[GlentoranMark (OP)]

I've read about the double slit experiment and understand the logic but how can we shoot 1 photon at a time?

What constitutes a photon? Can we shoot half a photon? May sound stupid but everything is stupid at this level  []

[syhprum]

Optical frequency photons are quite energetic photo multiplier tubes or their modern semiconductor equivalent can detect single photons.
Once you know your detector is responding to single photons (which you can count) you arrange a very weak light source to generate them and then use that source for your experiments

[GlentoranMark (OP)]

Can I have that in English?

What is a photon?

You said a detector can find a single photon, how do you know it's a single photon - or two photons or half a photon?

Sorry if it seems basic, I can't get my head round it.

[CliffordK]

I think there were some old psychology experiments that indicated that the visual perception limit of the human eye was seeing a single photon (in otherwise complete darkness).

Oops, sorry, it was 3 photons.
http://books.google.com/books?id=vw20LEaJe10C&pg=PA120&lpg=PA120&dq=three+photons+photon

Or, was it one photon?   []

Vision: A candle flame seen at 30 miles on a clear night (A single photon of light).

I haven't found the original reference paper though.  I'm curious on how this single photon flash was actually presented to the subject, and verified.

[syhprum]

You couple the output of your photomultiplier to an amplifier/loudspeaker system and you hear a series of clicks similar to what you would hear from a Geiger counter when it is recording particles from a radioactive source.
You then adjust the intensity of your light source until the clicks are easily distinguishable say one a second then you know your light source is emitting photons at that rate.

http://sensl.com/Products/?_kk=photon%20detector&_kt=9bece660-e952-4f5a-9086-340a1f0ae5bb&gclid=CI7269S-zqYCFc0f4Qod4z7kGg

[Soul Surfer]

The basic supplementary question was how do we know that photons exist.  We knew this well before individual photons could be detected because if the studies of the way hot bodies radiate energy.  We all know for example that an iron poker put in a flame will get red hot and when you take it out it will continue to glow red hot and cool down ass it looses energy by radiation.

Around 1900 physicists were studying this.  They knew that light and heat was electromagnetic radiation just like radio waves and was subject to maxwells equations.  They also knew that atoms were electromagnetic in structure although atomic structure was in it infancy.  they realised that atoms as they heated up became more agitated and bounced off each other faster this is what heat is so as the atoms hit each other and rebounded like tennis balls they should generate electromagnetic radiation as radiated heat (infra red or light if things were hot enough)  Using a spectrometer and a sensitive energy detector (a bolometer)  they could measure how much energy a hot body radiated at each frequency.  Stefan had used a simple model to show that the total energy radiated should increase at the fourth power of the absolute temperature and this was tested and found to be accurate.  So Wien built a more detailed mathematical model that predicted how much energy would be radiated at each frequency and it fitted the curve very well at the low frequency end of the spectrum but it went up without limit at the high frequency end which was clearly wrong because the spectrum had a peak and went down to zero as the frequency rose. 

Planck tried all ways to prove this should happen with a more detailed model of what was going on and in desperation he tried a model in which he forced the energy to be radiated in specific lumps that he called "quanta" and that these quanta got bigger as the frequency of the radiation got higher according to the simple relationship, energy equals a constant times the frequency.  He did not like this because he could see no good reason for it but it worked exactly!  So the fact that electromagnetic radiation came in quanta was proved well before anyone had thought of quantum theory

[lightarrow]

how can we shoot 1 photon at a time?

Taylor did it in 1909:
www.wbabin.net/historical/taylor.pdf   

[Soul Surfer]

That was using extremely low light intensity equivalent to small numbers of photons per unit time but the photons were detected collectively using a low quantum efficiency detector (photographic plates). modern high quantum efficiency detectors can detect individual photons as separate events.  I also understand that it is now possible in effect to detect the passage of an individual photon through a slit without destroying it but I am not sure of the techniques used to do this.