Friday, March 07, 2008

The Wave-Particle Duality of Light: A Demonstration Experiment

Other than the fact that I don't quite like the title, this is an excellent demonstration paper that was published recently in AJP. Very much like the J.J. Thorn et al. paper on the which-way experiment, these profound phenomena can actually be performed in an undergraduate physics lab.

First, the exact citation:

T.L. Dimitrova and A. Weis, Am. J. Phys. v.76, p.137 (2008).

They basically performed a Mach-Zehnder interferometer experiment using very low intensity light so much so that only one photon is in the apparatus at any given time. They also have a second stronger laser beam that traverse the same apparatus, but slightly displaced that exhibit the clear wave-like interference pattern.

So far, this is fine and dandy, and it would not have caught my eye because it would be a nice, undergraduate physics lab exercise. But at they end, they did something simple, yet, can be quite profound to a student. I'll quote what they said:

The demonstration, whose result is astonishing for students, is realized in the following way. First the fringe pattern is locked to a photodiode as explained in Sec. IV B, and the photomultiplier is moved to a fringe minimum, as characterized by a low photon count rate which can also be displayed acoustically. If now path A of beam 1 is blocked inside the interferometer, it is possible to hear (and see) a distinct increase of the click rate. This result demonstrates that if we give each photon the choice of taking either path A or path B, it has a low probability to appear at the detector. In contrast, if we force the photon to follow a specific path by blocking the other path, then the probability to arrive at the detector is much higher. The puzzling fact that a two-path alternative for each photon prevents it from reaching the detector, while blocking one of the paths leads to a revival of the clicks, is most intriguing for beginning students. This experiment is well suited for illustrating this remarkable quantum mechanical effect, which can be explained only if we assume that each photon simultaneously takes both paths A and B; that is, each photon, in the phrasing of Dirac, "interferes with itself."


It is something we know would happen, but the way this is demonstrated is so clear that I would say this is an experiment worth doing at every undergraduate level. Well done to the authors!!



Peter Morgan said...

Proves that there are waves, insofar as interference fringes prove so much; proves that there are "particle detectors"; but not, for a skeptic, that there are particles per se. When did you last observe a particle without a "particle detector"?

"Wave-particle duality" falls short of the instrumental side of the history of QM by suggesting too much. These are "particles" and "waves" that don't have properties unless we force them to show themselves; what kind of particles can they be? The question of what kind of waves they can be is different, suggesting an asymmetry to the duality that we ought to honor.

I'm lately trying out "sensitive apparatus" as a less leading term for all the carefully engineered objects (photographic plates, photomultipliers, cloud chambers, bubble chambers, etc.) we use that experience different rates and patterns of thermodynamic transitions depending on very small changes of their experimental environment. "Sensitive apparatus" seems a slightly better, less prejudiced match for the "preparation apparatus" of a Margenau-style instrumental interpretation (although I have no objection to speaking of measurements of preparation apparatus instead of measurements of quantum systems). Does an experimentalist object?

ZapperZ said...

Er... huh?

If you think a bit more carefully, EVERYTHING that you "see", not just these "wave-particle", requires your observation! Furthermore, they DO have properties before they are observed! These properties are in a superposition! So where is this "they don't have properties" nonsense?

Good luck with your "trying out" phase. When you have them published in a peer-reviewed journal, give me a call. Till then, you'll understand if I don't put too much credibility in what you just wrote.


Peter said...

Oh. I guess you do object.

Wave-particles have properties, in superposition. That's a nice way of presenting QM, provided we keep to the forefront the ways in which "in superposition" is not much like the classical idea of properties, whether of waves or of particles. I quite like the way the word "superposition" immediately forces us into the mathematics of Hilbert space. Do you take "properties in superposition" to be of individual particles, of ensembles, of wave-particles, of "quantum systems", or do you have another term that you prefer?

I believe I was referring to no-go theorems such as Kochen-Specker and Bell-EPR, and also to the EPR argument itself, as reasons to think that an attribution of properties to particles in a way that is at all close to classical is problematic. Your "in superposition" of course gets you out of that. I believe we ought to take care when we appropriate language that limitations of the relationships of the old usage and the new are fairly apparent.

