In 1996, researchers at the Boulder, Colo., laboratories of the Commerce Department's National Institute of Standards and Technology confirmed the belief that a single atom can be simultaneously located in two separated places (a physical state termed a quantum-mechanical superposition). Now, almost four years later, they have performed experiments to help explain why this works only at the atomic level and not in our world of everyday experience.
The latest findings by the research team in NIST's Time and Frequency Division are published in the Jan. 20, 2000, issue of Nature, a British scientific journal.
To understand the concept of superposition, the scientists say, imagine a marble in a large, shallow, round-bottomed bowl. In the superposition state, the marble can be simultaneously at opposite sides of the bowl, rolling from side to side and through itself at the center. But when the bowl is shaken ever so slightly, the marble ends up either on the left or the right side of the bowl exclusively. This ends the dual-position existence.
"We found that the shakes—the influence of the environment on the delicate superposition—quickly become a factor as you increase the distance between the two states of the atom. This dependence on separation tells us why it's impossible for us to see superpositions in our macro-sized world," explains David J. Wineland, a member of the NIST team.
The current experiment expands on the 1996 work in which the researchers isolated a single beryllium ion (an atom with one of its two outer electrons stripped away) in an electromagnetic trap and cooled it nearly to absolute zero with precisely tuned laser beams. This confined it to a tiny region of space less than a millionth of a centimeter across, where it rested almost without motion.
The ion's single remaining outer electron can be in two internal quantum states: "up" and "down." These states correspond to different orientations of the spin of the electron. If the two states both exist and are, in a sense, sort of stacked upon one another, the electron is said to be in superposition. Until an outside agent disturbs the particle, there is an equal probability that it is in either state, and thus it is considered to be in both states.
What the NIST team did differently from 1996 is attempt to separate the two states over a range of distances from almost overlapping to around 10 atomic widths apart. While extremely tiny from our perspective, this range is huge at the atomic level.
"As we increased the distance between the two states, the environmental effects on the superposition increased exponentially, so it didn't take long for the separated ion to collapse back into a single entity," says Christopher Monroe, another member of the NIST team.
In effect, the NIST researchers have step-by-step crossed the bridge between quantum mechanics (the study of matter at atomic and subatomic levels) and the real world—what we see in daily life.
"You need an experiment to cross over the boundary," Monroe says. "This is the first experiment where we can do this systematically."
Both the current and 1996 experiments have connections to the theories of Albert Einstein and Erwin Schroedinger, who in 1935 described hypothetical scenarios allowed by quantum mechanics that seemed to defy reality. Schroedinger considered the possibility that a cat could be made to be both dead and alive at the same time. "Schroedinger's cat" soon became a shorthand way to refer to a whole class of superposed atomic states.
It is said that Schroedinger later regretted making this analogy because it was so divorced from real life. Maybe the NIST team's latest findings will give the late physicist some peace at last.
In addition to Wineland and Monroe, other NIST researchers who contributed to the Nature paper include Chris J. Myatt, Brian E. King, Quentin Turchette, Charles A. Sackett, David Kielpinski and Wayne M. Itano.
As a non-regulatory agency of the U.S. Department of Commerce's Technology Administration, NIST strengthens the U.S. economy and improves the quality of life by working with industry to develop and apply technology, measurements and standards through four partnerships: the Measurement and Standards Laboratories, the Advanced Technology Program, the Manufacturing Extension Partnership and the Baldrige National Quality Program.