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Chris Lee-January 14, 2011 at 12:36 PM UTC

Okay, so I was wrong, which is really shocking. You see, a few years ago, I participated in a lecture on using classical light to make quantum "things"-classical light is the kind of light you get from the sun. The light emitted by the sun at any time and place has no resemblance to the light emitted by any other time and place. During the conversation, my main objection was that the light researchers use in their work is not the kind of light you get from a bulb—on the contrary, it is just laser light that has been irradiated with a laser. A small amount. Light still has all its laser-like properties, so quantum properties abound. Therefore, you can play quantum tricks with this kind of light, which is not surprising to me.

Obviously, I am not the only one who has such objections, because recent experiments have been more cautious about turning the laser into a pseudo-classical light source. In the plenary session of the Quantum Electronic Physics (PQE) conference, Yanhua Shih discussed some of his recent theoretical and experimental work on making classical light behave like quantum light.

I should note that since then, my own research has made me think carefully about the difference between classical light and quantum light. My conclusion is that there is no difference. This difference is usually because when we measure the quantum properties of a photon, we do it by comparing it with a second photon that is produced in the same way at the same time. Unfortunately, classic light sources will not let you do this easily. Since each photon is generated independently, you cannot gate the detector to ensure that you only compare the correct photons.

Facts have proved that some bigwigs in the quantum electronics world have reached the same conclusion before...well...yes...that was a while ago. Let's not go there, I need to maintain some pride.

In any case, what Shih does is to construct specific theories and design experiments for the quantum behavior of "classical" light so that you can clearly observe this behavior. In terms of experimentation, this involves using a light source that emits light in very short pulses, which usually means a laser. However, after some processing, it can be ensured that the lasers observed at different positions have a random relationship with each other, which is the basic property of classical light. This provides photons that can be time gated so that the correlation between these photons can be measured.

To my surprise—but obviously no one else—they turned out to be as related as pairs of entangled photons. This means that anything that previously belonged to the field of "need to entangle photons" may be done with a strong classical source.

In the second lecture of his lab members, Shih's team demonstrated this in a clear way: using sunlight for ghost imaging. Of course, sunlight is the ultimate classic light source, so this point is no longer controversial.

Ghost imaging involves creating images from entangled photons. The idea is simple, send one photon to the object you wish to view, and then send the second photon directly to the camera. Then, in some way, you build an image from only those photons that hit the camera while the partner photons hit the object. The key is that the image is made of light that has never been close to the object. 

Generally, the method of selecting photons is to detect the photons scattered from the object and use these detection events to select which photons hit the camera to keep and which to throw away. However, I think this is not absolutely necessary. If you can gate the camera at the right time with a narrow enough time window, you can still get the image.

Now, the usual argument is that you can use classic light sources for ghost imaging, but the effect is not very good. What Shih tells us is that it has nothing to do with the light source. On the contrary, our procedure for choosing the right photon is not good enough, and we erase the quantum nature of light. As we improve this, classical ghost imaging will be as good as quantum ghost imaging. The reason is that light is light, and if you look at it in the right way, it is essentially quantum mechanical.

After several years in the skeptical camp, I am now convinced. I'm very excited because the correlation of light also allows you to do some cool things, as far as resolution enhancement is concerned. That's me, always seeing tiny things...maybe there is hidden information in it?

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