# How do photon emitters and photon detectors work?

The interpretation of the double slit experiment is very strange and i want to understand how they did it before I give up my concept of reality. With the double slit experiment a interference pattern is created even when you emit just one photon. However, when you use photon detectors to find out which slit the photon went though, the photon goes in straight line.

So firstly I want to know how do the photon emitters work and secondly I want to know how these photon detectors work. How are you even meant to detect a photon without even touching it? also how big the the slits?

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to save you some trouble, keep in mind the double slit experiment has never been done with just one electron. What you state is just an interpretation of the quantum mechanics of the DS exp. Be sure there are others (like Bohm interpretation), that dont require that – Nikos M. Aug 19 '14 at 11:54
@NikosM. Really? As much as that particular thought experiment dominates discussions of intro quantum mechanics, I would expect either some folks did it in the 1950s or 1960s and saw only what was expected (and were thus forgotten) or that there's a well-known technical reason that the experiment is impossible. Certainly there have been many matter interferometer experiments, but tend to use a Mach-Zender geometry rather than Young's. – rob Aug 19 '14 at 12:07
@rob, i dont see where you are getting at? are you saying my comment is wrong? – Nikos M. Aug 19 '14 at 12:09
check this also as a related alternative: arxiv.org/ftp/arxiv/papers/0810/0810.1034.pdf – Nikos M. Aug 19 '14 at 12:31
Note that doing "the double slit experiment with just one <anything>" doesn't make sense as you can't show the development of a interference pattern. What does make sense is to do it with one <anything> at a time, and that is easy: it suffices to turn the <anything> intensity down until the expectation value for <anything>s on the optical path is much less than one. – dmckee Aug 19 '14 at 13:02

You're distracted by fancy language. Instead of "photon emitter" think "light bulb." (I suppose you could think "laser pointer" instead, since that's an easier way to make all the light exactly the same color than a light bulb plus a prism.) Instead of "photon detector" think "eyeball" or "camera."

You've probably noticed that you can't detect a photon without touching it. In Star Wars and GI Joe, a laser makes a beam of light that's visible along its entire length, perhaps moving at some quick-but-visible speed. But in a real laser pointer that you buy from the store, the beam is invisible, except for a bright spot where the beam is reflected diffusely from a wall. (I was very disappointed to learn this as a kid.) In order to see the whole beam you have to pass it through a cloud of dust or water droplets. Even the bright green lasers that star-party astronomers use to point out constellations are only visible because a small fraction of the light leaves the beam and scatters backwards; that's why they're so dangerous if they get pointed at an aircraft cockpit.

You can (and should!) do a double-slit experiment yourself, by splitting a laser pointer beam with a thin staple, and sending the split beam a few meters across a room so that the interference pattern is large enough to see. You'll see a row of bright and dark spots on the wall; you can measure the dot spacing and compare it to the staple-to-wall distance and find the wavelength of the light if you want to. But there you're seeing the scattered light from the interference pattern. If you wanted to distinguish which slit the light came from, there's a very simple way to do it: walk over to the wall, where the interference pattern is, and bend down and use one eye to look back into the laser. (Of course before you did such a thing you would have gotten a class-I laser that's safe to look into!) If the bright and dark spots were larger than your pupil, then there'd be places you could stand where you would see a lot of light from the staple, and places you could stand where the staple would be dark.

Note that this is an entirely classical explanation; it doesn't get strange until the light intensity gets low enough that you observe the light photon-by-photon rather than as a continuous excitation. But the main thing is not to let yourself get confused by fancy talkers. Light bulb. Eyeball. Super simple stuff.

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The interpretation of the double slit experiment is very strange and i want to understand how they did it before I give up my concept of reality.

The double slit experiment does not refute the idea that reality is objective. It does refute the idea that each photons goes through one slit or the other. Rather there are multiple versions of each photon that go down all of the possible paths from the screen with the slits to the detector. If you change what happens to those different versions of the photon, you change the interference pattern. See "The Fabric of Reality" by David Deutsch, Chapter 2.

With the double slit experiment a interference pattern is created even when you emit just one photon.

There is an interference pattern even if the probability of more than one photon going through the experiment at the same time is very low. Given that an experiment has dimensions of 1 metre, say, that means you could have $10^6$ photons going through the experiment each second and still only have less than one photon in the experiment at any one time.

However, when you use photon detectors to find out which slit the photon went though, the photon goes in straight line.

So firstly I want to know how do the photon emitters work

A quantum optics experiment would usually be done using a laser, possibly with neutral density filters in front of it to reduce the intensity.

The key point about the laser is that there is a constant phase relationship between the parts of the laser beam going through different slits.

and secondly I want to know how these photon detectors work. How are you even meant to detect a photon without even touching it?

What happens is the following. If you have no detectors in front of the slits you get a particular pattern of light and dark bars. If you have a detector in front of one of the slits you get a different pattern. The pattern you get with one detector is the same as it would be if you had an opaque card blocking the same slit. The only significance of the detector is that it blocks some versions of the photon from interfering with one another.

also how big the the slits?

Each slit is approximately 1-10 wavelengths wide.

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Thank you for you're reponse. How big is a wavelength? I didn't even know that was a size – Ray Kay Aug 19 '14 at 9:49
A wavelength is of course a length. For visible light this is between 400 nm and 700 nm, but the electromagnetic spectrum is very wide. The parts which we can easily manipulate go from about 0.1nm to a few hundred km. – Emilio Pisanty Aug 19 '14 at 9:53
And a lightyear is of course, also a length. – OrangeDog Aug 19 '14 at 12:18