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Watching The Quantum Jump

by John G. Cramer

Alternate View Column AV-26
Keywords: ion, trap, Penning, quantum, transition, jump, uncertainty
Published in the May-1988 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 10/16/87 and is copyrighted © 1987, John G. Cramer. All rights reserved.
No part may be reproduced in any form without the explicit permission of the author.


    How's this for a plot scenario? A giant meteor enters the solar system, collides with the planet Venus, and ejects it from its orbit around the Sun. The collision sends Venus in a new hyperbolic path that carries it far outside the solar system. Then to make things even more complicated, our planet Earth vanishes from its present orbit and simultaneously reappears in the orbit that Venus has vacated. At the same time a huge burst of radiation is emitted which contains the energy the Earth lost in moving into the orbit of Venus.

    That sounds like the worst kind of far-fetched rubber science, right? No self respecting science fiction writer would even consider writing a hard SF story using such an absurdly unphysical sequence of events. (Even TV writers might think twice.) But in atoms, which in many ways behave like miniature solar systems, an event just like the one described above would not be all that unusual. It's called a quantum jump. When an atomic orbit containing an electron is vacated by, for example, a collision, another electron from a higher orbit jumps to the newly vacant orbit while emitting a photon of light to carry away the energy difference between the high and the low orbit. In this process the electron, according to quantum theory, does not move in a continuous way between the first orbit and the second; instead it disappears from one orbit and appears in the other.

    In 1916 Neils Bohr first proposed his new model of the atom. The Bohr model describes the atom as having a small massive positively charged nucleus at is center with lighter negatively charged electrons orbiting the nucleus in fixed orbits. When an electron moves from one orbit to another, it must "jump" in an abrupt transition, since states of motion that are intermediate between one Bohr orbit and the next are not allowed in Bohr's model. Bohr's use of quantum jumps was disturbing to many prominent physicists of the day. They were slow to accept what was considered to be a radical idea. The notion that the electron could move from one place to another without explicitly travelling through the space between seemed more like magic than physics and so counter-intuitive as to violate the very spirit of classical physics.

    Later developments in quantum mechanics have left Bohr's description of quantum jumps essentially unchanged, and today the concept is routinely used in atomic physics. Indeed the phrase "quantum jump" has become a part of our everyday language. It has come to mean a dramatic qualitative change that happens very rapidly, with little in the way of an intermediate transition. It is ironical that a quantum jump in physics is often the smallest change possible, while in popular speech a quantum jump is a very large change.

    In this Alternate View column we are going to discuss quantum jumps because there has been an interesting new development in this area of atomic physics. Through the use of new "trapping" techniques a quantum jump in an isolated single atom has now been made visible to a human eye (slightly aided by a low power telescope). The trapping of atoms or sub-atomic particles with a Penning trap was discussed in a previous AV column ["Antimatter in a Trap", Analog, December, 1985], but let me remind you how the technique works.

    A Penning Trap looks rather like a metal hour-glass with a rounded knob poking into each end. The knobs are given a positive electrical charge, and the hour-glass is given a negative charge. The whole assembly is placed in the bore of a magnetic solenoid which produces a magnetic field pointing vertically along the axis of the hour glass. It is placed in a good vacuum and cooled to liquid helium temperature. Into this apparatus near the waist of the hour-glass one can inject a single positively charged atom. It will stay in that position, held in place by the electric and magnetic forces of the trap. The electric repulsion of the charged knobs keeps the atom from moving vertically, and the vertical magnetic field keeps it from escaping horizontally.

    Profs. Nagourney and Dehmelt, two of my colleagues in the University of Washington Department of Physics, have pioneered in using this kind of apparatus to study the behavior of an atom of the element barium (atomic number 56, chemical symbol Ba). They placed an ionized atom of barium (Ba+) in a Penning trap. This is done by turning on the trap's electric fields as the barium ion is near the center of the trap. The trapped ion, rather like a wild animal that has just been caught, is initially in a highly agitated state and moves rapidly around in its new cage.

