Two Slit Interference, One Photon at a Time

[brianbrane]

Hello

 

I watched the recent Horizon program on 'what is reality?' and I think they made a mistake.

 

I can see the experiment is taught generally, using one-photon at a time;

 

http://www.teachspin.com/instruments/two_slit/experiments.shtml

 

and I understand the concept.

 

However, they said that if the photons were tracked that the target pattern would change, so it would no longer be a wave pattern.

 

Has anyone done this in the lab?

Was the detector passive [what did you use]?

 

Cheers

[vargo100]

I see where you got miss leaded.

[Horza2002]

From how I understand quantum nature, the diffraction pattern exists because of the wave-particle duality of the photon. As it passes through the slits, it acts like a wave and so diffacts though the slits to give the pattern. During this time, the photon is a superposition of all the possible states. However, if you start watching one of the photons as it approachs the slit, it is no longer a superposition as you force it to be one single state. I think that is how it works in a hand-wavy not very accurate way...

[IM Egdall]

The quantum rule is this:

 

If, in principle, you cannot tell which path the particle takes, there is interference.

 

If, in principle, you can tell which path the particle takes, there is no interference.

 

All tests to date show this rule to be true. For example. in the double slit experiment, a single photon passes through both slits (like a wave). It interferes with itself and is detected in one location on the screen (like a particle). When a number of photons are put through the slits one at a time, we see an interference pattern build up.

 

Now what if we choose some way to detect which path each photon took; that is which slit it went through. Now we find there is no interference pattern on the screen.

 

There are many websites on this effect - just google double slit. Or go to: http://www.marksmodernphysics.com/ and click on Selected Animations, Quantum Entanglement #6 - Light through beam splitter.

Edited February 15, 2011 by I ME

[Horza2002]

Arr yes, thanks I Me. That is what I was trying to say but you've managed to say it far better than I did!

[steevey]

Hello

 

I watched the recent Horizon program on 'what is reality?' and I think they made a mistake.

 

I can see the experiment is taught generally, using one-photon at a time;

 

http://www.teachspin...periments.shtml

 

and I understand the concept.

 

However, they said that if the photons were tracked that the target pattern would change, so it would no longer be a wave pattern.

 

Has anyone done this in the lab?

Was the detector passive [what did you use]?

 

Cheers

 

Actually, I have done something like this at a college where I observed the wave effects of photons on a special back panel after the process has taken place.

But they are accurate in stating that tracking it would change the pattern

This is where the "weird" comes in.

Whenever you observe a particle, its inherit wave properties seemingly disappear, leaving you with what acts as a single point in space. So when you observe a wave particle, it acts like just a normal, non-wave particle, which changes the pattern to be just two slits.

This process is also called "determining" or "wave-function collapse".

When you observe a particle, it becomes determined, so it seemingly can take only one path if its determined to only be a single particle occupying a single point in space. When it's not a particle, but a wave, its position isn't determined, so it exists as a culmination of multiple possibilities at once.

Edited February 15, 2011 by steevey

[brianbrane]

This is where the "weird" comes in.

Whenever you observe a particle, its inherit wave properties seemingly disappear.

 

The qustion is, what was used to 'observe' the photon particles as they passed through the slits?

 

The experiment is; when firing single photons the wave pattern was still observed.

Nothing about observing them as they pass through the slits, a photon detection device is never mentioned.

You can observe other photons as they hit the one or other of the slits, but these are other photons, not the photons causing the diffraction.

Does anyone on this underachieving backward planet understand particle physics?