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# Observing single photons (may be future Instructable) Answered

In a thread on building a DIY Geiger counter, I mentioned that it's possible to do an experiment to see single photons directly with your eyes. This is a lab we did when I was an undergraduate, more than 20 years ago. I haven't done the setup myself since then, so I'm just going to describe it; if I have the opportunity run it again, then I'll make an I'ble.

If someone else decides to tackle it, please feel free to write it up yourself!

The human eye detects light via a family of proteins called opsins. Different forms of photopsins are sensitive to different wavebands, which is what gives us color vision. Rhodopsin is sensitive mainly to greenish-blue light, and provides us with monochromatic night vision. Rhodopsin works by changing its conformation when it absorbs a photon; that change of conformation allows ions to flow through the rod cell's membrane and generate a signal. The signal from each rod cell is processed through the retina and passed to the visual cortex, where a representation of the visual field is constructed.

Human rhodopsin has a quantum efficiency of about 25% (there's a 25% chance a single photon will be absorbed and produce the rod-cell signal). By comparison, cat rhodopsin is more than 90% quantum efficient. 25% QE is sufficiently high to be observable -- a source of single photons can be seen by a dark-adpated person with normal vision.

You'll need a steady source of well-collimated photons. A green laser pointer (~532 nm) will do nicely. But how many photons does it generate? A wavelength of 532 nm corresponds to 3.53×10-19 joules. So a small 1mW laser pointer puts out 2.8×1015 photons per second (watt = joule/s).

How do you reduce that to one photon at a time? With filters. An ND3 neutral density filter reduces the output light by 10-3 compared to the input, so a stack of just five ND3's in front of this laster pointer would result in (on average) just 2.8 photons per second! A stack of four ND4's would give you 0.28 photons/s on average.

If you don't have neutral density filters, you can make a decent approximation, by stacking sheets of black trash-bag plastic. To make this work you have to measure the attenuation yourself, so you'll need a photodetector, something which gives an output (voltage, resistance, current, whatever) proportional to the intensity of light.

Once you have your single-photon source, you need to set it up in a completely dark room. If you have access to an old-style photographic darkroom, use it. Otherwise, use thick (3-5 mm) black felt and gaffer's tape to seal any windows and doorframes. Put the laser on a table or stand pointed at your face, with the stack of NDs (or trash bags) in front of it. If you're doing this by yourself, you may want to have a piece of tape set up to hold the pointer's button down. Otherwise, your lab partner will take care of it.

Sit in the dark for 20 to 30 minutes. This will seem like forever, so you may want something to help you keep track of the time. A standard CD will be about half finished, or you can get through ten pop sons on your iPod, when your eyes become dark adapted.

Turn on the laser. You'll see intermittent flashes all coming from nearly the same place in your visual field; if you turn your head, the location will move in the opposite direction. If you've used filters to get down to a few flashes per second, POV will make them easier to see. At less than one photon per second, you'll see them individually.

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## Discussions

And I just recently acquired a laser pointer....alas, it is not a green one. Can I still get this to work?

Definitely! Your eyes can still see all colors in the dark (consider the use of red filters for darkrooms, night-time flashlights, etc.). The green/blue range just gives you roughtly three times as many retinal cells to hit than the red.

I do have a filter that is very dark, but is it an infrared filter...that would filter out all BUT the infrared....something I don't want (although I might be able to see it on my digital camera ? )

Hmmm...that might work; the laser pointer is not putting out any IR, after all :-) If you have a spec sheet for the filter, see if it gives you any information on the residual transmittance in the visible. If it's below one per mil (0.1%), then you can use it just like the ND3 filter I mentioned above (you'll need four or five of them, of course :-)

I am still thinking about this.....but I am more focused of late on my trip to NYC  (HOPE conference)....I finally got my Hotel reservations in  *big grin*

I don't have any information on the filter (I got it from American Science and Supplus; and they are not very good at having spec sheets :-) I don't remember if I have more than one of them or not......I will have to find where I put it / them last :-) It is a pretty thick filter if I remember correctly. Some army surplus item...

how can one determine the physical size of a photon?
and If photons move through space in a helical manner, what causes them to do this?
I am really stumped. any thoughts?

Following up on your first question (again), what do you mean by "physical size" of a photon?

-- Wavelength?  That's pretty obvious.

