This apparatus is extremely sensitive: when the laser is tuned to the cavity's resonant frequency, the passage of a single atom lowers transmission significantly. This phenomenon can be used to count atoms in the same way one currently counts cars or people intercepting an infrared light in front of a photodetector.
Although simple in principle, such an experiment is technically demanding. The cavity must be as small as possible because the frequency splitting is proportional to the vacuum-field amplitude, which is inversely proportional to the square root of the box's volume. At the same time, the mirrors must be very good reflectors so that the photon remains trapped for at least as long as it takes the atom and cavity to exchange a photon. The group at Caltech used mirrors that were coated to achieve 99.996 percent reflectivity, separated by about a millimeter. In such a trap, a photon could bounce back and forth about 100,000 times over the course of a quarter of a microsecond before being transmitted through the mirrors.
Experimenters have been able to achieve even longer storage times--as great as several hundred milliseconds-- by means of superconducting niobium cavities cooled to temperatures of about one kelvin or less. These cavities are ideal for trapping the photons emitted by Rydberg atoms, which typically range in wavelength from a few millimeters to a few centimeters (corresponding to frequencies between 10 and 100 gigahertz). In a recent experiment in our laboratory at ENS, we excited rubidium atoms with lasers and sent them across a superconducting cylindrical cavity tuned to a transition connecting the excited state to another Rydberg level 68 gigahertz higher in energy. We observed a mode splitting of about 100 kilohertz when the cavity contained two or three atoms at the same time.
There is a striking similarity between the single atom-cavity system and a laser or a maser. Either device, which emits photons in the optical and microwave domain, respectively consists of a tuned cavity and an atomic medium that can undergo transitions whose wavelength matches the length of the cavity. When energy is supplied to the medium, the radiation field inside the cavity builds up to a point where all the excited atoms undergo stimulated emission and give out their photons in phase. A maser usually contains a very large number of atoms, collectively coupled to the radiation field in a large, resonating structure. In contrast, the cavity QED experiments operate on only a single atom at a time in a very small box. Nevertheless, the principles of operation are the same.
Indeed, in 1984 physicists at the Max Planck Institute for Quantum Optics in Garching, Germany, succeeded in operating a "micromaser" containing only one atom. To start up the micromaser, Rydberg atoms are sent one at a time through a superconducting cavity. These atoms are prepared in a state whose favored transition matches the resonant frequency of the cavity (between 20 and 70 gigahertz). In the Garching micromaser the atoms all had nearly the same velocity, so they spent the same time inside the cavity.
This apparatus is simply another realization of the atom-cavity coupled oscillator; if an atom were to remain inside the cavity indefinitely, it would exchange a photon with the cavity at some characteristic rate. Instead, depending on the atom's speed, there is some fixed chance that an atom will exit unchanged and a complementary chance that it will leave a photon behind.
If the cavity remains empty after the first atom, the next one faces an identical chance of exiting the cavity in the same state in which it entered. Eventually, however, an atom deposits a photon; then the next atom in line encounters sharply altered odds that it will emit energy. The rate at which atom and field exchange energy depends on the number of photons already present--the more photons, the faster the atom is stimulated to exchange additional energy with the field. Soon the cavity contains two photons, modifying the odds for subsequent emission even further, then three and so on at a rate that depends at each step on the number of previously deposited photons.
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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.