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Physicists Stretch Quantum Superposition from Chicago to Minnesota

A long-distance experiment offers new verification for fundamental quantum mechanics.

by Michael Byrne
Jul 20 2016, 1:55pm

MINOS. Image: Reidar Hahn/Fermilab

A team of MIT physicists has observed quantum correlations extending 735 kilometers (456 miles) from Fermilab's MINOS experiment near Chicago to an underground detector at Soudan, Minnesota.

The span represents the largest distance that quantum mechanics has been tested to date, and an unusually macroscopic perspective on phenomena usually considered only at the finest subatomic scales. The group's work is to be published in the Physical Review Letters later this month and can be accessed now at the pre-print server.

More specifically, the physicists observed that neutrinos traveling in a beam fired underground from Fermilab to Soudan demonstrated statistical relationships indicating intact quantum superpositions. That is, distinct and would-be contradictory properties of the particles became correlated to the point of existing simultaneously. The result is a relationship that's not unlike, say, being both heads and tails at the same time—not a mix or mingling of the states, but both at once.

Quantum superposition is a key feature of the subatomic world, or so most physicists think. Beyond the superimposed properties of single particles, it's possible for even multiple particles to be brought together into a single state and then separated again while maintaining that single state. It's this feature that's responsible for much of what makes the quantum world so odd and is what's demonstrated by Schrödinger's feline thought experiment. If you'll recall, this is where a cat in a closed box with a 50 percent probability of being poisoned while hidden in the box is able to exist simultaneously in both "alive" and "dead" states so long as we don't open the box to look at it (thus interfering with the system and causing it to "decohere").


Schrödinger's cat does the job as an illustration, but only to a point. Usually, when you have collections of particles that are as big as cats (or even much, much, much smaller) you don't need to peek in the box to cause decoherence to occur. That is, contrary to a great deal of contemporary quantum woo (what is real, man?), there isn't anything special about observation when it comes to collapsing a quantum superposition—it's any sort of environmental interference that matters. Which could be the act of looking, but, in the case of the cat or any other macroscopic entity, decoherence would come courtesy of the environment itself. So, I actually lied above when I said that superposition is a key feature of the subatomic world—it's a feature of everything. It's just that in the macroscopic world superpositions are always getting washed out by interference before it can make much difference.

The result is a relationship that's not unlike, say, being both heads and tails at the same time—not a mix or mingling of the states, but both at once

This is the challenge in long-distance quantum superposition. It's not like we can just look away and our particles will stay superimposed no matter what. But, if any one variety of particle is capable of maintaining such a state over long distances, it would be the neutrino. Neutrinos have the neat property of interacting with their environment only via the weak force and gravity, which means most everything in existence is completely transparent to a neutrino. Hence, we can fire neutrinos a vast distance through solid earth and expect them to arrive intact at a detector.

"The data we collected lines up beautifully for quantum mechanics," David Kaiser, a study co-author and MIT physics professor, told me. "One of the things we found so exciting is that not only can we ask this question on typical quantum mechanical scales, like a nanometer, the size of an atom, but instead we can say what happens when we try to describe the neutrinos that traveled hundreds of miles."

The correlations in question have to do with the various "flavors" that neutrinos take on: electron neutrinos, muon neutrinos, and tau neutrinos. Each of these is associated with the electron, muon, and tau particles, respectively, and neutrinos are known to oscillate between the different flavors as they cruise through space at very near the speed of light (not much to slow them down). Like the spin of an electron or polarization of an photon, neutrino flavors are properties that can be superimposed together; that is, a neutrino may be simultaneously an electron neutrino and a tau neutrino, even though those things should be mutually exclusive. It's this sort of superposition that was maintained between Fermilab and Soudan.

Unlike the superimposed properties of electrons or other particles more prone to environmental interference, "the coherence length of neutrino oscillations—the length over which interference occurs and oscillations may be observed—extends over vast distances, even astrophysical scales," the physicists note.

This is more than a neat trick. The MIT experiments offer further evidence against an old anti-quantum mechanical idea positing that the seeming indeterminism of a superposition (alive and dead, etc.) isn't real. What appears to us as non-deterministic is really as certain as what we should expect intuitivity (alive or dead, etc.). The missing determinism is just present in some way that we haven't figured out how to observe. Einstein ("god does not play dice") and later Schrödinger (look at this weird cat, jeeze) preferred this view over the probabilistic alternative.

"The data we collected lines up beautifully for quantum mechanics."

"Hidden variable models are alternatives, rivals to quantum mechanics," Kaiser said. "They try to say that even if we don't make a direct measurement, our theory attributes to every bit of matter a definite property instead of these superpositions."

By observing the statistics of neutrino oscillations at very large distances, it's possible to test for the presence of hidden deterministic variables. Non-determinism in quantum mechanics is represented by waves, which, instead of representing definite values, represent probabilistic smears between those two (or more) values. Pick some point in time and the wavefunction will give you some odds of something being true at that point (the cat being alive, say).

If you have several properties superimposed together, you can imagine several waves together, with each one representing the probability of a certain property being true.

"The neutrinos are not in one state or another," Kaiser said. "They are in a wave and those two waves are interfering. That's different than what you should see if they had a unique identity at every moment of their journey."

With larger distances, the more reliably we can say that hidden variables are out of the question. Eventually, we may even be able to look at neutrinos arriving from deep space and make inferences further supporting the non-deterministic view of quantum mechanics (which is, by the by, now favored by physicists by an enormous margin). The fate of Schrödinger's cat depends on it.