Entangling Photons: A Brief Overview of Quantum Interference

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In summary: If you send two photons into a beam-splitter, you get two photons out. This is a very different process than the PDC described above.In summary, the primary technique for entangling photons is through parametric down conversion, where a laser input beam is sent into a non-linear crystal and undergoes a metamorphosis, resulting in two entangled photons. Other methods, such as using beam splitters and Young's slits, do not produce photon pairs with laser light.
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thenewmans
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How do you entangle photons?
 
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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
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:

http://scotty.quantum.physik.uni-muenchen.de/publ/achtbild.pdf [Broken]
 
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  • #4
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
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.
 

1. How do you entangle photons?

Entangling photons involves using a process called quantum entanglement, where two or more photons are linked in such a way that their properties are correlated and become dependent on each other. This can be achieved through various methods such as spontaneous parametric down-conversion, quantum dots, or using nonlinear crystals.

2. Why is entangling photons important in quantum computing?

Entangling photons is important in quantum computing because it allows for the creation of qubits, which are the basic units of information in a quantum computer. By entangling photons, we can achieve superposition and use them to perform complex calculations and solve certain problems much faster than traditional computers.

3. How do you measure entangled photons?

Measuring entangled photons involves using detectors that can detect the properties of individual photons, such as their polarization or phase. These detectors are usually optical devices such as polarizers, beam splitters, or interferometers.

4. Can entangled photons be used for secure communication?

Yes, entangled photons can be used for secure communication through a process called quantum key distribution (QKD). This involves sending entangled photons between two parties, and any attempt at eavesdropping or interception would disrupt the entanglement, thus alerting the parties to a potential security breach.

5. What are the challenges in entangling photons?

There are several challenges in entangling photons, including the difficulty in creating and maintaining entangled states, as well as the issue of decoherence which can cause the entanglement to break down. Additionally, entangling multiple photons at once can be challenging, as it requires precise control and manipulation of the photons' properties.

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