Physicists Detect Neutrinos in Superposition Hurtling Through Spacetime

Physicists have verified a central tenet of quantum mechanics—that quantum particles can occupy a so-called superposition—using neutrinos separated by 450 miles, the farthest such test to date.

A group of physicists from MIT shot a beam consisting of one “flavor” of neutrino, a tiny subatomic particle that rarely interacts with other matter, but hundreds of miles away, they detected a different flavor of neutrino. This indicates the neutrinos were traveling in a quantum superposition, a midflight mixture of flavors, as reported last week in Physical Review Letters.

The neutrino detector in Minnesota.

The study adds credence a long-held theory that says that subatomic particles can be in superposition while hurtling through spacetime.

Many physicists are pleased with this affirmation of quantum mechanics, but they were also tickled by the way in which it was done. Here’s Adrian Cho, reporting for Science:

“If you had told me 10 years ago that we would use neutrinos to study quantum foundations, I would have said that you’d been smoking something very exciting,” says Andrew White, a physicist at the University of Queensland, St. Lucia, in Brisbane, Australia, who was not involved in the work. “The result is utterly unsurprising and yet utterly attractive because it tells us that there’s a new system for testing quantum foundations.”

Erwin Schrödinger highlighted this sort of weirdness that’s inherent to quantum mechanics with his famous feline thought experiment. Inside of a box, he imagined, there’s a cat and a radioactive particle. The cat will die if the particle decays. After exactly the half-life of the radioactive particle, there’s a 50% chance the particle has decayed and a 50% chance it has not. So, the cat is in limbo, equally likely to be alive or dead. The cat is in a quantum superposition, both alive and dead…sort of. Just like a quantum particle, the cat is only in a superposition until it’s observed. When you open the box, the cat is either alive or dead—not both.

If multiple particles like neutrinos are in a superposition together, their states can be entangled. In Schrödinger’s experiment, if there are two cats and we open the first box, we immediately know whether the second cat is alive or dead, without observing it directly. This is true even if the cats are around the globe from each other, an effect that Einstein referred to as “spooky action at a distance.”

Physicists demonstrated quantum mechanical these effects by firing the neutrino from the outskirts of Chicago to the northern edge of Minnesota—adding another convincing data point yet in support of the theory that that quantum superposition can happen over infinite distances.