Rice University physicists have built an accurate model of part of the solar system inside a single atom of potassium.
They’ve made an electron in an atom orbit the nucleus in exactly the same way that Jupiter’s Trojan asteroids orbit the sun – bearing out a prediction made in 1920 by Danish physicist Niels Bohr about the relationship between quantum mechanics and Newton’s laws of motion.
“Bohr predicted that quantum mechanical descriptions of the physical world would, for systems of sufficient size, match the classical descriptions provided by Newtonian mechanics,” says lead researcher professor Barry Dunning.
“Bohr also described the conditions under which this correspondence could be observed. In particular, he said it should be seen in atoms with very high principal quantum numbers, which are exactly what we study in our laboratory.”
The team began by using an ultraviolet laser to create a Rydberg atom, which contain a highly excited electron with a very large quantum number. In the Rice experiments, potassium atoms with quantum numbers between 300 and 600 were studied.
“In such excited states, the potassium atoms become hundreds of thousands of times larger than normal and approach the size of a period at the end of a sentence,” says Dunning. “Thus, they are good candidates to test Bohr’s prediction.”
Electrons exist as both particles and waves. To ‘locate’ an electron, physicists calculate the likelihood of finding it at different locations at a given time.
These predictions are combined to create a ‘wave function’ that describes all the places where the electron might be found. Normally, an electron’s wave function looks like a diffuse cloud that surrounds the atomic nucleus, because the electron could be anywhere at any time.
The team had already used a tailored sequence of electric field pulses to collapse the wave function of an electron in a Rydberg atom, limiting where it might be found to a localized, comma-shaped area called a ‘wave packet’. While this localized wave packet orbited the nucleus of the atom much like a planet orbits the sun, the effect lasted only a short time.
In the new experiments, though, the Rice team managed to use radio frequency waves to capture this localized electron and make it orbit the nucleus indefinitely without spreading out, by applying a radio frequency field that rotated around the nucleus itself. This field ensnared the localized electron and forced it to rotate in lockstep around the nucleus.
A further electric field pulse was used to take a snapshot of the wave packet – destroying the delicate Rydberg atom in the process. After the experiment had been run tens of thousands of times, all the snapshots were combined to show that Bohr’s prediction was correct: the classical and quantum descriptions of the orbiting electron wave packets matched.
Indeed, the classical description of the wave packet trapped by the rotating field parallels the classical physics that explains the behavior of Jupiter’s Trojan asteroids.
Jupiter’s 4,000-plus Trojan asteroids have the same orbit as Jupiter, and are contained in comma-shaped clouds that look very similar to the localized wave packets created in the Rice experiments.
And, just as the wave packet in the atom is trapped by the combined electric field from the nucleus and the rotating wave, the Trojans are trapped by the combined gravitational field of the sun and orbiting Jupiter.
The researchers are now working to localize two electrons and have them orbit the nucleus like two planets in different orbits.
“The level of control that we’re able to achieve in these atoms would have been unthinkable just a few years ago and has potential applications in, for example, quantum computing and in controlling chemical reactions using ultrafast lasers,” says Dunning.
