion billions.
"We have made it incredibly unlikely that a local realistic theory of the physics of the universe could underlie," said Guth.
There is still a small opening to the election gap. To further limit it, the team has ideas ready to look further back in time. However, sources such as cosmic microwave background photons emitted as residual radiation immediately after the Big Bang represent a multitude of new technical challenges.
"It's fun to think about new kinds of experiments that we can design in the future, but for the moment we are very happy that we could tackle this particular gap so dramatically," Kaiser said. "Our experiment with quasars poses extremely narrow constraints on various alternatives to quantum mechanics. As strange as quantum mechanics may seem, it still fits in with any experimental test we can develop."
The research was published in Physical Review Letters (doi: 10.1103 / PhysRevLett.121.080403).