Can anyone give an example for a physically real and common process of entanglement?

Please not "only" a mathematical expression. I need a physically real example.

  • $\begingroup$ Particles can have their wave functions overlap in a way that both particles's wave functions can not be described without the other. There is no macroscopic equivalent process than can be used to describe this. $\endgroup$ Sep 15, 2017 at 17:19
  • 1
    $\begingroup$ "I need an example which I can rebuild for me" Do you mean the experiment you can perform at home? $\endgroup$
    – OON
    Sep 15, 2017 at 17:23
  • $\begingroup$ exactly .... or in any labor $\endgroup$ Sep 15, 2017 at 17:35
  • $\begingroup$ @ingeniosus The thing is that it may cost quite a lot for an individual. Like just a cheap BBO crystal may cost already 500$ and that's not talking about lasers and detectors. There are some possibilities that use radioactive sources that may be cheaper but then there are legal constraints. $\endgroup$
    – OON
    Sep 15, 2017 at 18:27
  • 2
    $\begingroup$ see blogs.scientificamerican.com/critical-opalescence/…. Pretty sure spending more than 30 seconds on Google will produce additional examples. $\endgroup$ Sep 15, 2017 at 19:32

2 Answers 2


Certain crystals can split a laser beam of a visible light unto two beams of infrared. For example, hypothetically, each photon of the 0.5um wavelength (green) can be split into two entangled photons of 1.0um (near infrared). BBO or beta-barium borate is one example of such a crystal.

  • $\begingroup$ I can see that some property of the original photon (like angular momentum) might be split between the two products so that measuring one gives the value of the other, but what is magical about that? $\endgroup$
    – R.W. Bird
    Dec 31, 2021 at 16:05
  • $\begingroup$ @R.W.Bird Since your comment is not specific to my answer, please feel free to post a separate question. $\endgroup$
    – safesphere
    Dec 31, 2021 at 19:26

If you want an eyeball situation of entanglement you can do that with an ultraviolet diode flashlight and a cup of coffee. The UV light has a wavelength of about $350nm$ which by the simple formula $c~=~\lambda\nu$ means the frequency is $\nu~=~8.6\times 10^{14}Hz$. These photons interact with coffee, and similarly with some minerals, so each photon is split into two photons. This pair of photons is an entanglement pair. The UV light on the coffee produces a green light. So the photon is split into a photon with $\lambda_1~\simeq~500nm$, or frequency $\nu_1~\simeq~6.0\times 10^{14}Hz$ so the remaining photon has a wavelength of about $\nu_2~\simeq~1.1\times 10^{-6}m$ or in the IR band. If you have an IR radiometer you could detected the IR.

  • $\begingroup$ "Each photon is split into two photons" - Are you claiming that spontaneous parametric down conversion (SPDC) happens in a cup of coffee? Coffee is photoluminescent under UV light, but it does not produce entangled photons, which requires SPDC. $\endgroup$
    – ACuriousMind
    Sep 16, 2017 at 12:36
  • $\begingroup$ What's IR? Your example is funny. In serious way you can explain the green color on your coffee by absorbed energy of coffee. But where is your generate a new Photon? You underlay entanglement - that's too phantastic. $\endgroup$ Sep 17, 2017 at 22:08
  • $\begingroup$ I knew I was a going a bit out on a limb, but luminesence involves orders of photon absorption and emission, including of second order. This means there are spontaneous emission and down conversion physics involved. Entangled states actually occur in many generic things, such as why in sunlight your fingernails have little colored sparkles. I leave it to the reader to find out how that is the case. $\endgroup$ Sep 17, 2017 at 23:07
  • $\begingroup$ I doubt each photon is split into two. I wonder if someone more clever than I can work out the probability of the two entangled photons escaping the coffee cup under reasonable assumptions (say cylindrical glass cup). $\endgroup$ Sep 19, 2017 at 18:54

Not the answer you're looking for? Browse other questions tagged or ask your own question.