Editor’s note (10/9/2012): We are making the text of this article freely available for 30 days because author Serge Haroche is one of the winners of the 2012 Nobel Prize in Physics. The full article with images, which appeared in the April 1993 issue, is available for purchase here.

Fleeting, spontaneous transitions are ubiquitous in the quantum world. Once they are under way, they seem as uncontrollable and as irreversible as the explosion of fireworks. Excited atoms, for example, discharge their excess energy in the form of photons that escape to infinity at the speed of light. Yet during the past decade, this inevitability has begun to yield. Atomic physicists have created devices that can slow spontaneous transitions, halt them, accelerate them or even reverse them entirely.

Recent advances in the fabrication of small superconducting cavities and other microscopic structures as well as novel techniques for laser manipulation of atoms make such feats possible. By placing an atom in a small box with reflecting walls that constrain the wavelength of any photons it emits or absorbs—and thus the changes in state that it may undergo—investigators can cause single atoms to emit photons ahead of schedule, stay in an excited state indefinitely or block the passage of a laser beam. With further refinement of this technology, cavity quantum electrodynamic (QED) phenomena may find use in the generation and precise measurement of electromagnetic fields consisting of only a handful of photons. Cavity QED processes engender an intimate correlation between the states of the atom and those of the field, and so their study provides new insights into quantum aspects of the interaction between light and matter.

To understand the interaction between an excited atom and a cavity, one must keep in mind two kinds of physics: the classical and the quantum. The emission of light by an atom bridges both worlds. Light waves are moving oscillations of electric and magnetic fields. In this respect, they represent a classical event. But light can also be described in terms of photons, discretely emitted quanta of energy. Sometimes the classical model is best, and sometimes the quantum one offers more understanding.

When an electron in an atom jumps from a high energy level to a lower one, the atom emits a photon that carries away the difference in energy between the two levels. This photon typically has a wavelength of a micron or less, corresponding to a frequency of a few hundred terahertz and an energy of about one electron volt. Any given excited state has a natural lifetime—similar to the half-life of a radioactive element—that determines the odds that the excited atom will emit a photon during a given time interval. The probability that an atom will remain excited decreases along an exponential curve: to one half after one tick of the internal clock, one quarter after two ticks, one eighth after three and so on.

In classical terms, the outermost electron in an excited atom is the equivalent of a small antenna, oscillating at frequencies corresponding to the energy of transitions to less excited states, and the photon is simply the antenna's radiated field. When an atom absorbs light and jumps to a higher energy level, it acts as a receiving antenna instead.

If the antenna is inside a reflecting cavity, however, its behavior changes—as anyone knows who has tried to listen to a radio broadcast while driving through a tunnel. As the car and its receiving antenna pass underground, they enter a region where the long wavelengths of the radio waves are cut off. The incident waves interfere destructively with those that bounce off the steel-reinforced concrete walls of the tunnel. In fact, the radio waves cannot propagate unless the tunnel walls are separated by more than half a wavelength. This is the minimal width that permits a standing wave with at least one crest, or field maximum, to build up—just as the vibration of a violin string reaches a maximum at the middle of the string and vanishes at the ends. What is true for reception also holds for emission: a confined antenna cannot broadcast at long wavelengths.

3 Comments

1. 1. billsmith 11:03 PM 10/9/12

   "spontaneous emission of a photon by an excited atom is in a sense induced by vacuum fluctuations."
   
   "Induced"? I knew about vacuum fluctuations as an explanation for the Casmir effect, but I'd not heard this theory. While the beginning of this article is written in a tone of "old news" (which is always the best kind of science to deeply reflect on), I wonder if this theory is still new enough to be controversial.
   
   I'm especially curious about the "in a sense" part. Can someone here explain that to me? Such complications are frequently omitted and/or horribly abused by pseudo-scientific types who fail to realize that quantum physicists (when deprived of their wave functions) often resort to loose metaphors. I'd like to know where this metaphor breaks down.
   
   
   ---
   I'm impressed by the technical feat of a grad student putting a beryllium ion 80 nanometers away from itself (on the order of a thousand times its stationary diameter and ten times the wave packet's width).
   http://www.nist.gov/pml/div688/grp10/upload/bkthesis.pdf
   
   And likewise impressed by the measurement of decoherence over time.
   http://phd.fisica.unimi.it/assets/docs/PC_and_Seminars/0910/SlideHaroche.pdf
   
   Perhaps those quantum optics x-ray lasers could one day be used for fusion ignition?
   http://www.nature.com/nature/journal/v481/n7382/full/nature10721.html
   
   By the way, why weren't Rydberg atoms defined until halfway through the article? If I'd not already known, I would have been a bit confused. And some of the superscripts are missing; for example, 10 to the 23 was rendered as 1023.

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2. 2. europamoon100 11:39 AM 10/13/12

   "And some of the superscripts are missing; for example, 10 to the 23 was rendered as 1023."
   
   I agree: Presenting numbers without their proper superscripts and subscripts is dumb, and it should never be allowed to happen under any circumstances. Yes, it was allowed to happen out of human carelessness, and nobody or nothing else can be blamed.
   If there is any difficulty in typesetting and presentation, then the people of Scientific American should use 10^23, which is generally understood for exponentiation.
   There is also "ten to the 23rd power" - just use Plain English. Never allow it to be presented as a four-digit number.
   
   

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3. 3. europamoon100 11:54 AM 10/13/12

   "Atoms and photons in small cavities behave completely unlike those in free space."
   
   "completely unlike" ?? Very questionable.
   If even one similiarity can be found, then that is a false statement. I am sure that some similarities can be found.
   For a quick example of one, the rest mass of a photon is zero, no matter what.
   As for the atoms, their electrons continue to "orbit" the nuclei of the atoms and the energy levels of those electrons continue to take on discrete levels according to the rules of quantum mechanics.
   The nuclei of the atoms continue to behave completely the same, whether the atoms are in cavities, in free space, or in condensed matter.
   
   You remind me of the TV commercials that advertize the products as "perfect" or "ideal". Well, no - No commercial product is perfect or ideal. We don't even have ideal gases here, nor ideal simple machines such as levers.

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