Observing Single Photons
Intro: Observing Single Photons
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
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
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.
SAFETY WARNINGEven 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
STEP 6: Fire Away!
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
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 :-)
64 Comments
ΣΚΡΟΥΤΖΜΑΚΝΤΑΚ 8 years ago
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 !
a120439 10 years ago
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.
kelseymh 10 years ago
kelseymh 10 years ago
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.
rameshrai 10 years ago
Can you tell us the name of laser diode you had used?
thanks
kelseymh 10 years ago
Higgs Boson 12 years ago
Higgs Boson 12 years ago
kelseymh 12 years ago
ikyiky 12 years ago
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.
kelseymh 12 years ago
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.
ikyiky 12 years ago
Facts:
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.
kelseymh 12 years ago
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.
Higgs Boson 12 years ago
kelseymh 12 years ago
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.
Higgs Boson 12 years ago
kelseymh 12 years ago
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.
Higgs Boson 12 years ago
kelseymh 12 years ago
Higgs Boson 12 years ago