Observing Single Photons




Introduction: Observing Single Photons

About: I've been an experimental high-energy physicist for 20 years (since I started graduate school in 1988). I got my BS in physics from UCLA, my Ph.D. at Caltech, and did a post-doc at UBC before moving to SLAC...

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 (QE) 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.

Step 1: Not Ready for Prime Time

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 can take pictures and publish this as an I'ble.

If someone else decides to tackle it, please feel free to contact me and I'll make this a collaboration.

Step 2: Producing Single Photons

NOTE:  I have modified the ND notation below to refer explicitly to the optical density, rather than fractional lens area.  I was unware of the confusion until Instructables user jefc_uk pointed it out.  Thanks!

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). You can use a red laser pointer, but your eye will be less efficient (see intro). Estimate the number of photons your pointer produces given its wavelength and power rating.

How do you reduce that to one photon at a time? With filters. An ND3.0 neutral density filter (optical density of 3.0) reduces the output light by 10-3 compared to the input, so a stack of just five ND3.0's in front of this laster pointer would result in (on average) just 2.8 photons per second! A stack of four ND4.0's (each reducing the output by 10-4)would give you 0.28 photons/s on average.

ThorLabs sells both ND 3.0 and ND 4.0 filters. They have mounted ($67, for use on a camera) and unmounted ($54) versions.

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.

Shine the laser on your detector with no filters in place, and record the output. Do the same with one, two, etc. filter layers, and make a plot of output (on a log scale) vs. number of filters. Hopefully, you get down to 0.001 or 0.0001 with just a few layers. with the log plot, you can draw a straight line to extrapolate how many layers you need to get down to a few photons/s.

Ideally, you'd also like a single-photon counter, something like an avalanche photodiode, connected to a piezo-speaker, so that you can hear "clicks" each time a photon comes through the final stack of filters, and confirm that the rate is as low as you expect. Building such a thing is a whole separate project in itself, so I'm just going to assume that you have one.

Step 3: Make a Dark Room -- or a Dark Box

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. When you turn off the lights, you should not see any light coming in through cracks or edges. If you do, fix them and check again.

I'm coming to the conclusion that this is too complicated for the problem at hand. It may turn out to be better to build a light-box with a gasketed hole at one end for the laser pointer, and a viewing window with draped headcover at the other.

Step 4: Set Up Your Photon Gun

Attach the ND filters (or plastic sheets) to the front of your laser, sealing around the edges with gaffer's tape. You don't want any light scattering out the sides.

Put the laser on a table or stand pointed at where you'll be sitting.


Even a 1 mW laser pointer can damage retinal cells from direct impact. Not enough to blind you, but enough to potentially contribute to vision issues later in life. Don't point the laser at your face until you have the ND filters installed.

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.

Step 5: Sit in the Dark

The human eye requires 20 to 30 minutes to fully dark adapt. Turn off all the lights in your room and wait. 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.

Step 6: Fire Away!

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.

If you have a lab partner, you can make a real experiment out of it. Sit quietly, and have them turn the laser on and off without telling you. If things are working correctly, then you should be able to identify when the laser has been turned on, and when it's off. Keep track of hits and misses for many (at least 20) trials, and measure whether or not what I've described actually works.

Step 7: Other Single-photon Experiments

Now that you have a reliable source of (on average) single photons, you can use it to explore some of the other weirdness in quantum mechanics.

How about "self-interference" and wave-particle duality with a double-slit experiment?

If you are really ambitious, and have access to a proper optical table and research equipment, you could even try measuring entanglement, or trying a delayed-choice experiment.

Of course, if you have access to that kind of equipment, you're probably already a postdoc or facult in quantum optics, and are about to write a long comment about all the stuff I've gotten wrong :-)



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    65 Discussions

    Hi. This sounds like an awesome experiment. I've recently done a home experiment where I observed a diffraction pattern using a $2 red laser going through a handmade thin grater and was very proud that I've experienced this wavelike emanation of photons (I have the video somewhere too).

    I'll surely try yours in a dark room and let you know how it went. Unfortunately, don't have a green laser, so I might use a camera to try detect those tiny devils.

