Can the wave function of an electron be divided and trapped?

30-Oct-2014 - USA

New research by physicists from Brown University puts the profound strangeness of quantum mechanics in a nutshell — or, more accurately, in a helium bubble.

Experiments led by Humphrey Maris, professor of physics at Brown, suggest that the quantum state of an electron — the electron's wave function — can be shattered into pieces and those pieces can be trapped in tiny bubbles of liquid helium. To be clear, the researchers are not saying that the electron can be broken apart. Electrons are elementary particles, indivisible and unbreakable. But what the researchers are saying is in some ways more bizarre.

In quantum mechanics, particles do not have a distinct position in space. Instead, they exist as a wave function, a probability distribution that includes all the possible locations where a particle might be found. Maris and his colleagues are suggesting that parts of that distribution can be separated and cordoned off from each other.

"We are trapping the chance of finding the electron, not pieces of the electron," Maris said. "It's a little like a lottery. When lottery tickets are sold, everyone who buys a ticket gets a piece of paper. So all these people are holding a chance and you can consider that the chances are spread all over the place. But there is only one prize — one electron — and where that prize will go is determined later."

If Maris's interpretation of his experimental findings is correct, it raises profound questions about the measurement process in quantum mechanics. In the traditional formulation of quantum mechanics, when a particle is measured — meaning it is found to be in one particular location — the wave function is said to collapse.

"The experiments we have performed indicate that the mere interaction of an electron with some larger physical system, such as a bath of liquid helium, does not constitute a measurement," Maris said. "The question then is: What does?"

And the fact that the wave function can be split into two or more bubbles is strange as well. If a detector finds the electron in one bubble, what happens to the other bubble?

The new research is published in the Journal of Low Temperature Physics.

Electron bubbles

Scientists have wondered for years about the strange behavior of electrons in liquid helium cooled to near absolute zero. When an electron enters the liquid, it repels surrounding helium atoms, forming a bubble in the liquid about 3.6 nanometers across. The size of the bubble is determined by the pressure of the electron pushing against the surface tension of the helium. The strangeness, however, arises in experiments dating back to the 1960s looking at how the bubbles move.

In the experiments, a pulse of electrons enters the top of a helium-filled tube, and a detector registers the electric charge delivered when electron bubbles reach the bottom of the tube. Because the bubbles have a well-defined size, they should all experience the same amount of drag as they move, and should therefore arrive at the detector at the same time. But that's not what happens. Experiments have detected unidentified objects that reach the detector before the normal electron bubbles. Over the years, scientists have cataloged 14 distinct objects of different sizes, all of which seem to move faster than an electron bubble would be expected to move.

"They've been a mystery ever since they were first detected," Maris said. "Nobody has a good explanation."

Several possibilities have been proposed. The unknown objects could be impurities in the helium—charged particles knocked free from the walls of the container. Another possibility is that the objects could be helium ions — helium atoms that have picked up one or more extra electrons, which produce a negative charge at the detector.

But Maris and his colleagues, including Nobel laureate and Brown physicist Leon Cooper, believe a new set of experiments puts those explanations to rest. know."

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