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Chris Lee - Jan 14, 2011 12:36 pm UTC

Okay, so I was wrong, what a shocker. You see, a couple of years ago, I attended a talk that made a great deal of noise about doing quantum "stuff" with classical light—classical light is the sort of light that you get from the sun. The light emitted by the sun at any time and place bears no resemblance to light emitted at any other time and place. At the time of the talk, my main objection was that the light the researchers used in their work wasn't really the sort of light that you would get from your light bulb—instead, it was just laser light that had been played with a bit. The light still had all of its laser-like properties, hence quantumness abounded. Therefore, it wasn't very surprising to me that you could play quantum tricks with that light.

Obviously, I was not the only person to have this sort of objection, because more recent experiments have been more careful about how they turn laser light into a pseudo classical light source. In a plenary session at the Physics of Quantum Electronics (PQE) conference, Yanhua Shih discussed some of his recent theoretical and experimental work in getting classical light to behave like quantum light.

I should note that, since then, my own research has caused me to give some serious thought to what the difference between classical and quantum light is. My conclusion was that there is no difference. The distinction usually comes about because, when we measure the quantum properties of a photon of light, we do it by comparing it with a second photon that was produced at the same time and in the same way. Unfortunately, classical light sources don't let you readily do that. Since every photon is produced independently you can't gate your detector in such a way as to ensure that you are only comparing the right photons.

It turns out that some of the bigwigs of the quantum electronic community had come to the same conclusion back in.... um.... yeah... It was a while ago. Let's not go there, I need to keep some pride.

Anyway, what Shih has done is to construct the specific theory for quantum behavior of "classical" light and design experiments that allow you to unequivocally observe this behavior. In terms of the experiment, what this involves is using a light source that emits light in very short pulses, which generally means a laser. But, after some manipulation, it's possible to ensure that laser light observed at different locations has a random relationship to one another, which is the essential property of classical light. This provides photons that can be time-gated in such a way that the correlations between these photons can be measured.

To my surprise—but no one else's, apparently—it turns out that they are as correlated as pairs of entangled photon. Meaning that anything that has previously been the domain of "requires entangled photons" can, perhaps, be done with intense classical sources.

At a second talk, given by a member of his lab, Shih's group demonstrated this in an unequivocal way: ghost imaging using sunlight. Sunlight is, of course, the ultimate classical light source, so there can be no arguing that point anymore.

Ghost imaging involves creating an image from entangled photons. The idea is simply that one sends one of the photons to the object that you wish to view and the second straight to the camera. Then, by some means, you build up the image only from those photons that hit the camera at the same time as the partner photons hit the object. The point being that the image is constructed from light that has never been anywhere near the object. 

Usually, the means of choosing photons is by detecting photons scattered from the object and using those detection events to select which of the photons that hit the camera to keep and which to throw away. But, I don't think this is absolutely necessary. If you can gate the camera at the right time and with a narrow enough time window, then you would still get an image.

Now, the argument that usually get raised is that you can do ghost imaging with classical light sources, it just isn't quite as good. What Shih is telling us is that this has nothing to do with the light source. Instead our procedure for selecting appropriate photons isn't good enough and we smear out the quantum nature of light. As we improve this, classical ghost imaging will be as good as quantum ghost imaging. The reason being that light is light, and, if you look at it the right way, it is all quantum mechanical in nature.

After spending a few years in the skeptical camp, I am now convinced. And I am pretty excited, because the correlated nature of light allows you to do some pretty cool things as far as resolution enhancement goes, as well. That's me, always going on about seeing tiny things... Maybe there is a hidden message in that?

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