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Schrödinger's cat explained with neutrinos

Physicists show quantum weirdness of neutrinos over longest distance yet

By Katyanna Quach, 20 Jul 2016

Physicists have found that neutrinos keep their quantum weirdness over the longest distance that quantum mechanics has been tested to date.

Superposition is a fundamental theory in quantum mechanics. The idea that particles can exist simultaneously in many different states was famously compared to a thought experiment devised by Erwin Schrödinger.

A cat in a sealed box exposed to a vial of poison was both dead and alive at the same time, until somebody opened the box and checked, Schrödinger said.

Before the outcome is determined, the cat occupies both states: dead or alive, according to the laws of quantum mechanics.

Instead of using cats, however, a team of physicists from the Massachusetts Institute of Technology (MIT) have proved the strange effect of superposition with neutrinos.

Neutrinos are ghostly particles that stream through the Earth and go largely undetected because they interact so weakly with matter.

Neutrino flavours

The experiment builds on the idea of neutrinos changing flavours. Many experiments that hunt for these particles in vats of chlorine liquid or scintillation detectors have proved difficult. Theoretical calculations that predicted the yield of neutrinos in detectors did not match what was found in reality.

Last year's Nobel Prize in Physics was awarded to physicists who solved the mystery of the missing neutrinos. Two different research groups working in Canada and Japan found that on the way to the detectors, the neutrinos could sneakily change identities.

The different identities, known as "flavours," mean neutrinos can be split into an electron, muon or tau neutrino. Physicists had underwhelming results because they were only looking for one flavour.

The MIT researchers took data from the Main Injector Neutrino Oscillation Search (MINOS) experiment from Fermilab, a particle accelerator laboratory in Illinois.

Neutrinos were produced as other high-energy particles were scattered from the MINOS experiment and were beamed to a detector 456 miles away in Minnesota. Although the neutrinos left Illinois as one flavour, they arrived in Minnesota as a completely different flavour.

The researchers proved that in-between the journey, the neutrinos were suspended in a superposition state of all the flavours – and had no definite identity.

Quantum reality

A mathematical relation called the Legett-Garg inequality was used to analyse the data produced from the neutrinos in the MINOS experiment.

The Legett-Garg inequality can be used as a test for quantum superposition. It can find out whether a system with two or more distinct states acts in a quantum or classical manner.

The distributions of different neutrino flavours arriving at the detector in Minnesota should look different if they had a definite identity (classical physics) or were in superposition (quantum physics).

When the researchers compared the data to their predictions, it matched the distribution of flavours detected if the system was in superposition.

They ran statistical analyses of their results and found that the chances of a classical system giving the distribution of neutrino flavours they detected was “something like one in a billion”.

The quantum weirdness of superposition was undisturbed even as the neutrinos were travelling hundreds of miles close to the speed of light.

"What gives people pause is, quantum mechanics is quantitatively precise and yet it comes with all this conceptual baggage," said David Kaiser, co-author of the study and professor of physics at MIT.

"That's why I like tests like this: Let's let these things travel further than most people will drive on a family road trip, and watch them zoom through the big world we live in, not just the strange world of quantum mechanics, for hundreds of miles. And even then, we can't stop using quantum mechanics. We really see quantum effects persist across macroscopic distances."

The preliminary paper is to be published in the journal Physical Review Letters later this month. ®

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