One could inject perhaps a dozen or so photons into a cavity and then launch through it, one by one, Rydberg atoms whose velocity is fixed at about a meter per second. The kinetic energy of these atoms would be greater than the atom-cavity potential energy, and they would pass through the cavity after experiencing a slight positive or negative delay, depending on the sign of the atom-cavity detuning. To detect the atom's position after it has passed through the cavity, researchers could fire an array of field ionization detectors simultaneously some time after the launch of each atom. A spatial resolution of a few microns should be good enough to count the number of photons in the cavity.
Before measurement, of course, the photon number is not merely a classically unknown quantity. It also usually contains an inherent quantum uncertainty. The cavity generally contains a field whose description is a quantum wave function assigning a complex amplitude to each possible number of photons. The probability that the cavity stores a given number of photons is the squared modulus of the corresponding complex amplitude.
The laws of quantum mechanics say that the firing of the detector that registers an atom's position after it has crossed the cavity collapses the ambiguous photon-number wave function to a single value. Any subsequent atom used to measure this number will register the same value. If the experiment is repeated from scratch many times, with the same initial field in the cavity, the statistical distribution of photons will be revealed by the ensemble of individual measurements. In any given run, however, the photon number will remain constant, once pinned down.
This method for measuring the number of photons in the cavity realizes the remarkable feat of observation known as quantum nondemolition. Not only does the technique determine perfectly the number of photons in the cavity, but it also leaves that number unchanged for further readings.
Although this characteristic seems to be merely what one would ask of any measurement, it is impossible to attain by conventional means. The ordinary way to measure this field is to couple the cavity to some kind of photodetector, transforming the photons into electrons and counting them. The absorption of photons is also a quantum event, ruled by chance; thus, the detector adds its own noise to the measured intensity. Furthermore, each measurement requires absorbing photons; thus, the field irreversibly loses energy. Repeating such a procedure therefore results in a different, lower reading each time. In the nondemolition experiment, in contrast, the slightly nonresonant atoms interact with the cavity field without permanently exchanging energy.
Quantum optics groups around the world have discussed various versions of quantum non-demolition experiments for several years, and recently they have begun reducing theory to practice. Direct measurement of an atom's delay is conceptually simple but not very sensitive. More promising variants are based on interference effects involving atoms passing through the cavity--like photons, atoms can behave like waves. They can even interfere with themselves. The so-called de Broglie wavelength of an atom is inversely proportional to velocity; a rubidium atom traveling 100 meters per second, for example, has a wavelength of 0.45 angstrom.
If an atom is slowed while traversing the cavity, its phase will be shifted by an angle proportional to the delay. A delay that holds an atom back by a mere 0.22 angstrom, or one half of a de Broglie wavelength, will replace a crest of the matter wave by a trough. This shift can readily be detected by atomic interferometry.
If one prepares the atom itself in a superposition of two states, one of which is delayed by the cavity while the other is unaffected, then the atomic wave packet itself will be split into two parts. As these two parts interfere with each other, the resulting signal yields a measurement of the phase shift of the matter wave and hence of the photon number in the cavity. Precisely this experiment is now under way at our laboratory in Paris, using Rydberg atoms that are coupled to a superconducting cavity in an apparatus known as a Ramsey interferometer.
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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.