The no-photon interference effect arises because the fluctuations of the vacuum field, like the oscillations of more actual electromagnetic waves, are constrained by the cavity walls. In a small box, boundary conditions forbid long wavelengths--there can be no vacuum fluctuations at low frequencies. An excited atom that would ordinarily emit a low-frequency photon cannot do so, because there are no vacuum fluctuations to stimulate its emission by oscillating in phase with it.
Small cavities suppress atomic transitions; slightly larger ones, however, can enhance them. When the size of a cavity surrounding an excited atom is increased to the point where it matches the wavelength of the photon that the atom would naturally emit, vacuum-field fluctuations at that wavelength flood the cavity and become stronger than they would be in free space. This state of affairs encourages emission; the lifetime of the excited state becomes much shorter than it would naturally be. We observed this emission enhancement with Rydberg atoms at the École Normale Supérieure (ENS) in Paris in one of the first cavity QED experiments, in 1983.
If the resonant cavity has absorbing walls or allows photons to escape, the emission is not essentially different from spontaneous radiation in free space--it just proceeds much faster. If the cavity walls are very good reflectors and the cavity is closed, however, novel effects occur. These effects, which depend on intimate long-term interactions between the excited atom and the cavity, are the basis for a series of new devices that can make sensitive measurements of quantum phenomena.
Instead of simply emitting a photon and going on its way, an excited atom in such a resonant cavity oscillates back and forth between its excited and unexcited states. The emitted photon remains in the box in the vicinity of the atom and is promptly reabsorbed. The atom-cavity system oscillates between two states, one consisting of an excited atom and no photon, and the other of a de-excited atom and a photon trapped in the cavity. The frequency of this oscillation depends on the transition energy, on the size of the atomic dipole and on the size of the cavity.
This atom-photon exchange has a deep analogue in classical physics. If two identical pendulums are coupled by a weak spring and one of them is set in motion, the other will soon start swinging while the first gradually comes to rest. At this point, the first pendulum starts swinging again, commencing an ideally endless exchange of energy. A state in which one pendulum is excited and the other is at rest is clearly not stationary, because energy moves continuously from one pendulum to the other. The system does have two steady states, however: one in which the pendulums swing in phase with each other, and the other in which they swing alternatively toward and away from each other. The system's oscillation in each of these "eigenmodes" differs because of the additional force imposed by the coupling--the pendulums oscillate slightly slower in phase and slightly faster out of phase. Furthermore, the magnitude of the frequency difference between the two eigenmodes is precisely equal to the rate at which the two pendulums exchange their energy in the nonstationary states.
Researchers at the California Institute of Technology recently observed this "mode splitting" in an atom-cavity system. They transmitted a weak laser beam through a cavity made of two spherical mirrors while a beam of cesium atoms also crossed the cavity. The atomic beam was so tenuous that there was at most one atom at a time in the cavity. Although the cavity was not closed, the rate at which it exchanged photons with each atom exceeded the rate at which the atoms emitted photons that escaped the cavity; consequently, the physics was fundamentally the same as that in a closed resonator.
The spacing between the mirrors was an integral multiple of the wavelength of the transition between the first excited state of cesium and its ground state. Experimenters varied the wavelength (and hence frequency) of the laser and recorded its transmission across the cavity. When the cavity was empty, the transmission reached a sharp maximum at the resonant frequency of the cavity. When the resonator contained one atom on average, however, a symmetrical double peak appeared; its valley matched the position of the previous single peak. The frequency splitting, about six megahertz, marked the rate of energy exchange between the atom and a single photon in the cavity.
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3 Comments
Add Comment"spontaneous emission of a photon by an excited atom is in a sense induced by vacuum fluctuations."
Reply | Report Abuse | Link to this"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.
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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.
"And some of the superscripts are missing; for example, 10 to the 23 was rendered as 1023."
Reply | Report Abuse | Link to thisI 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.
"Atoms and photons in small cavities behave completely unlike those in free space."
Reply | Report Abuse | Link to this"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.