How big is a visible photon?

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This video is actually not about photon size but about coherence length. In this video I discuss the behavior of electromagnetic radiation, especially the aspect of interference. The experiment shows that there is no such thing as individual photons in EM radiation. The photon only exists as an energy exchange between radiation and matter.

0:00 General Intro
0:47 What do others say?
1:21 About wavelength and size
2:10 Interference in light
3:08 Electromagnetic waves and detection
5:25 Things that make you go Hmmm...
7:36 New experiment and setup
10:23 Calculation of single photon level (boring)
11:59 Result of the new experiment
12:41 Discussion of the result
16:29 About "shot noise"
17:16 EM field strength and probability of detection
19:18 So how big is it then?
20:02 Deleted scene

At 3:08 the Electric and Magnetic field components have been swapped accidentally.

If you wish to use any copyrighted material from this video for purposes of your own that go beyond 'fair use', you must obtain expressed permission from the copyright owner.17 U.S. Code § 107

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I've always kept in mind how one lecturer described it to me: _light propagates like a wave and interacts like a particle._ You can't go too far wrong with that. Glad to see the video comes to the same conclusion.

ps
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I love it! I never knew that coherent light would have additional assumptions compared to spontaneous emission in interference experiments. Maybe light really is a wave. The "duality" certainly seems less 50/50 and more like a wave with some extra features. Thanks so much for your videos.

AppliedScience
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One of the most original, well spoken, informative and amazing videos in YouTube. 1 million likes by me!

antonyskountzos
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The virtue of this video is not that it answered any questions, but that it intelligently and honestly raised some interesting ones, and showed how hard it is to do these kinds of experiments. Thanks for providing so much food for thought.

hhewer
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This is one of the best science videos on Youtube. I cannot imagine how much time, work and thinking have gone into making this amazing video. For me, this is a better way of communicating science to people than publishing in Nature or Science.
Thank you very much. Keep up the good work of educating people like me. Thanks.

mandardeodhar
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I've been searching over a year for a video like this, that actually performs the experiment and shows the results, not just an animation. Wonderful. Subscribed.

andthorn
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Wow, this is was a super concise and delightful presentation on something I've often seen ignored in modern POPSCI YouTube. You took a stab at explaining something I always imagined as black magic and I left with a greater intuition and understanding of something I don't even use day to day in my work. However it did make me think differently about EM in general, which is an amazing accomplishment for a such a short video.

rallokkcaz
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I love your silliness. It feels like I'm watching a 1940s scientist instead of a post 2000s one. There's a more human touch on this channel than on a lot of others, while still having legitimate science and engineering.

EllyCatfox
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Excellent video, this is the first time I've listened to such a clear explanation in such a short time of the seemingly paradoxical behavior of photons. The graphics and narrative are just what is needed to explain this topic in a super-efficient way. Thanks so much for sharing!

mau_lopez
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I REALLY like the calm way you present these awesome experiments! ;) A nice contrast to show and shine channels like many others...

jackmclane
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“Let’s not keep you in the dark any longer” ..very light on your puns there, really brightens my day.

linkin
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Wow. A logically consistent and scientifically consistent explanation of quantum mechanics. I've had trouble fully understanding the double slit phenomenon for years and now I suddenly think, "well duh, of course it works that way. It's obvious." You're an incredible teacher sir.

knurlgnar
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Hello!

Grad student working in quantum optics here. Great video!


The coherence length of a photon is the same coherence length as the beam. Single photon experiments can be done with kilometers of distance between the photons, given a sufficiently coherent laser. The explanation you have given is spot on.

Also, the attenuation method is not reliable to produce only single photons states. You're still in the right range, but the resulting beam will be "bunched", where you have a high probability of two-photon events. (think Bose-Einstein statistics) Either way, you're actually measuring the interference of the photons with your experiments, even if they are bunched up.

Generating single photon states is an area of ongoing research, but the main methods are 1) quantum dots, or 2) SPDC single photon heralding. Both of those require expensive equipment. The second method is totally attainable if you can get a single photon detector second hand from somewhere on the cheap. They tend to be very expensive.

Another interesting avenue to get single photons would be the SPDC part, where you could generate photon pairs from a single pump photon through interaction with a nonlinear cristal (like the ones found in green laser pointers). If you know about second harmonic generation, it's basically that process in reverse. Since you have photolithography equipment, you could DIY produce periodically poled materials that would even allow for colinear polarisation-entangled photon pairs to be generated.

acebulf
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I've watched all your videos after discovering your channel two weeks a go. I'm addicted! Your explanations are superb!

juliankandlhofer
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Fantastic presentation. I have been studying optical phenomena for many years (as an amateur) and thought I was alone in wondering how long a photon might be. Also, a great explanation as to the reduction in EM passing through filters without violating Planks law due it to being coherent & unconstrained by quantisation.

troymeister
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Wow! Awesome to see the double-split actually demonstrated like this, whereas most discussions just illustrate it with graphics. Makes it seem even more mind blowing.

duprie
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That's pretty cool man. When it comes to extrapolating certain information from results or trying to infer a mechanism, it usually ends up necessitating at least a dozen different, highly controlled experiments all with their own interpretations as well. It usually makes me feel like we are trying to see the universe through a single pinhole in a 1 square mile cloth.

Hopefully, our curiosity and individuality can be maintained after our physical death so we can prove for answers to these questions. In fact, I'd like to talk with Werner Heisenberg about some of these newer findings but I feel like he'd just say "uh, I don't know." Fitting for an uncertain bastard...

DrewishAF
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What a wonderful experiment and presentation. As a “radio person” the concept of a Photon has never been easy to swallow. I can appreciate that an emission of light is produced as a quantum event (photon) but from there the electromagnetic field created propagates without quanta limitations just as your experiment demonstrates. The same quanta effect applies to detection or attenuation as you describe so we end up detecting a photon from the wave. It is time for a resurgence in science to reconsider what a photon is and how light and radio waves propagate and I think you are on to it. Perhaps we need to set up quantum de- radicalisation classes for Photon believers. Well done.

paulcopeland
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Beautiful!
Gravitational lensing, in particular, Einstein rings, is an example where the size of the photon is greater than the size of the galaxy.
In other words, the distributed chunks of masses in the galaxy (stars, etc) gravitationally interacts with the very distant photon's wave function to make it become focused on a telescope here on Earth.
In fact, due to the low luminescence, you can argue that the ring is a bunch of single photon interference experiments!

henryD
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I would have loved to see the results of the experiment with the florescent light. The insight about the continuous nature of the laser beam was very interesting, I wonder why I don't here that talked about more when people describe these experiments.

monkeygame