    So it must be cooled. This is done by directing at the atom a beam of laser light tuned to a frequency just below some frequency that the atom selectively absorbs. Over a period of time the absorption and reemission of this light slows or "cools" the trapped atom. This happens because the kinetic energy of the atom's motion is consumed through the Doppler effect in making up the energy difference between the laser light and the light absorbed. The barium atom becomes very "cold" as its motion in the trap slows to a near standstill. If the temperature of a single atom is a meaningful concept then a barium atom cooled in this way reaches the lowest temperature ever achieved in a laboratory.

    Now, with the barium atom at rest in the trap, we can examine it. The laser light that cools the atom also makes it visible. The atom alternately absorbs photons from the laser beam and then re-emits them in random directions. To the eye of an observer looking through a low power telescope (used for gathering light) the single trapped barium atom can be seen as a tiny point of light. The idea that a nearly unaided eye can actually see a single atom is quite remarkable in itself. But wait, Folks, there's more ...

    Atoms can be placed in a number of "states" which are essentially different arrangements of its orbiting electrons. Each state has a characteristic energy. The state with the lowest energy is called the "ground state" of an atom. The normal condition of an atom is to be in this state. If an atom is excited to a higher energy state (called an "excited state"), it will eventually emit one or more photons and return to the ground state. In such a process one of the atom's electrons makes a quantum jump into a new orbit, and then falls back into its original orbit.

    Barium has a special excited state with a very long lifetime. If an ionized barium atom is put into this state, it remains there for about 30 seconds. In this excited state it cannot absorb the laser light that is used to cool the ground state. And the barium atom atom can be placed in this long-lived state by shining on it laser light of a higher frequency.

    So now suppose that we watch the isolated barium atom through the telescope and observe the tiny point of light from the absorbtion and re-radiation of its cooling light. Then we turn on a second laser which drives it in a quantum jump from its ground state into its long-lived excited state. The point of light in the telescope vanishes. We wait for about half a minute. Finally the atom makes a second quantum jump back to the ground state. The point of light in the telescope reappears. We have "seen", in some sense of the word see, a single isolated atom make a quantum jump from ground state to an excited state and then make another quantum jump to return. Quantum jumps, which has been a thing of conjecture and quantum weirdness are now visible to the nearly unaided eye.

    This experiment, performed essentially as described above, was reported last year in Physical Review Letters (see the reference below) and immediately received considerable attention in science news articles in The New York Times, Science, Science News, Nature, and The Scientific American. The idea that one could actually see a quantum jump and its effects in a single atom seemed to fire the imaginations of the science reporters. But it is fair to ask what this effect might be used for, other than to produce a concrete demonstration of an esoteric effect.

    Improvements in our ability to measure increments of time with high precision have played a very important role in technological progress, and have also made possible certain fundamental investigations. For example, atomic clocks are now accurate enough to detect the time shift which one clock undergoes due to the effects of general relativity when it is carried around the Earth in a commercial passenger jet when compared with another otherwise identical clock which remains at rest in the laboratory. Such time measurements use atoms as the basic time keepers, employing the natural resonance frequencies of selected atoms like the vibrations of a tuning fork to mark off small increments of time. A device that does this is called an atomic clock.

    But there is a fundamental problem with atomic clocks. The clock's basic timing elements, atoms placed in a definite state of vibration, are always in thermal motion. The atoms rattle around and bump together, and their motions fuzz out the precise time structure of their vibrations. But in a Penning trap the single barium ion can, by laser cooling, be placed in a state where its thermal motion is reduced to a value below the quantum limit. The result is that the thermal effects vanish and the atom becomes a "perfect" tuning fork.

    The trapping technology therefore offers the possibility of making new and more perfect atomic clocks, of measuring time with several orders of magnitude more precision than has been possible in the past. At this level many previously unfeasible fundamental tests of theories like general relativity and quantum electrodynamics move into the range of possibility. Confinement and study of trapped atoms offer us completely new ways of testing our understanding of how the universe works.


Ion Trapping:
P. Ekstrom and D. Wineland, Scientific American 243 #2, 105 (August, 1980).

Quantum Jumps:
W. Nagourney and H. G. Dehmelt, Physical Review Letters 56, 2797 (1986).

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