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Double Slit Question

dogjones said:
How small do the slits have to be in order to let only one photon through??
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They don't filter out photons by making the slit smaller, they use a source that can be "pulsed" in very short durations to only emit one photon at a time.
 
Ahhhhhhh! Making more sense. Although how near do the slits have to be to each other to ensure it goes through both at the same time?
 
And can you explain the mechanics of being able to fire one photon at a time?
 
dogjones said:
Ahhhhhhh! Making more sense. Although how near do the slits have to be to each other to ensure it goes through both at the same time?
Click to expand...

I'm curious about the result, too, since my understanding is that the distance between slits is dependent on the scale of the wave component of the object passing through.

I was trying to do the math backwards: how far apart do apertures have to be spaced to produce interference with fullerines? How about for planets?
 
godofpie said:
And can you explain the mechanics of being able to fire one photon at a time?
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It's possible to do it straightforwardly. Take (say) a green LED emitting 1 microwatt of light---that's enough to see---or 1.5x10^12 photons per second. Put it behind a screen with a tiny pinhole in it; so now 10^6 photons per second come through the pinhole. Now, instead of turning the LED on continuously, turn it on in 10-nanosecond pulses. Each pulse has a 99% chance of launching zero photons through the pinhole, 1% of launching one photon, 0.01% of launching two photons, etc. So that's a reasonable sort of single-photon source. A photon-counting detector (like a PMT) can confirm that this is how it behaves.

Fancier sources can be arranged using quantum physics tricks. There are certain atomic states that are guaranteed to emit exactly one photon; you can prepare one atom at a time, confirm that there's only one of it, and trigger its emission. Certain solid-state quantum systems, like quantum dots, can be made to do the same thing---that's a fairly recent innovation though. The classic single-photon experiments were done with very faint light beams as above.
 
I forget how the math works, but I know that if you coat a microscope slide with tar, and put two razor blades together to make a double slit in the tar, they are the right distance apart to diffract light.
 
godofpie said:
And can you explain the mechanics of being able to fire one photon at a time?
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They aren't fired exactly, they attenuate a laser beam to a low enough intensity that the mean distance between photons is extremely large (on the order of kilometers), essentially making it a pulsed source. The point is that they slow down the photon rate at the source, not by filtering them using the slit.
 
Madalch said:
I forget how the math works, but I know that if you coat a microscope slide with tar, and put two razor blades together to make a double slit in the tar, they are the right distance apart to diffract light.
Click to expand...

If I recall correctly, the result is something like this. If D is the distance from the slits to the screen, d is the slit separation and l is the wavelength then the bright fringes on the screen are a distance Dl/d apart. This assumes that both l and d are much smaller than D. For your molecule beam, the wavelength can be found from the momentum using the basic relation: l = (Planck's constant)/(momentum). Sorry for the rubbish formatting, I'm posting from my phone :-)
 
dogjones said:
Ahhhhhhh! Making more sense. Although how near do the slits have to be to each other to ensure it goes through both at the same time?
Click to expand...

There is no maximum. However, the farther apart they are, the more likely that the photon will just hit the wall between them. So it's more of a probability issue: how often do you want the photon to pass through the slits?

As a practical matter, though, the size is generally determined by the size of the diffraction pattern you want to create, not by intensities or probabilities of transmission. The farther apart the slits are, the smaller the separation between diffraction fringes, which can make them hard to see. You can make that distance larger in one of three ways: move the screen farther away from the slits, use longer wavelength light, or move the slits closer together. The first two have obvious limits in any real experiment, which means that you'll end up with a maximum slit separation distance based on the desired diffraction pattern.
 
dogjones said:
Ahhhhhhh! Making more sense. Although how near do the slits have to be to each other to ensure it goes through both at the same time?
Click to expand...


Just watched a programme on the BBC tonight explaining the experiment. As I understood it the slits and gap may vary. In the instance shown the slits were approximately a hairs width and the spacing the same.

The programme was on measuring a length of string and ranged from using a rule at the shop the string was bought from, to how long it took light to travel its length.
Similar to measuring a coastline, it was not possible. It just became longer the more accurately you measured it.
Very interesting and well explained.
 
edd said:
Tish and fipsy. The UK's Ordnance Survey has long been defying mathematics to do just that.
http://mapzone.ordnancesurvey.co.uk/mapzone/didyouknow/whatis/q_12_69.html
(Despite enquiries, they've never told me how exactly they did that)
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Presumeably, very quickly, before the tide changed, and with a tiny rule!

Just had a daunting thought. I am planning a walk around the coast of Ireland (as near as i can get to it) If its gona take an infinite length of time im not going. Think I will take a rule and assess at half way.
Doh! theres no half way!
 
learner said:
Just watched a programme on the BBC tonight explaining the experiment.
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Me too. I've known of the experiment for years and never really understood how you could make things small enough, especially considering when it was first done. It was therefore quite a surprise to see it being done and using stuff that was all pretty big. The target was about the size of a postage stamp and it just had two thin slits in it, thin but not microscopicly thin. The laser was a meter or so away from the target and the wave pattern was projected onto a wall!
 
Do not worry. If you take steps that are almost a meter long then the length of a coast is finite. However if you want to produce an infinite length try answering this problem.

Give the total length of the triangle produced by this process.

1. Generate a triangle with each side of length one.
2. For every straight length add another triangle in the middle such that all four lines are of equal length (the four lines are the two sides of the triangle and the two parts of the remaining original line).
3. Repeat infinitely step 2 for every straight line

NB Step 2 turns a straight line into this shape: _/\_ Now you have four straight lines. For each one of those four lines you need to repeat the step.

The result is a simulated coastline.

I hope the above makes sense. It is bedtime for me and I am rather tired.


Edit. This is in response to posts 12,13,14.
 
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rjh01 said:
Do not worry. If you take steps that are almost a meter long then the length of a coast is finite. However if you want to produce an infinite length try answering this problem.

Give the total length of the triangle produced by this process.

1. Generate a triangle with each side of length one.
2. For every straight length add another triangle in the middle such that all four lines are of equal length (the four lines are the two sides of the triangle and the two parts of the remaining original line).
3. Repeat infinitely step 2 for every straight line

NB Step 2 turns a straight line into this shape: _/\_ Now you have four straight lines. For each one of those four lines you need to repeat the step.

The result is a simulated coastline.

I hope the above makes sense. It is bedtime for me and I am rather tired.


Edit. This is in response to posts 12,13,14.
Click to expand...

Um, I'm not really sure what's going on here. This seems an elaborate way of saying "if you want to produce an infinite length, then keep drawing?"
 
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