Scientists overhear two atoms chatting

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      On a large scale, this kind of exchange of information between atoms can lead to fascinating phenomena. A classic example is superconductivity: the effect where some materials lose all electrical resistivity below a critical temperature. While well understood for the simplest cases, nobody knows exactly how this effect comes about in many complex materials. But it’s certain that magnetic quantum interactions play a key role. For the purpose of trying to explaining phenomena like this, scientists are very interested in being able to intercept these exchanges; to overhear the conversations between atoms.

      In Otte’s team they go about this rather directly: they literally put two atoms next to each other to see what happens. This is possible by virtue of a scanning tunneling microscope: a device in which a sharp needle can probe atoms one-by-one and can even rearrange them. The researchers used this device to place two titanium atoms at a distance of just over one nanometer—one millionth of a millimeter—apart. At that distance, the atoms are just able to detect each other’s spin. If you would now twist one of the two spins, the conversation would start by itself.

      Usually, this twist is performed by sending very precise radio signals to the atoms. This so-called spin resonance technique—which is quite reminiscent of the working principle of an MRI scanner found in hospitals—is used successfully in research on quantum bits. This tool is also available to the Delft team, but it has a disadvantage. “It is simply too slow,” says Ph.D. student Lukas Veldman, lead author on the Science publication. “You have barely started twisting the one spin before the other starts to rotate along. This way you can never investigate what happens upon placing the two spins in opposite directions.”

      So the researchers tried something unorthodox: they rapidly inverted the spin of one of the two atoms with a sudden burst of electric current. To their surprise, this drastic approach resulted in a beautiful quantum interaction, exactly by the book. During the pulse, electrons collide with the atom, causing its spin to rotate. Otte: “But we always assumed that during this process, the delicate quantum information—the so-called coherence—was lost. After all, the electrons are incoherent: the history of each electron prior to the collision is slightly different and this chaos is transferred to the atom’s spin, destroying any coherence.”

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      Scott Crowder
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        NV Wino
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