-- Spatial extent?  That depends on the time duration (equivalent to frequency spread) of the pulse.  If you're interested, I can go into detail.

-- Range of influence?  Since photons are massless, their range is infinite.  Not a particularly interesting answer :-)

What do you mean by "physical size"?

Why do you think photons follow helical trajectories?  Could you give me a reference to where you got this idea?

Neither of these questions make any sense to me, so I can't give you any guidance toward an answer.  If you can help me get a clearer understanding of what you mean, maybe I can point you toward useful resources.

Last year, I also put together an Instructable on how to make a double-slit interference setup.  It is also woefully lacking in pictures, but I've gone ahead and published it anyway.

I did turn this into an Instructable, and finally published it today.  It'll be a while before I can actually build and document it with pictures, so the I'ble is not great quality.

If anybody out there does do the experiment, please take some pictures.  I would be most grateful to add you as a collaborator and include (with credit) pictures or and comments.

<> listen to a Ipod, the display turned on and there went the night vision.. but after a while you can see all kinds of things while waiting, like the CFL lights phosphor coating glowing faintly ( the daylight type from philips seem to glow the longest ) and the small static sparks from walking around and touching grounded objects become very visible, especially when you touch the CFL's...

I have a reallly fuzzy blanket on my bed, and the other night I discovered that I can generate enough static electricity (just tossing and turning, I guess) to make a very visible spark-like an inch or two long-that HURTS.

Quite a shock (ha-no pun intended) to wake up to...

When I was younger, I remember one night the top fitted sheet came of the bunk bed and we (my brother and I) saw sparks through the sheets ! There was, unbeknownst to us, a plastic liner under there creating quite the static storm :-)

Wow! :D It must have really freaked you guys out. I know it did me.

The first time I noticed that (I was like 8), I yelled for my mom. I was like "Mom, do you see that?...Yes?... Good, At least I'm not losing my mind!..Now, What is it??!!"

:-) even at that young an age I was curious and always looking for answers, and I came up with the idea it must be static....but my younger brother was a bit scared about it.....wanted a night light on from that time onwards.

Yes it did. We must have spent half an hour running our hands over the new sheets and watching the light show, before I figured out is must be static electricity. :-) Gave a whole new meaning to "electric blanket" LOL

wow actually in conditions like that i sometimes saw phosphorescence of things that dont normally - like white plastics etc can a LED be dimmed with resistors to the level of single photons ? is video tape or the black thing from floppy disks good replacement for the filters ?

You ask, "can a LED be dimmed with resistors to the level of single photons?" Theoretically, I guess it could, but not in practice. The number of photons it's putting out is of the same order as I computed for a 1 mW laser diode -- something like 1015 per second. You'd need to control the voltage (i.e., the resistance) to that precision. You just aren't going to do that with 10% resistors :-)

Black mylar would probably work fine. Just as with LDPE (trash bag material), you'll need some kind of photodetector in order to measure the attenuation, and then calculate how many layers you need to get down to the right level.

As with any physics experiment, you should expect to do some math!

Theoretically, I guess it could, but not in practice

I don't think it is practical, but under the conditions i.e. temperatures, that produce quantum effects, one could. But that is a bit beyond most home hobbiests :-)

you can connect higher and higher resistors to a LED and itll get dimmer and dimmer. it looks like there is no high enough resistor to shut it off completely so if resistor X makes singlephotons then 2X will make single photons too ! (just less frequent). this is way more forgiving than 10 %

Yeah, but do the math and ask about practicality. Let's keep the same round numbers I derived earlier, and say that in normal operation an LED puts out 1015 photons/sec. You'd need a billion 1 Mohm resistors to drop that to 1 photon/sec. I think you can see how that might be a bit difficult to build :-)

AC + capacitance between 2 wires in the air as a resistance ?

With AC you are cutting the duty cycle to %50, but is that overly helpful?

i can PWM a led with very high frequency and very short duty cycle maybe i can put a resistor that allows say 1000s photons / sec (is this easier ?) if i make the duty cycle 1/1000s and frequency as high as possible then i can hope that in average the amount of photons per second (lots of cycles) is like the frequency then i can shrikmthe duty cycle more (by 1/f) and make in average 1 photo / sec ? is this correct ?

