What's Wrong with "Atom Laser"

There's a news piece in Physics World this week titled "Atom laser makes its first measurement" and you might think this would be right up my alley. Mostly, though, it serves to remind me that the term "atom laser" has always kind of pissed me off.

This is somewhat ironic, as it's a beautiful piece of "framing," the sort of thing I've spoken in favor of numerous times here. I have a principled technical objection to the term, though, in that I think the analogy it draws is deliberately misleading.

I should stress that there's really nothing wrong with the analogy on the face of it. The basic idea is that there is a parallel to be drawn between a Bose-Einstein condensate (BEC) and a laser-- a BEC is a macroscopic number of atoms occupying a single quantum state (at least, that's the high school physics explanation), while a laser can be thought of as a macroscopic number of photons occupying a single quantum state (in this case, a single mode of the electromagnetic field, with a particular frequency, direction, and polarization). The process by which atoms in a vapor form a BEC is directly analogous to the stimulated emission process by which the photons end up in a single laser mode, and you can draw valid analogies between various laser components and the elements of a BEC experiment (with the important caveat that all "atom lasers" to date have been Q-switched lasers).

The problem has to do with the way this analogy is deployed, and the meaning of the word "macroscopic."

"Macroscopic" in the context of atoms in a BEC means "a few million atoms." "Macroscopic" in the context of laser physics is vastly bigger-- a cheap laser pointer will put out something like 1015 photons per second. That's nine orders of magnitude difference, assuming you can make a BEC every few seconds.

This is the source of the problem with "atom laser," for me. The analogy is most often used in press materials and the like, where it's intended to evoke the vast range of technologies that depend on optical lasers. This makes people sit up and say "Wow! Lasers are good for lots of things, so atom lasers must be great stuff."

The problem is, most of the technologies that make ordinary lasers useful rely on the laser as nothing more than a really bright source of light. The laser UPC scanner in the grocery store doesn't depend on the coherence properties of the light in any significant way-- you could do the same thing with lamps, lenses, and color filters. It's just using the laser as a convenient way to get a bright, narrow beam of light.

This is exactly the point where the "atom laser" analogy is weakest, though. An optical laser that only produced a million photons per second would be pretty much useless as a technology. The number of atoms in a typical BEC just isn't large enough to be the basis for lithography techniques, or anything like that. You can get vastly higher throughput from a collimated atomic beam apparatus (the equivalent of using lamps, lenses, and filters for light) than you can from a BEC, and that's still not nearly enough for, say, lithographic chip manufacture.

So, as a way of publicizing BEC research, the term "atom laser" has always seemed a little more deceptive to me than the usual lies-to-children explanations we use in press releases and the like. There's no denying that it's effective, but the analogy breaks down exactly at the point where you would want it to be strongest.

Ironically, this is exactly the sort of narrow, technical objection that leads lots of people to recoil from the notion of "framing" scientific results (those who are arguing in good faith, anyway). I tend to think that this is a more pernicious error than the usual "framing"-induced mistakes, but then, I'm sure most of the people complaining about "framing" feel the same way about whatever they're complaining about.

Then again, most of the actual experiments that have been done with BEC's don't use the coherence properties of the condensate any more than a supermarket scanner uses the coherence properties of optical lasers-- they're just using it as a convenient source of really cold atoms. So, maybe it's not as bad an analogy as I've been thinking...

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Also the whole "first measurement" thing is kinda totally off too. A) One could make the argument that any BEC is an atom laser, regardless of the trap. Isn't a photon laser in a cavity still a laser? B) I seem to recall an experiment based on your old apparatus that measured g using out-coupled condensate atoms.

My problem is the semantic quibble that laser stand for light amplification by stimulated emission of radiation, none of which describe a BEC?

Of course, every time I say what laser means, one movie quote always pops into my head:

Mitch: ". . . light amplification through stimulated emission of radiation, it's coherent light."
Mitch's dad: "Oh, so it talks?"

I think you're right, Chad. This quibble is exactly the kind of quibble other people have when they object to framing. There will always be such quibbles when something is framed in an accessible way without all the gory details in the title.

Have you ever noticed that people believe nearly everything they read in the newspaper, except that when they have any personal knowledge of the events described, they're always pointing out what the article got wrong? I think you're supporting framing of other people's work and not of your own.

Jeff: Also the whole "first measurement" thing is kinda totally off too. A) One could make the argument that any BEC is an atom laser, regardless of the trap. Isn't a photon laser in a cavity still a laser?

Well, it probably technically is a laser, but you wouldn't be able to do much of anything with it...

B) I seem to recall an experiment based on your old apparatus that measured g using out-coupled condensate atoms.

Yeah, I thought about mentioning that. Of course, that measurement managed about the same precision that you can get with a stopwatch, so I didn't think it was worth making much fuss.

My silly thought on this is to wonder if atom lasers retroactively fix one of the many absurd lines in The Rocky Horror Picture Show

"Exactly, Dr. Scott. A laser capable of emitting a beam of pure anti-matter."

You physicists sure are picky! I noticed you didn't suggest a better explanation for laymen.

Keep in mind that most people don't even know what bosons are. The laser analogy seems like the obvious one to me. That's the way I explained Bose-Einstein condensates to my daughter when she was in 5th grade, and she said "Let's do it for the science fair!"

By Riesz Fischer (not verified) on 09 May 2008 #permalink

They said the laser was a solution in search of a problem.

Right now "atom lasers" are just in search of.....something.

As a "what lie is least objectionable", the atom laser thing is not so bad.

Actually for all the great work in BEC, most of it involves just using it as a cold dense ball of atoms (like for slow light) or to measure things that are in Stat Mech books as theory. Now the interface between condensed matter and lattices is really neat. Its all neat, I think the coolest was the Phillips group doing the four wave mixing.

perry: As a "what lie is least objectionable", the atom laser thing is not so bad.

It's a perfectly good analogy, and I'm not saying they shouldn't use it to explain the physics of BEC (though it's not like the man on the street has a deep intuitive grasp of stimulated emission, either). It just bugs me a little when it's used to invoke the wonders of laser technology, because that's where I think it crosses the line from "useful analogy" to "hype."

Actually for all the great work in BEC, most of it involves just using it as a cold dense ball of atoms (like for slow light)

Don't get me started on "slow light"...

or to measure things that are in Stat Mech books as theory. Now the interface between condensed matter and lattices is really neat. Its all neat, I think the coolest was the Phillips group doing the four wave mixing.

I think the quantum phase transition stuff (Mott insulator, etc.) is pretty cool as well. There's been some neat stuff done with solitons, too.

I have to admit to a certain amount of puzzlement regarding vortices, though. I'm not quite sure why everybody gets so fired up about them.

yeah, I forgot vortices.

Slow light is just coupled oscillators. Neat but not quantum really.