April 10, 2009 — Picture the textbook atom.
It would resemble a miniature solar system — an atomic nucleus orbited by electrons, drawn in nice tidy elliptical orbits — like planets orbiting the sun. This is a reasonable classical depiction of an atom, but it is completely at odds with the usual quantum description of an atom.
In the quantum energy states of a one-electron atom, the electron does not move in an orbit, but is described by a wave function, which, when squared, produces a probability cloud about the nucleus which does not change in time.
The electron can be in any given place at any given time, and at all places at once. That is quantum mechanics, an arena of physics so strange and complicated, even physicists admit it is hard to picture.
But University of Virginia physicist Tom Gallagher and his colleagues have engineered, in a sense, the classical picture-perfect textbook atom.
The physicists used a weak microwave field to lock together the time-dependent phase evolutions of the wave functions of several energy states. If only one energy state's wave function is present, its phase is of no consequence; but if there are two or more, the phases matter.
At any given time the wave functions add in one region of space and cancel in another. When the composite wave function is squared, the probability is localized, and it moves, just like the classical atom we picture.
Gallagher and his team recently published their results in Physical Review Letters (volume 102, page 103001).
Researchers at the University of Rochester originally suggested that making such classical atoms might be possible, noting the similarity to Lagrange points — regions
of space where gravity from a variety of points, such as planets, affect the orbits of other bodies in space, and can cancel out distant sources of gravity.
To realize such atoms in the laboratory, Gallagher and his team struck upon the idea of first locking the motion of an electron to a linearly polarized field, producing an atom in which the electron oscillates along a line, and then altering the microwave polarization to circular. The electron orbit follows the changing polarization, becoming a circular orbit.
"We honestly were quite surprised by how well we could manipulate the atom with our technique," Gallagher said. "We demonstrated that we can change the state of the atom in a way that was once considered impossible."
Carlos Stroud, a physicist at the University of Rochester, marveled at the quality of Gallagher's work in a commentary for the publication Physics; and he later described the experiment as "a beautiful piece of physics" for the magazine New Scientist.
Gallagher's co-authors are U.Va. graduate student Joshua Gurian and Haruka Maeda, now of the Japan Science and Technology Agency.