I am not sure of the "reaction" time of an LED. How long does it take from the moment of the application of power, to the point when the device actually activates?

can be determined using photodiode and oscilloscope i'd expect all leds except white to be quite fast

Yes, but anything short of instantaneous can still skew results, I would think.

if i understood well you need more or less one photon per second. it does not need to be precise and anyway its random

. IIRC, LEDs have a threshold where they jump from zero photons out to many. I may be thinking of lasers. You should be able to track down a LED voltage/current to luminance chart/table.

Well a laser uses a form of LED so I would think it would be very similar. I know they don't operate below specific power levels, so this is unlikely to work for him.

Yes, well, I still think it might be rather difficult to go that route, but try it and see, right ? :-)

and.. . i need then 2 leds in parallel (reverse directions) so the capacitance won't charge up ad clog itself

. Video tape and floppy disks have an opaque iron oxide coating. Might work if you scrape the oxide off.

I so want to do this. This is very neat. I never would have thought you could see an individual proton! I thought they were too small. Glad to be wrong! I can't wait to find a place to try it out...

...Wups, I didn't catch the typo, Adrian. These are "photons", the quanta of electromagnetic radiation, that you can see with fully dark-adapted eyes. Protons, the positively charged particles in the nucleus, are too small to see directly(*). Sorry if I caused confusion!

(*) Though under just the right conditions "you" might see them indirectly.

Astronauts have reported seeing poorly localized "flashes" while in orbit, and the very early accelerator physicists reported similar flashes when they stuck their heads/eyes into beamlines. I wouldn't do that if I were you :-/

The interpretation of these effects are that high energy protons (such as cosmic rays) passing through the vitreous humor produce Cherenkov radiation which is picked up by the retina.

Yes, I know they are photons-it was a typo! I know what light from a green laser pointer is made of!-sorry about that.

Ooo. Bad. Sounds sort of like the Curies separating radium without any protection...

But my real question is, what effect does the luminiferous aether have on this experiment?? ;-)

Okay, just making sure (look at some of the other comments I've gotten about my radiation-related topics :-). Yeah, sticking your face into a particle beam is extremely stupid. There's a substantial probability of getting skin or brain tumors as a result. But the guys who built those things in the first place didn't really have our experience to know what the effects would be. As for the &aether, it should have no effect at all. Since you and the laser are at rest relative to one another, you wouldn't know about it :-)

Heh, it's not like it's ridiculous that I'd make that mistake though.

:-( That's sad. I sometimes wonder what scientists (or just regular people) are doing today that people will look back on and shake their heads and say, "Poor ninnies...they never stood a chance after doing that...". (Like kelsey actually getting inside the accelerator to service it! :D j/k)

The aether was a joke...yuk, yuk... :D

I am beginning to suspect that the way we've been dealing with buckballs, carbon nanotubes, and whatnot may turn out the same way.

The only reason I was able to climb into my detector (check out what I had to do to fix it) was because the accelerator sections had been removed :-)

I hope you don't mind me asking, but does that scare you? That you're working on the bleeding edge of this stuff, and just don't know for sure yet what is safe and what is not?

The work I do doesn't scare me. I understand electronics, and I understand radiation. Our lab (actually, the whole freaking Department of Energy complex) has gone overboard about Safety, but I take safety issues seriously where they matter -- high voltage, radiation exposure, confined spaces (asphyxiation).

The "nanotechology" stuff I mentioned was more meant to be a general answer to your comment about "what are people in the future going to think?"

As for my own work, the detector and accelerator are very well shielded (several feet of concrete block), and no one is allowed anywhere inside while the beams are on. When we were running, we monitored everything on computers (that scene from the NOVA episode? that was the BaBar control room).

Now it's my turn to thank you for a detailed explanation! :) Seriously, I always have wondered about that. Thanks.

Hey, I've been rethinking the implementation of this experiment. In my lab, we did in a light-sealed room, probably because we already had one. I think for the I'ble, it'll be easier to have the user make a light-tight box with a viewer (maybe binoculars or opera glasses?) which can seal against their eyes.

Exactly so. This is just getting a bit more complex than I had originally envisioned.

Hmmm. I hate unnecessary complexity. What if they just got in a very dark (but not lightproof room) and then put their head in the box (a big box, with the laser stuck in a hole at the end) and put something over the whole shebang to get rid of the last of the light? Although that isn't all that much progress is it...