    Entanglement is most likely out of reach, but it's ok.

    It's always fun to play with quantum physics, even if we are left with few phenomena to observe.

    I wonder how many quantum phenomena can be observed with cheap equipment that one can buy from Ebay/Online...That would be a cool project...Keep it up and thanks !

    Sorry, my good fellow...the probability of the mammalian visual system detecting a single photon is, exactly, zero. The flaw in your argument is either the assumption that it only takes the activation of 1 molecule of rhodopsin to generate a signal on the optic nerve detectable by the brain neural network, or the assumption that 1 photon can activate more than 1 rhodopsin molecule. Whereas in reality, the reason that NO mammal can detect 1 photon is that the cone cells contain millions of rhodopsin molecules, and that it takes the activation of millions of rhodopsin molecules to generate a big enough signal to make it all the way down the (million-molecule-wide) optic nerve and then be detected by the (billion-molecule-wide) brain neural network.

    As a check the logic, all that is necessary is to look around. There are birds, and cats, and nocturnal animals of all varieties, and starlight night-vision goggles, which all can detect far lower photon densities than humans. If the human visual system, or even cheap commodity CCD (charge-coupled device) cameras, could detect single photons, then laboratories and militaries would not spend millions of dollars on expensive CCDs and photon detectors to perform that function. And if you could capture a single photon activation on photographic film, it would take you longer than your lifetime to find the 1 molecule it activated out of the other trillions that remained unactivated.

    It would be great if you figured out a cheap single-photon source. But the only valid proof MUST come from expensive detection equipment. "Humans seeing flashes", if you believe in science, must have a different explanation.

    2 replies

    Also see Phan et al. 2013 (http://arxiv.org/abs/1308.0670), accepted by PRL 29 April 2014.

    Please see Baylor 1998 (http://www.pnas.org/content/93/2/560.short), Reike and Baylor 1998 (http://rmp.aps.org/abstract/RMP/v70/i3/p1027_1), Granath 1981 (http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=1456292).

    Your analysis is reasonable but incomplete, as you perhaps don't realize that rod cells include significant amplification. In particular, the absorption of a photon by a single rhodopsin molecule is sufficient for the rod cell to fire, producing a signal which will propagate out of the retina and up to the visual cortex.

    Sorry. I didn't use a specific laser diode. I used a simple laser pointer I already had (green, for eye sensitivity), and just taped down the little button to keep it turned on.

    I got five of the ND3.0 neutral density filters, and I stacked them and mounted them on a laser pointer which was advertised as being only 1 mw. but in the dark even before my eyes were dark adapted I could see light coming from the laser through the five filters. I looked at the laser and realized that its power was actually anywhere from 0 to 5 mw. It said on the label max output power: <5mw. I am guessing the laser was too powerful. where could I get a laser which is exactly 1mw? or is there another solution?

    Bravo experiment, I learned a lot from it, thank you!

    thorlabs also sells the ND3.0 neutral density filter in smaller sizes for around $20. Is there any reason why you would need the larger size?

    1 reply

    Not at all. Since this design uses a diode laser (pointer) with a small aperture, the smaller the better. I didn't notice that the filters come in multiple sizes. Thanks!

    ...I think that you're in a mistake.
    For demonstrate that you have a single photon emitter, Could you do a HBT(Intensity interferometry) experiment for probe it?
    If you use filter only decrease density but we have in this case a poissonian distribution. This implicates that you could have in one second(using you disertation) an indeterminated quantity of photon (mean could be 1 by second but no each second 1 photon).
    Please don't say that this is way for observe single photons because you're promoting ignorance.

    3 replies

    This is not in any sense a research-quality single-photon system, nor is it meant to be. It is, however, a reasonable do-it-yourself demonstration, a point which I think I make clear in the introduction.

    This demonstration, in fact, was adapted from an actual undergraduate physics lab experiment I did at UCLA when I was there (1984-1988). We first did measurements using a small photomultiplier tube and scaler, to verify that the single-photon rate was about 1 Hz. After that, we did the dark-adapted observation, using a "clicker" to count the flashes we thought we observed.

