Buckyball Interference, classical/quantum border?

[Q_Goest]

In '99, Markus Arndt published his paper in which single buckyballs (primarily C60 molecules) were fired at a diffraction grating and quantum interference was found. Arndt's experiment used single molecules that had a velocity of roughly 220 meters per second. The molecules traveled roughly 1 meter to the target.

Now imagine that the rate at which these molecules were fired at the diffraction grating was slowly increased such that instead of just one molecule being in the apparatus at a time, 2, then 3, then many more would actually be moving through the apparatus at any moment. For example, if the molecules were shot out at a rate of 220,000 moelcules per second, the molecules would be 1 mm apart and there would be roughly 1000 molecules transiting through the apparatus at once. One could then imaging even closer spacing between molecules, spacing so close, the molecules might even begin interacting.

At some point, I would think the molecules would be so close that they would interact with each other and/or you might even be able to see a thin stream of them. As the molecules got closer and closer together, is there a possibility that they would stop behaving as quantum particles and start acting like classical particles (ie: the diffraction pattern would disappear)?

 

[vanesch]

At some point, I would think the molecules would be so close that they would interact with each other and/or you might even be able to see a thin stream of them. As the molecules got closer and closer together, is there a possibility that they would stop behaving as quantum particles and start acting like classical particles (ie: the diffraction pattern would disappear)?

Why would you think so ? After all, you can have light show interference without having it to be "counted as photons" but where you can "see a thin stream of light".

Of course, it might be that the specific interaction of the buckyballs makes them decohere, or makes the interference not work in the same way as without it, but I wouldn't consider that as some kind of indication of a "quantum-classical" transition. Just that the experimental setup which was conceived for individual buckyballs is now perturbed by an extra interaction that wasn't initially taken into account (and was negligible).

 

[Q_Goest]

Hi vanesch,
I'm assuming, perhaps incorrectly, that when these molecules interact, either with other molecules or something in the environment, the interference pattern will disappear. Take for example, an expanding cloud or thick stream of molecules. I wouldn't expect gasses such as air for example, to produce an interference pattern. I'd expect air molecules to act as classical particles if they are directed at a diffraction grating. That is, if an air hose is used to direct air at a diffraction grating, one would detect two, independent jets as opposed to an interference pattern because they are constantly bumping into one another as they traverse the distance from the air hose, through the diffraction grating, to some point of detection.

Similarly, I'd expect C60 molecules would do the same thing as we increase the density of the stream. We could increase that stream density by only increasing the rate as if the molecules were in a single file, or we could increase also the breadth of the stream so it's more like shooting molecules from an air hose.

Thanks for any help in trying to understand this... Luckily I'll never be taking a final exam on any of this - it's too late for me!

 

[vanesch]

I'd expect air molecules to act as classical particles if they are directed at a diffraction grating. That is, if an air hose is used to direct air at a diffraction grating, one would detect two, independent jets as opposed to an interference pattern because they are constantly bumping into one another as they traverse the distance from the air hose, through the diffraction grating, to some point of detection.

Actually, if the air molecules are still bumping into one another before, "during" and a bit after they pass a grating, then I'd expect them not even to show up as two jets! If their interaction is so strong as to be able to change significantly their "trajectory", there's no point in expecting two separated jets afterwards either, but only some bump.

It might of course be that there is a level of interaction which can blur already the expected single-particle interference pattern, without completely blurring the two-jet picture.

 

[Q_Goest]

It might of course be that there is a level of interaction which can blur already the expected single-particle interference pattern, without completely blurring the two-jet picture.

Thanks vanesch. Yes, that's really what I'm wondering. It almost seems like an easy experiment to do, just increase the number of molecules being sent through the experimental apparatus till we see the interference pattern begin to disappear.

But I'd assume there is some ability to predict under what conditions that 'blur' should occur. Maybe not so easy?