As to getting this kind of stuff published, it takes lots of time and thought to get it into the Physics journals. My web-page shows that I have managed to get some of my approach to QFT published in good Physics journals, even though it is completely different from any other interpretation, but I don't expect to get this new thinking published for a few years at least. It took six years of revisions to get my paper on Bell inequalities for random fields published in JPhysA.

I was genuinely interested in an experimentalist's (as I take you to be) response to a relatively hard-line instrumentalism.

Anonymous said...

Why is this experiment so astounding? If you send photons through a double slit, there will be very few photons detected at the interference minima. If you block one of the slits, there will be more photons detected there. This has been known for decades.

ZapperZ said...

There's a difference between "knowing" and actually seeing the effects. If there's no difference, why bother even having students do any experiments at all since everything they will be doing is "well known" already.

The fact that the students get to really see that having 2 paths with all the photons getting through produces less intensity than having just one path (and having only half of the photons getting through)really drives home the point that this is not what classically is expected. That's why they found this "astounding".


Edwin Monter said...

Peter Morgan is on to something here. Yes, this experiment proves “there are waves.”

And yes, a particle is an inference based upon energy received by a “particle detector.”

But why infer that the energy the received is from a “particle?” Two reasons/assumptions seem obvious here.

First, if you receive energy at a space point it must be due to a particle (projectile) impact. Second, the only way to transmit energy from point A to point B is via a particle-projectile.

Obviously the second assumption is a lazy extrapolation of common experience and can be dismissed as lacking universal validity. The first assumption, that you have “impact” and it must be from a particle is also suspect.

Why equate the collapse of the wave front with impact of a projectile? Should we not then equate the origin of the wave front with the launching of a projectile? And if so, what creates this projectile, an electron slipping from one orbit to another? And if you have a projectile to start and projectile at the end, why is there wave behavior in between?

The origin of the photon wave front is emission. The demise of this photon wave is absorption as the reverse of emission. Why conflate emission/absorption with particle creation/impact? Others have also attacked this assumption, for example.

This “demonstration experiment” indicates that the photon-as-wave takes both paths; the weakest part of the argument is to assume that photon termination equals particle impact.

ZapperZ said...

That makes even LESS sense, because you are ignoring the fact that photon-antibunching experiment, for example, cannot be explained at all with wave picture, regardless of what you consider as the detector.

Waves do not produce which-way effects. Period. This is independent of the detection mechanism.


Carl said...

Reading Edwin Monter's post I don't see where he denies that photons are quantized in terms of their energy. It's not clear why one needs the particle concept to explain photon antibunching from resonance fluorescence.
As for which-way effects, more information needed beside an ex cathedra pronouncement.
You might also consider the following:

"The photoelectric effect was the most famous effect to demonstrate that light can have particle character,” said Mathias Richter of the Physikalisch-Technische Bundesansalt in Berlin, and lead author of the study published Monday in Physical Review Letters. “Now we come and say, even the photoelectric effect is better described in the wave picture of light if you apply these high intensities.”

ZapperZ said...

All you need to do is point to me the exact formulation not using the photon picture that can describe those antibunching phenomenon. Period. Otherwise, all you're doing is making empty claims.

The same can be said about the photoemission phenomenon. This phenomenon has gone BEYOND the naive photoelectric effect. People kept claiming that the classical stochastic picture can reproduce the photoelectric effect, but completely ignored the fact that there are no such formulation done for the more detailed form of this phenomenon. I have see ZERO formulation of the angle-resolved photoemission phenomenon, the resonant photoemission phenomenon, the multiphoton phenomenon, etc.. Continuously clinging on to the photoelectric effect is similar to claiming that you can model a cow as a sphere when looked at it from very far, so the cow must be a sphere! This is silly.

Pay attention to the details of the phenomenon. Unless there are exact formulation to describe all of those phenomenon beyond just the naive version, then it is totally illogical to proclaim that something is valid.