# Single Photon Interference

1. Mar 31, 2012

### Cmertin

I'm trying to write up an experiment that I have done on single photon interference. The design is as follows:

• Class 2 HeNe laser
• 25 micron pinhole
• double slit
• output via CCD
I have all the data, but I cannot find anything that says that having a small pinhole about 80cm from the double slit allows only one photon through at a time. I cannot find anything that says by what degree the photons "spread" after going through the pinhole or their distribution (if it's even all over or if most of them are forward while the little number of photons that aren't going straight go off at more of an angle). I've come to you guys looking for help to see if it's just an accepted idea that I wouldn't need to cite or if there is something that can explain better what is happening.

Thanks,
Cmertin

2. Mar 31, 2012

### vanhees71

If you have simply a laser, you don't produce single-photon states but coherent states. If the laser is dimmed somehow, the mean photon number is small (maybe even less then 1!), but then you still do not have a single photon but a coherent state which is a superposition of infinitely many Fock states. For a very low-intensity beam the main component is the vacuum and a single-photon state.

3. Mar 31, 2012

### Bill_K

OK, then how do you prepare a state that has a single photon?

4. Mar 31, 2012

### Cthugha

The typical way relies on using some saturable transition like the elecronic states of a simple two level atom. If you excite it resonantly using a cw beam you will find that the two-level atom will show some emission into all directions. If you now have a look at the emission at right angle to your exciting laser beam (so you can distinguish between the emitted ligt and the light used for excitation) you will find that it consists of single photon states. The reason is simply that the atom returns to the ground state during an emission process and it takes some time to get it into the excited state again so that further emission events are ruled out for a brief period of time. In principle similar processes are possible for any fermionic two-level system. Unfortunately this is definitely not something for use at home.

One may also use single artificial atoms (like quantum dots) or defect centers in diamond. The latter are best for use at room temperature and the only realistic single photon source one might be able to use in an experiment at home. Nevertheless it will still be quite cumbersome to get it to work.