# Trying to Generate Entangled Photons

Strange_matter
TL;DR Summary
I'm trying to generate entangled photons using a KTP laser pointer.
I've been trying to generate entangled photons using KTP laser pointer that normally produces green light at 532 nm. I was hoping that after removing the IR filter, I could add a reflective coating (right now the coating I have is a glittery metallic green nail polish) to reflect the green light and mix with the IR light to essentially create a optical parametric amplifier and produce entangled light that way. Unfortunately, now that the filter has been removed, it looks like it's only producing IR light. Are there any possible solutions? I think the laser pointer is pulsed if that makes a difference with the production of green light.

## Answers and Replies

Science Advisor
Gold Member
Laser light is not entangled. Why do you think mixing 2 beams will create any useful entanglement?

I assume you know that to get entangled pairs from a laser: you commonly use a suitably cut BBo crystal to convert a single input photon (from the input beam of 1 color) to produce entangled pairs of a different color.

vanhees71
Science Advisor
Summary:: I'm trying to generate entangled photons using a KTP laser pointer.

I've been trying to generate entangled photons using KTP laser pointer that normally produces green light at 532 nm. I was hoping that after removing the IR filter, I could add a reflective coating (right now the coating I have is a glittery metallic green nail polish) to reflect the green light and mix with the IR light to essentially create a optical parametric amplifier and produce entangled light that way. Unfortunately, now that the filter has been removed, it looks like it's only producing IR light. Are there any possible solutions? I think the laser pointer is pulsed if that makes a difference with the production of green light.

While one may indeed create continuous variable entanglement in above-threshold optical parametric oscillators using feedback on the down-converted light, you will find that already in the first experimental demonstration of above-threshold continuous variable entanglement in twin beams (which should be this paper to the best of my knowledge: https://arxiv.org/abs/quant-ph/0506139 ), the authors emphasize the important role of phase matching. Although your KTP certainly has a high $\chi^{(2)}$-nonlinearity, it is most likely cut for phase-matching in second-harmonic generation to get the beam at 532 nm. It is quite unlikely that the same crystal will have an adequate geometry to achieve phase matching in a reflective OPO or OPA geometry.

It would be better to use a different and dedicated KTP crystal for creating twin beams and even in that case a standard laser pointer is not a good light source for such an experiment.

DrChinese and vanhees71
Strange_matter
While one may indeed create continuous variable entanglement in above-threshold optical parametric oscillators using feedback on the down-converted light, you will find that already in the first experimental demonstration of above-threshold continuous variable entanglement in twin beams (which should be this paper to the best of my knowledge: https://arxiv.org/abs/quant-ph/0506139 ), the authors emphasize the important role of phase matching. Although your KTP certainly has a high $\chi^{(2)}$-nonlinearity, it is most likely cut for phase-matching in second-harmonic generation to get the beam at 532 nm. It is quite unlikely that the same crystal will have an adequate geometry to achieve phase matching in a reflective OPO or OPA geometry.

It would be better to use a different and dedicated KTP crystal for creating twin beams and even in that case a standard laser pointer is not a good light source for such an experiment.
In other words, since the crystal is likely not cut for this application, I won't be able to generate an appreciable amount of entangled photons with my setup. Thanks for letting me know; I wouldn't want to continue wasting time on something that won't work. If you don't mind my asking, as I understand it if SHG is possible the reverse reaction should also be possible; is the lack of entangled photons in this scenario then the result of the crystal cut favoring a certain phenomenon or am I misunderstanding something? Also, do you have any idea why it would stop producing green light?

Science Advisor
If you don't mind my asking, as I understand it if SHG is possible the reverse reaction should also be possible; is the lack of entangled photons in this scenario then the result of the crystal cut favoring a certain phenomenon or am I misunderstanding something? Also, do you have any idea why it would stop producing green light?

The reverse process is possible, but in this case the crystal is most likely cut such, that both of the photons will end up in the same mode, which is also the mode of the strong IR beam. Usually you would like to have entangled photons in two different modes (momentum, polarization or whatever) to do something useful.

With respect to SHG not taking place, anymore. This could be everything. If you just slightly rotate the crystal out of its standard position, phase matching will not work and there will be no green light anymore. Any small change in distance, focus, crystal orientation or even dust and dirt on the crystal might be the reason for this.

Just as a disclaimer: some light emitted from laser pointers may be quite harmful for the eye, when the pointer is dismantled. Please use appropriate eye safety equipment.

Strange_matter
The reverse process is possible, but in this case the crystal is most likely cut such, that both of the photons will end up in the same mode, which is also the mode of the strong IR beam. Usually you would like to have entangled photons in two different modes (momentum, polarization or whatever) to do something useful.

With respect to SHG not taking place, anymore. This could be everything. If you just slightly rotate the crystal out of its standard position, phase matching will not work and there will be no green light anymore. Any small change in distance, focus, crystal orientation or even dust and dirt on the crystal might be the reason for this.

Just as a disclaimer: some light emitted from laser pointers may be quite harmful for the eye, when the pointer is dismantled. Please use appropriate eye safety equipment.
Wait, does this mean that it may be possible to get it to generate entangled photons, if I'm careful and can produce a laser pointer that can still go through SHG? You said that it would be in the same mode as strong IR beam. Would that mean that entangled photons would be indistinguishable from non-entangled photons, or would be some regions such that pairs of entangled photons would veer off into cones, thus separating them from the IR pump? I want to generate a relatively large number of position-momentum entangled photons that would be distinguishable from the pump. Would it be feasible to do that with my laser pointer?

Science Advisor
Wait, does this mean that it may be possible to get it to generate entangled photons, if I'm careful and can produce a laser pointer that can still go through SHG? You said that it would be in the same mode as strong IR beam. Would that mean that entangled photons would be indistinguishable from non-entangled photons, or would be some regions such that pairs of entangled photons would veer off into cones, thus separating them from the IR pump? I want to generate a relatively large number of position-momentum entangled photons that would be distinguishable from the pump. Would it be feasible to do that with my laser pointer?

SPDC is the opposite process of sum frequency generation and SHG is the special case of degenerate sum frequency generation. A crystal for SHG is usually cut such, that two photons moving along the same direction orthogonal to the surface of the crystal get converted into one photon of higher energy. Accordingly, such crystals would give you down-conversion into two photons that share the same wavelength, which do not travel along cones, but also along exactly the same direction. This is also exactly the mode of the IR pump beam, so for such a crystal all you would get is that a part of the SHG photons may become converted back into the pump beam again, so that the photons are completely indistinguishable from the pump beam.

What you want is a crystal oriented/cut for sum-frequency generation, which would create SHG for two photon beams arriving at different angles to the crystal or for photons with two different energies. In that case, the reverse process would be the kind of SPDC you have in mind. In some cases, this is just a matter of geometry, so the most reasonable way would be to use two different crystals (one for SHG from the pump beam and a second one to perform the SPDC), anyway.