Hydrogen atom orbitals. Image: FlorianMarquardt/Wikimedia

Electrons cooled close to absolute zero reveal their quantum nature

We can finally see things more clearly.

FIONA MACDONALD
10 NOV 2016
 

Scientists have discovered that electrons cooled close to absolute zero slow down so much that they can be studied individually - allowing us to see the world in a whole new level of detail.

At those temperatures, electric current stops flowing. Instead, electrons trickle through a conductor like grains of sand in an hourglass, finally revealing their quantum state and allowing us to study them one at a time.

 

"These extremely low temperatures open up an unexpected richness of detail," said lead researcher Christian Ast from the Max Planck Institute in Germany.  

Electrons are curious subatomic particles - they form the atoms that make up the world around us and flow as electric current. But even though we know a lot about them and how they work, they're incredibly hard for us to really nail down and study.

That's because when electric current is flowing, it's impossible to distinguish individual electrons. It's just like water - when water is flowing from a tap, it feels like it's homogenous (or all one medium) and it's impossible to distinguish between individual water molecules.

Similarly, flowing electric current feels like a single medium, and scientists aren't able to distinguish individual electrons in the flow, even though quantum mechanics says they should exist.

That means we also don't really understand how electrons behave on an individual level, which limits our understanding of the world around us.

But now researchers have finally managed to slow things down enough to get electrons to reveal their quantum state - which is the understanding of a single entity within an isolated quantum system. 

 

In other words, they slowed things down enough to study individual electrons as they flow through a conductor.

To do this, the team cooled a scanning tunnelling microscope down to a fifteen-thousandth of a degree above absolute zero, which is roughly –273.135 degrees Celsius (–459.65 degrees Fahrenheit).

The scanning tunnelling microscope works works by running a tiny, pointed tip across the surface of a sample without actually touching it, encouraging current to flow from the tip into the sample.

Under normal temperatures, that current is pretty tiny - we're talking less than one-billionth of the current that flows through a 100-watt light bulb. But even in that small amount, billions of electrons still flow every second, and it's impossible for scientists to distinguish individual electrons.

Electron-webMPI for Solid State Research

It wasn't until the team cooled the microscope down to almost absolute zero that they were finally able to see that the electric current consisted of individual electrons - it became a granular medium.

At this low temperature, they were able to detect new and unexpected structures in the electric feedback recorded by the microscope.

"We could explain these new structures only by assuming that the tunnelling current is a granular medium and no longer homogeneous," said Ast.

The discovery could finally confirm a theoretical hypothesis developed more than 20 years ago that suggested this type of examination would be possible.

"The theory on which this is based was developed back at the beginning of the 1990s," said one of the researchers, Joachim Ankerhold, from the University of Ulm.

"Now that conceptual and practical issues relating to its application to scanning tunnelling microscopes have been solved, it is nice to see how consistently theory and experiment fit together."

This isn't the first time electrons have revealed their quantum nature - they already move through things like quantum dots one at a time - but it is the first time a scanning tunnelling microscope has been shown to have reached its quantum limit.

The researchers are hopeful that this ability will lead to new discoveries in the quantum world, and will "allow us to understand superconductivity and light-matter interactions much better", said Alt.

After all, the better we understand matter itself, the better we'll be able to understand the Universe.

The research has been published in Nature Communications.

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