    Using neutral-density filters reduces the intensity of the beam, while still preserving the Poissonian emission distribution, exactly as you say, but your conclusion is not correct.

    The probability distribution means that on average, the time interval between successive individual photons will be longer than human POV resolution. For an individual observation, there is some probability (which you can calculate) that two photons reached your eye in a time interval shorter than human optical resolution (POV).

    The design of the experiment is to get the Poisson interval to be long enough that the probability of a two-photon event is small. This is not "ignorance," it is mathematics.

    This isn't a demonstration this is only a wrong concept.
    laser: coherent emitter with poissonian distribution.
    Laser diode: have fluctuations. Because this you can't calculate density in each time.
    Improper definition of this demonstration. If anybody do this, he are doing a LOW DENSITY EMITTER SYSTEM, never you've a single emitter. If you want a single emitter you can use a Single Quantum Dot structure. Actually in science, work with single emitters is considered a top research and you need a very expensive lab for work properly.
    Be carefully: NEVER use your eyes for see laser emition. NEVER NEVER. And please DON'T PROMOTE THAT SOMEBODY DO THIS.
    If you want to build a single emitter you need a sub-poissonian distribution. You can check this in many articles that works with Single emitters.
    This diode have fluctuations if you are so crazy for see this flashes, you're detecting this fluctuations. not photons.
    For other part, Please think in this: If we can detect single photons with our eyes, then when we are outside in a sunny day, why we don't become blind?
    I propose to you that, erase this demonstration or rename properly. Because you're promoting ignorance.

    I'm curious about your background and current research area. Are you a knowledgeable layman, a graduate student, or university faculty in quantum optics?

    You are correct that laser diodes (in fact any laser) have fluctuations such that you can only calculate a probability for single-photon detection, with no guarantees. QDs or trapped ions (e.g., in a MOT or Penning structure) can provide guaranteed single photon emission, which is required for true research-level work.

    You are highly overstating the danger of laser emission. It is not significantly different than any other high-intensity light. The primary danger is from infrared lasers because the human blink reflex is only triggered by visible light. Consequently, the eye may be exposed to even a low-power IR laser for a relatively long time, resulting in significant thermal damage.

    For visible light lasers, the danger is specifically graded according to intensity (power output). Below one milliwatt output (i.e. a full intensity green laser pointer) the hazard is considred "negligible." With the filtering described here, the visible power on target is femtowatts.

    Finally, the human eye does detect single photons (if it didn't, we couldn't see anything at all). Under normal conditions, there are many, many single photon detections all happening at once, each within a different specific rhodopsin molecule. It is the temporal correlation of all those detections which forms the "scene" we see.

    This is absolutely awesome! now to preform the double slit experiment all you would need is the detector screen. (and I guess another sensor to make the photons want to become particles.)

    5 replies

    Well, you can do the double slit experiment with just a piece of photographic film (if that stuff even exists any more ;->). Film has a quantum efficiency of only a few percent, so since you've gotten the intensity down to ~one photon at a time, you "know" (can infer) that you'll get just one exposed silver halide grain per photon.

    With both slits open, a many-hour exposure will get you a pattern of dots on the film in the form of bands (interference fringes). Covering one slit will get you a different pattern.

    Yeah I'm thinking using a webcam in a dark box with the shudder open behind the slits while the laser is running through the filter would be a good setup. I'm thinking about trying it. I think you'd have to remove the eye sight from the cam first though which will be a pain, but worth it if it works.

    Sounds excellent! You'll want to put the sensor relatively close (few cm) from the slits, since it has a small area.

    If you do build this, please take lots of pictures. I can add you to this I'ble as a collaborator, so you can attach pictures and narrative, or I can cross-link to your own I'ble if you decide to write one.

    Oh I forgot to ask, Whether there is a cheaper option than using the 5 filters stacked on top of each other. is there another kind of cheaper filter that would work? if not it is fine, but it would be better for me if there was.

    Not as far as I know. With real research optical systems, you can turn down the laser intensity, allowing you to use fewer filters. I did mention the really cheap option -- a thick stack of black trash bag pieces -- but it's completely uncalibrated.