Treating Big Molecules Like Electrons: "Real-time single-molecule imaging of quantum interference"

Richard Feyman famously once said that the double-slit experiment done with electrons contains everything that's "'at the heart of quantum physics." It shows both particle and wave character very clearly: the individual electrons are detected one at a time, like particles, but the result of a huge number of detections clearly traces out an interference pattern, which is unambiguously a wave phenomenon. The experiment has been done lots of times, but a particularly nice realization of it comes from Hitachi's R&D department, where you can see both still images and video of their experiment, with arriving electrons making little dots on a fluorescent screen.

Of course, an interesting question in all this is just how big you can make some material object and still see interference. I've research-blogged before about experiments in Austria using big floppy organic molecules with up to 430 atoms. Those didn't have the "arrive one at a time" feature of the best electron interference experiment, though. But now, the same group in Austria has a new paper in Nature Nanotechnology doing just that. The paper is at least temporarily free to access, making it a good target for a new ResearchBlogging post.

OK, so what's interesting about this again? If people already did interference with large molecules, and already did interference with electrons, how is this in a Nature journal? Well, the previous interference experiments using molecules used slow detectors that had to collect signal for a long time. They couldn't see an individual molecule showing up at a particular spot at a particular time. This leaves open the possibility that what they see is some collective effect of lots of molecules going through their apparatus at the same time. What they did here detects the individual molecules, one at a time, closing that notional loophole, and making this experiment essentially equivalent to the double-slit experiments with electrons. You can see the buildup of the pattern in this image:

(Figure 3 in the paper, also used in their press release)

Each individual dot in the figure represents a single molecule detected at the end of their apparatus. Images a)-d) show the slow buildup of the pattern from many individual detections, and part e) is the end result. You can compare this to the corresponding figure from Hitachi's electron experiment:

I dunno, dude. The electron picture is a whole lot better. It's cleaner, sure, but then people have been using electron beams and fluorescent screens for the better part of a century. The single-molecule detection technology is a new thing, and a little noise in the image is only to be expected.

So, how do they detect single molecules, anyway? Some kind of super fluorescent screen that lights up when molecules hit it? No, because the molecules don't have the energy needed to do that. What they do instead is much more clever: they let the molecules hit a thin glass window, where they stick, at least temporarily.

So, these images are smudges on glass? No, because there aren't enough molecules to make much of a mark. Instead, they shine a laser onto the glass that's tuned to a frequency the molecules will absorb. When a molecule hits a spot on the glass and sticks, it absorbs and re-emits light from the laser over and over, and shows up as a bright spot on a microscope image of the glass window.

So you could look through the microscope and watch this happen? If you were very, very patient, maybe. Probably not, though, because they do some optical filtering of the light, and end up detecting it with a CCD camera capable of picking up a single photon. So your eye isn't likely to register an image that's quite this sharp.

This is also a very slow experiment-- in the figure above, image a) is a blank field (no molecules sent onto the glass yet), and image e) represents 90 minutes of data collection. So, you know, you'd be squinting through that microscope for a long time.

So, what are these molecules you're being all vague about, anyway? They did this with two different molecules: : phthalocyanine PcH₂ (C₃₂H₁₈N₈), and a fluorinated version of the same thing, F24PcH2 (C₄₈H₂₆F₂₄N₈O₈) that roughly doubles the number of atoms involved and the mass of the molecules. These aren't as big as the molecules used in the experiment mentioned back at the start of this post (contrary to some claims being made about the experiment), but, again, the point is that they detect these one at a time.

If you really care, I could reproduce the chemical diagrams showing the structure, but the important point is that they're organic molecules with 58 or 114 atoms in them.

Don't bother with the diagrams. All those little hexagons make my eyes water. Anyway, this is pretty awesome. Are you going to be adding this to the undergrad demo roster? Um, no. The experiment is tremendously complicated, and pushing the limits of a few different technologies. They need to work really hard to make this work at all, because even subtle effects can mess the whole thing up and obscure the interference pattern. For example, they had to make their gratings for this out of silicon nitride membranes only 10nm thick, which is about 100 atoms (give or take) because otherwise interactions between the molecules in their beam and the atoms in the walls smeared the pattern out. (The gratings were made by a nanotech research group in Tel Aviv.)

I guess that's why it's in Nature Nanotechnology, then? It's probably part of the reason, yes. There's also the fact that this is demonstrating the quantum nature of single molecules, and that's something anybody who's really serious about molecular-level nanotech needs to be thinking about.

The point is that this, like everything out of the Austrian quantum optics community, is not only a beautifully clear demonstration of the quantum nature of the universe, but also a technical tour de force. These guys are awesome.

Of course, there's still one problem, here... What's that?

You started out talking about a double-slit experiment. But you used the word "grating" above, suggesting there are lots of slits, not just one. Ture enough. The experiment uses an array of lots and lots of little slits, not just a single pair. This is because it's hard enough to get molecules through at all that they need the extra openings to make sure they get enough molecules to finish the experiment within the normal lifetime of a graduate student. And while the double-slit is conceptually simpler (which is why everyone uses it to talk about quantum phenomena), mathematically, there's not much difference. If it works for a grating, it'll work for a double slit. Just, you know, more slowly.

Still, it's something for them to work on, no? I suspect they'll probably go for trying to do even bigger molecules instead, but on the off chance that they read this, I'm sure they'll make a note that a single pair of slits would be nice to see.

Juffmann, T., Milic, A., Müllneritsch, M., Asenbaum, P., Tsukernik, A., Tüxen, J., Mayor, M., Cheshnovsky, O., & Arndt, M. (2012). Real-time single-molecule imaging of quantum interference Nature Nanotechnology DOI: 10.1038/nnano.2012.34