Researchers shed new light on atomic "wave function"

Physicists have devised a new method for obtaining the essential details describing an isolated quantum system, such as an atom gas, by direct observation. The new method provides information on the probability of finding atoms at specific locations in the system with unprecedented spatial resolution. With this technique, scientists can get details on a scale of several tens of nanometers, less than the width of a virus.

The experiments conducted at the Joint Quantum Institute (JQI), a research partnership between the National Institute of Standards and Technology (NIST) and the University of Maryland, use an optical network (laser light array suspending thousands of atoms individual) to determine the probability that an atom could be at a given location. Because each atom of the network behaves like all the others, a measure on the whole group of atoms reveals the probability that an atom is at a given point in space.

Posted in the journal Physical examination X, the JQI technique (and a similar technique published simultaneously by a group from the University of Chicago) may give the probability that the locations of atoms are well below the wavelength of the light used for illuminate the atoms – 50 times better than the limit of what optical microscopy can normally solve.

"It's a demonstration of our ability to observe quantum mechanics," said Trey Porto, of JQI, one of the physicists behind the research effort. "That was not done with atoms with nowhere near that accuracy."

To understand a quantum system, physicists frequently speak of its "wave function". This is not just an important detail; it's all history. It contains all the information needed to describe the system.

"That's the description of the system," said JQI physicist Steve Rolston, another author of the newspaper. "If you have information about the wave function, you can calculate everything, such as the magnetism of the object, its conductivity and its probability of emitting or absorbing light. . "

While the wave function is a mathematical expression and not a physical object, the team method can reveal the behavior described by the wave function: the probabilities that a quantum system behaves one way or the other. In the world of quantum mechanics, probability is everything.

Among the many strange principles of quantum mechanics, there is the idea that before measuring their positions, objects may not have precise location. The electrons surrounding the nucleus of an atom, for example, do not move in regular planetary orbits, unlike the image that some of us had taught in school. Instead, they act as wave waves, so that an electron itself can not be considered to have a defined location. Rather, the electrons reside in fuzzy regions of space.

All objects can have this wave behavior, but for anything large enough to be observed without help, the effect is imperceptible and the rules of classical physics are in effect – we do not notice that buildings, buckets or bread crumbs spread like waves. But isolate a tiny object such as an atom, and the situation is different because the atom exists in a size domain where the effects of quantum mechanics are supreme. It is not possible to say for sure where it is, but only that it will be found somewhere. The wave function provides the set of probabilities of finding the atom at a given place.

Quantum physics is quite well understood – in any case by physicists – for a fairly simple system, experts can calculate the wave function from the first principles without having to observe it. Many interesting systems, however, are complicated.

"There are quantum systems that can not be calculated because they are too difficult," said Rolston, such as molecules made up of several large atoms. "This approach could help us understand these situations."

As the wave function only describes a set of probabilities, how can physicists get a complete picture of its effects quickly? The team's approach is to simultaneously measure a large number of identical quantum systems and combine the results into a single overall image. It's a bit like throwing 100,000 pairs of dice at the same time: each throw gives a single result and contributes to a single point on the probability curve, displaying the values ​​of all the dice.

The team observed the positions of some 100,000 ytterbium atoms suspended by the optical network in its lasers. The ytterbium atoms are isolated from their neighbors and restricted to a back and forth along a one-dimensional straight line segment. To obtain a high resolution image, the team found a way to observe the narrow slices of these segments, as well as the frequency with which each atom appears in its respective slice. After observing one area, the team measured another, until it had a complete picture.

Rolston said that although he has not yet thought of a "revolutionary application" that would take advantage of the technique, the mere fact that the team has directly imaged something central to the research quantum fascinates him.

"It's not entirely clear where it will be used, but it's a new technique that offers new opportunities," he said. "We have been using an optical network to capture atoms for years and it has now become a new type of measurement tool."