# How do you entangle photons?

1. Aug 4, 2008

### thenewmans

How do you entangle photons?

2. Aug 6, 2008

### ZapperZ

Staff Emeritus
Since no one is answering this...

One of the common technique of getting entangled photos is by using something called a "parametric down conversion". This is where one photon comes in into a crystal and is split into two. The "daughter" photons together obey the conservation properties of the original photon. That property is what is being "entangled" between the two resulting photons.

I thought I had a webpage that gives a rather simple, non-technical explanation of this, but now I can't seem to find it. A google search will give you plenty of technical papers on this topic, though.

Zz.

3. Aug 7, 2008

### DrChinese

As ZapperZ says, the primary technique is through PDC (parametric down conversion).

There are 2 types of PDC, Type I and Type II. They both rely on a similar underlying principle. A laser input beam of a known wavelength in sent into a non-linear crystal (basically like a small lens) tuned for the input wavelength. Most of the input photons pass through unchanged - and these are ignored. Perhaps one in a million, however, undergoes a metamorphosis. It emerges as 2 photons. Because of conservation of energy, momentum, spin, etc, they are each half the energy and twice the wavelength of the original. Total spin is conserved, so knowing the polarization of one tells you the polarization of the other. They are entangled because they exist as a superposition of states until they are observed. Because these entangled photon pairs emerge off-angle from the original input beam, they can be captured and pulled aside for testing - often using fiber optics and lensing mechanisms to manipulate.

Below is a technical link, unfortunately just about any description of the process gets complicated and confusing very quickly:

Type II Parametric Down Conversion

4. Aug 8, 2008

### wawenspop

Does that mean that beam splitters and Youngs Slits DO NOT produce photon pairs with laser light? I have always been unsure about this. Anyone know the correct answer?

5. Aug 8, 2008

### Staff: Mentor

That is correct. Those experiments do not change the number of photons. If you send one photon into a beam-splitter, you get one photon out. Until that photon interacts with a detector, it is indeterminate, which way the photon went. If the two paths are reunited before the photon is detected, you get interference. If the photon is detected first, you don't get interference.