A team of physicists in the Netherlands and Germany recently placed a bunch of titanium atoms under a scanning tunneling microscope. Those atoms were in constant, quiet interaction with each other through the directions of their spins. In a clever feat, the researchers were able to home in on a single pair of atoms, zapping one with an electric current in order to flip its spin. They then measured the reaction of its partner.
When two atoms have spins that are interdependent, they are considered quantumly entangled. That entanglement means that the behavior of one atom has a direct impact on the other, and theory says this should remain true even when they are separated by great distances. In this case, the titanium atoms were a little over a nanometer (a millionth of a millimeter) apart, close enough for the two particles to interact with one another but far enough away that the interaction could be detected by the team’s instruments.
“The main finding is that we have been able to observe how atomic spins behave over time as a result of their mutual interaction,” said co-author Sander Otte, a quantum physicist at the Kavli Institute of Nanoscience at the Delft University of Technology in the Netherlands. Otte explained in an email that scientists previously have been able to measure the strength of various atomic spins and the influence of that strength on the atom’s energy level. But this experiment allowed them to observe that interaction over time.
One big hope of experimental physics is that someday researchers will be able to simulate quantum interactions at will, tweaking a quantum system as they see fit and observing how the quantum mechanics play out. The researchers, in effect, did that, triggering a specific action in one atom and observing how the atom next door reacted.
“This is a very nice demonstration of a very simple ‘quantum simulator,’” said Ella Lachman, a quantum physicist at UC Berkeley who wasn’t involved in the new study. “By controlling the positions of the atoms, we can theoretically build a replica of a lattice or any system we want to study the dynamics of.”
The team chose to work with titanium atoms because they have the fewest possible options for their spin—either up or down. The titanium atoms were bound on a magnesium-oxide surface, holding them in place for inspection. Stuck to that surface, which was kept in a near-vacuum at just one kelvin, or -457.87 Fahrenheit, the atoms could be individually picked out by the physicists under the tip of the microscope (here’s a neat video showing how that works). They could then reverse the atom’s spins by zapping one atom in a pair with an electric pulse, prompting an immediate reaction by its neighbor. These reactions are predictable, Otte said, through the laws of quantum mechanics. (If you say “knock knock,” you can be damn sure the next particle over will respond “Who’s there?”) The entire process took about 15 nanoseconds, or 15 billionths of a second. Their research was published today in Science.
There are other means of reading into the quantum world. Scientists are able to conjure interactions between atoms by altering the spin of one, but that intercommunication happens so fast that typical means of observation, like the spin resonance technique, can’t pick it up. Quantum researchers often use microwave pulses to get the atoms to change states or otherwise observe quantum mechanics, but this electric pulse approach gave the team the ability to sense the most minute of minute interactions; the equivalent of an atom-to-atom DM.
Methods like the spin resonance technique are “simply too slow,” said Lukas Veldman, a quantum physicist at the Kavli Institute of Nanoscience at the Delft University of Technology, in a Delft release. “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.”
The real magic of this line of research has yet to come, Otte said. While this detection mapped the bouncing of spins between two atoms, the situation becomes much more complex with each atom you add to the equation. You could think of a game of Telephone where participants can both pass the message along while also whispering it back the way it came. Messages coming from different directions would start to intersect, garbling the communiques.
“As always, the toy models are nice, but once we add to them the complexity we are truly interested in, the questions of measurements and interpretations of them become more complicated,” Lachman said. “Can they do the same experiment with three atoms while only measuring one? Probably yes, but the interpretation of the measurement becomes more complicated. How about ten atoms? Twenty? Time and ingenuity will tell if this is a cool experimental demonstration of a toy model or something deeper. The potential is there.”
Otte also emphasized the mind-bending challenges of going beyond a simple system of two atoms. “If we increase to 20 spins, my laptop could no longer calculate what happens. At 50 spins, the best supercomputers in the world give up, and so forth,” Otte said. “If we ever want to understand precisely how the complex behavior of certain materials comes about (an excellent example is superconductivity), we would have to ‘build’ materials from scratch and see how the laws of physics play out when increasing from 10 to 100 to 1,000 atoms.” Superconductivity refers to materials that can transmit electricity with zero resistance, something only possible for now at very cold temperatures. That’s why the development of a room-temperature superconductor is a holy grail of physics. It would completely change the world.
But it’s at these bigger numbers that you begin to get a sense of the ultimate prizes. Instead of listening in on one atomic heart-to-heart, researchers could eventually hear the murmur of quantum conversations with many atoms as they flip to-and-fro. We’ll need better computers for such quandaries, of course, but even the smallest interactions have their own intimate importance, as starters of a bigger conversation.