Last year, physicists at MIT, the University of Vienna, and elsewhere provided strong support for quantum entanglement, the seemingly far-out idea that two particles, no matter how distant from each other in space and time, can be inextricably linked, in a way that defies the rules of classical physics.
Take, for instance, two particles sitting on opposite edges of the universe. If they are truly entangled, then according to the theory of quantum mechanics their physical properties should be related in such a way that any measurement made on one particle should instantly convey information about any future measurement outcome of the other particle—correlations that Einstein skeptically saw as “spooky action at a distance.”
In the 1960s, the physicist John Bell calculated a theoretical limit beyond which such correlations must have a quantum, rather than a classical, explanation.
But what if such correlations were the result not of quantum entanglement, but of some other hidden, classical explanation? Such “what-ifs” are known to physicists as loopholes to tests of Bell’s inequality, the most stubborn of which is the “freedom-of-choice” loophole: the possibility that some hidden, classical variable may influence the measurement that an experimenter chooses to perform on an entangled particle, making the outcome look quantumly correlated when in fact it isn’t.
On Jan. 11, 2018, in a new experiment to test quantum entanglement, MIT’s David Kaiser and other team members gathered on a mountaintop in the Canary Islands and began collecting data from two large, 4-meter-wide telescopes: the William Herschel Telescope and the Telescopio Nazionale Galileo, both situated on the same mountain and separated by about a kilometer.
One telescope focused on a particular quasar, while the other telescope looked at another quasar in a different patch of the night sky. Meanwhile, researchers at a station located between the two telescopes created pairs of entangled photons and beamed particles from each pair in opposite directions toward each telescope.
In the fraction of a second before each entangled photon reached its detector, the instrumentation determined whether a single photon arriving from the quasar was more red or blue, a measurement that then automatically adjusted the angle of a polarizer that ultimately received and detected the incoming entangled photon.
“The timing is very tricky,” Kaiser says. “Everything has to happen within very tight windows, updating every microsecond or so.”
The researchers ran their experiment twice, each for around 15 minutes and with two different pairs of quasars. For each run, they measured 17,663 and 12,420 pairs of entangled photons, respectively. Within hours of closing the telescope domes and looking through preliminary data, the team could tell there were strong correlations among the photon pairs, indicating that the photons were correlated in a quantum-mechanical manner.
The team performed a more detailed analysis to calculate the chance, however slight, that a classical mechanism might have produced the correlations the team observed.
They calculated that, for the best of the two runs, the probability that a mechanism based on classical physics could have achieved the observed correlation was about 10 to the minus 20—that is, about one part in one hundred billion billion—outrageously small.
Sorry, Professor Einstein—it looks like “spooky action at a distance” is proving to be a reality.