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I have heard it is possible to create entangled laser beams using a process roughly similar to this:

  1. Pass the beam through a crystal that splits each photon into two, each of half the energy of the original and opposite polarization.
  2. Pass the beam through a special prism that separates light according to polarization rather than color sending the entangled beams on different paths.

I'm sure the actual process is more complex but my question is this: How much would it cost to do this experiment? Would it be practical to do at home or would it require specialized laboratory conditions of some kind? If it could be done at home is there or why is there not an off the shelf product one can purchase to do this?

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The hard part is that non-linear processes require quite high energy densities to happen. But I can see how you can make it work under 1000$:

  1. You take cheapest possible passive Q-switched laser: Nd:YAG or Nd:YVO4 with Cr:YAG saturable absorber. If you find any Nd:YAG or Nd:YVO4 laser with "open" resonator you can stick Cr:YAG crystal inside and it will continue working. But you should operate it at minimal possible power level to reduce the risk of damaging the optics.
  2. You take 10mm KTP crystal to convert 1064->532nm (if laser is not 532nm already).
  3. Filter out remaining 1064nm radiation with a 1064nm filter
  4. Use second 10mm KTP crystal to convert 532 back to 1064 with entangled photons.
  5. Do a polarization split with polarizing beam splitter or as simple as glass plate at Brewster angle.

So to build all this you'll need:

  • Chinese 532nm lasers with open resonators could be found around 500\$ which will partially cover #1..#3
  • CrYAG from Eksma will take 150\$.
  • 1-2x KTP crystals from EKSMA are 130\$ each (only 1 is needed if laser already emits 532nm).
  • Brewster glass plate is free.

This all is not a commercial product because not many people need it. Academic institutions do not target lower possible cost, and it might be more interesting to build from scratch rather than getting a kit. I am not sure there is much commercial use in this as is.

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    $\begingroup$ and don't forget that you need single photon detectors for this to actually see the entanglment $\endgroup$ Commented Sep 28, 2020 at 18:07

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