In a Roger Penrose book I’m reading (don’t remember the title, he’s arguing against algorithmic AI) he mentions that some experiments in quantum mechanics involve a single (one, count ‘em, one) photon. In the same book he mentions, by way of contrast, that a 60 watt light bulb generates about 10^20 photons per second (!).
My question is: HTF do they generate a single photon?
You don’t usually generate a single photon – you generally generate lots of photons and then use lots of filters to make certain that the photon flux is exceedingly low, amounting to only one photon at a time over long periods of time. That., I understand, is how they did those experiments such as Young’s double-slit experiment with only one photon at a time. The result, if you held to the particle rather than the wave model, shows that, even when only a single photon at a time encounters the slits, you still get the same photon statistics and diffraction pattern as when lots of photons are there (over time, of course). So the photon doesn’t need to “interfere with” another photon as it passes through the slit. *
Recently there have been other improvements, such as Mike Feld’s “single atom laser”, which must, of necessity, emit only a single photon at a time. I’m not up on exactly how that’sa built and prepared, but any time you have an isolated excited atom, you’ll only get a single photon out. At an unpredictable time.
*The way the Double Slit experiment is described is pretty misleading. People make a big deal about the photon going through one slit or the other, as if that’s the big mystery. It isn’t. You’ll get interference effects with three or more slits. Heck, you’ll get a diffraction pattern using a single slit, but which changes its shape as the slit width changes. In that case you know which slit the photon has gone through. The question then might be how the photon “knows” how wide the slit is, when photons are much smaller than the slit. Asking which slit the photon goes through – or even how the photon senses slit width – is a pretty crude way of asking how you can reconcile a localized particle model with non-localized wavelike behavior.
Or just flip the light switch on and off really fast.
You can have a room bright enough to read in, which still contains only one photon at a time. Yes, many are created each second, but at the speed of light, it also doesn’t take long to hit a wall.
Chronos, do you disagree, then, with the nomber quoted above that a 60W light bulb generates on the order of 10[sup]20[/sup] photons per second? Because if that’s true, then a light bright enough to read by (I’ll say one to two orders of magnitude less than that 60W bulb) would generate more than 10[sup]18[/sup] photons per second. And so each photon would have only 10[sup]-18[/sup] seconds to get out of the way before the next one was generated. that’s only enough time to travel a billionth of a foot. Pretty small room. But again, that’s based on the 10[sup]20[/sup] number being accurate.
Regardless, you just blew my mind with that post.
I’d call BS on this if I knew enough about physics to argue 
Can you please explain this statement.
It would have to be a really small room?
“Bright enough to read in” could be more than several orders of magnitude less than a light bulb. With a dark-adapted eye, you could conceivably still read with a children’s room glow-in-the-dark sticker held right above the page–not nearly as many photons there. That might scale you past your room volume to achieve only one-photon in the room at a particular instant.
Haven’t run the numbers, but a fun speculative post from Chronos that I’d never thought of
I’ll second that.
If the distance from the light source to the wall is 10’, then it takes a photon 10.018 nanoseconds to reach the wall. Therefore, you can only have 98.208 million photons per second, which is probably not enough to read by (that’s 12 orders of magnitude fewer photons than the 60W bulb generates per second).
The 60 watt bulb emits photons in practically all directions, whereas your 98.2 million photons are the number emitted in only one single direction. At least I think that’s how you calculated it, I haven’t bothered to check your math.
No, I just calculated it as if the bulb was suspended in the middle of a 10’ sphere, and only one photon is allowed to be in flight at any time.
Sorry. I also missed that Chronos said there would only be one photon in the entire room at a time. I hope he reenters the scene. He’s very rarely wrong.
OK, running the numbers, it turns out I was dead wrong. For a flight path of 3 meters, and blue light, one photon at a time would mean about 5e-11 Watts.
Dangit, I know I saw a statistic like that somewhere, but obviously, I horribly mangled it. What was it I was thinking of, then?
Ok, as I said above, you just can’t trust Chronos he’s very rarely right. 
OK, this has been fun (Really. That’s why I ask questions on this board; I get much more than asked for) but does anyone have any answer to the OP? Is the super heavy filter the answer? Seems a rather hit-or-miss approach.
But it’s far and away the easiest, and perfectly adequate for many of the experiments they used to do.
With the advent of quantum cryptography and other phenomena, though, people are looking for something better. Check out this recent article on the topic in Laser Focus World:
http://www.laserfocusworld.com/articles/257230
(Bolding mine)
For my undergraduate project, about 15 years ago, I did the double split experiment with a CCD camera. CCD was quite new and exciting then and this expensive camera with PC capture card had been purchased for the observatory but ended up in my hands.
I captured hundreds of images of a single photon and then added them together one at a time to produce a series of still images, each one having one more dot than the last. These were then fed out to VHS (remember that?) with each image lasting half a second. The final result was a 3 minute video showing the diffraction pattern appearing one photon at a time.
These days this is probably a standard teaching method to first years, but at the time I believe I was the first to use this technology in this way. Rock and roll!
Anyway, I can confirm that the laser beam was reduced to a single photon at a time by the use of a bucket load of filters.
I’ve always wondered if you could make a holgram this way, one photon at a time.
One photon is always coherent with itself right?
Of course each photon would still have to be in a very narrow range of wavelengths as well.
What about a 10’x10’x10’, or 10’ Dia. Sphere, Hermetically Sealed, Mirrored room…and a single photon?
At the speed of light, wouldn’t it be pretty much everywhere all at once?
How many Photons does it take to make Light visible at normal 20/20 vision?
(the visible spectrum)
I’m not quite sure if this is what you’re asking, but in a darkened room the human eye can detect a single photon.