# B How do scientists work with nonlinear crystals

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1. Jan 17, 2016

### sciencejournalist00

When I point a 5 milliwatts red laser at a pile of barium borate crystals, all I get is red speckles of scattered light.
When I point a 100 milliwats blue laser at the same pile of barium borate crystals, I get blue speckles.
When I point a 2000 milliwats green laser at the pile of barium borate crystals, I get green speckles.

Scientists say barium borate should decrease the frequency of transmitted laser light by 50%.

How come spontaneous parametric down conversion I do not observe?

Whether I use a single crystal or more, I get the same result. Speckles of scattered light of the same colour.

2. Jan 17, 2016

### sciencejournalist00

I bought the crystals a year ago. I tried every day and I got the same result. No frequency change

3. Jan 17, 2016

### sciencejournalist00

Did you ever work with these crystals in the right way? Can you tell which laser intensity you used to get a frequency change

4. Jan 17, 2016

### Mentz114

5. Jan 17, 2016

### sciencejournalist00

I know how the phenomenon is called and that it requires laser light and special crystals, but I do not understand how to do it practically. What intensity is needed to observe the change of light colour from higher to lower frequency for barium borate?

6. Jan 17, 2016

### sciencejournalist00

https://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion

In a commonly used SPDC apparatus design, a strong laser beam, termed the "pump" beam, is directed at a BBO (beta-barium borate) crystal. Most of the photons continue straight through the crystal. However, occasionally, some of the photons undergo spontaneous down-conversion with Type II polarization correlation, and the resultant correlated photon pairs have trajectories that are constrained to be within two cones, whose axes are symmetrically arranged relative to the pump beam. Also, due to the conservation of energy, the two photons are always symmetrically located within the cones, relative to the pump beam. Importantly, the trajectories of the photon pairs may exist simultaneously in the two lines where the cones intersect. This results in entanglement of the photon pairs whose polarization are perpendicular
SPDC is stimulated by random vacuum fluctuations, and hence the photon pairs are created at random times. The conversion efficiency is very low, on the order of 1 pair per every 1012 incoming photons.However, if one of the pair (the "signal") is detected at any time then its partner (the "idler") is known to be present. The output of a Type I down converter is a squeezed vacuum that contains only even photon number terms. The output of the Type II down converter is a two-mode squeezed vacuum.

Too low conversion efficiency?

7. Jan 17, 2016

### kith

If you search the arxiv for "spontaneous parametric down-conversion" you get a lot of papers. Let's take this for example. In the section "experimental results" they state that they use an Omicron LDM405.120.CWA.L.WS laser. Googling this leads to the manufacturer website where the specifications of the laser are stated. So they seem to use 300 mW @405nm (cw).

8. Jan 17, 2016

### sciencejournalist00

Both types of crystals are thin (0.8 mm) in comparison to the Rayleigh length and compared to the transvers eextent of the imaged rings - a requirement for polarization entanglement. As the crystals become thick, which-path information begins to degrade the entanglement because the rings become distinguishable.
cause the rings are elliptical, they no longer perfectly spatially overlap around the entire ring, leading to potential degradation in entanglement-purity, and limiting the
ability to multiplex many channels around the down-conversion ring. Lack of spatial overlap is the most obvious way in which which-path information is
revealed. A smaller overlap integral represents lower entanglement quality because the dierence in spectra between the crystals means it is possible
to distinguish which crystal the photons came from.

http://arxiv.org/pdf/1503.04676v2.pdf

So that is why I got speckles

9. Jan 17, 2016

### sciencejournalist00

I still get speckles.I tried it over all range of sizes of crystals. I do not get frequency conversion. I really do not know what is happening.

I bet those scientists cannot see the converted photons with their naked eye among unconverted photons. Do they use detectors instead?

10. Jan 17, 2016

### Staff: Mentor

That's right.
And even detectors are not sufficient, between sensitive detectors also respond to pretty much every photon that comes by. If you read the actual scientific papers (such as the one that DrChinese pointed you at in your other thread) you'll find a description of the fairly sophisticated methods (often involving coincidence counters) used to pick the detections of converted photons out from the all the rest.

11. Jan 17, 2016

### Staff: Mentor

The downconversion happens with a tiny fraction of the total number of photons. You will never see this with your eye, you need some very effective way to block the light from the laser wavelength to have some chance to see anything.
You might get the conversion, but you just don't see it. A related example: stars are present during daylight, but you also don't see them without specialized equipment.

12. Jan 17, 2016

Staff Emeritus
Right. And if you downconvert red light, you won't see it no matter how much of it you have.

13. Jan 17, 2016

### DrChinese

Here is a great reference for the details of creating entangled pairs. This is as basic a "how to" as you will likely find.

http://arxiv.org/abs/quant-ph/0205171

We use polarization-entangled photon pairs to demonstrate quantum nonlocality in an experiment suitable for advanced undergraduates. The photons are produced by spontaneous parametric downconversion using a violet diode laser and two nonlinear crystals. The polarization state of the photons is tunable. Using an entangled state analogous to that described in the Einstein-Podolsky-Rosen paradox,'' we demonstrate strong polarization correlations of the entanged photons. Bell's idea of a hidden variable theory is presented by way of an example and compared to the quantum prediction. A test of the Clauser, Horne, Shimony and Holt version of the Bell inequality finds S=2.307±0.035, in clear contradiciton of hidden variable theories. The experiments described can be performed in an afternoon.

And as with the other thread: you seem to be following much of what you are reading but are missing key items. You made a comment about seeing entangled photons with the naked eye, and then fail to ask how you would pick out the 1 in a trillion that meets the entangled criteria in that case. The answer to that is to use filters (to allow only the entangled wavelengths to pass); off-angle collection (as entangled pairs are bent off angle slightly); high intensity; etc.

Also: not all BBo crystals work equally well with all lasers.

14. Jan 17, 2016

### Spinnor

15. Jan 17, 2016

### Mentz114

16. Jan 18, 2016

### sciencejournalist00

I got it - I cannot see the converted photons because the intensity of the converted light is very low and cannot be seen with the naked eye. I have to use detectors.

However I wanted to entangle multiple photons at the same time, so if the beam splitter were the only thing needed, I already have a multi-port beam splitter ordered from China that can make photons follow many trajectories at the same time, making them interfere.

17. Jan 18, 2016

### sciencejournalist00

But infrared can be absorbed by my skin and be felt as heat. However the red laser is also absorbed by my skin and felt as heat, so I dont make any difference between the heat sources.

18. Jan 18, 2016

Staff Emeritus
I doubt you can feel the heat from a 5 mW laser. (5 mW is ~ 4 calories per hour). Since energy is conserved, downshifting doesn't change the heat produced.

19. Jan 18, 2016

### sciencejournalist00

I have both 5 mW, 100 mW and 2000 mW lasers and I can feel heat from all of them.

Down-converted photons are infrared if the laser is red and I cannot distinguish the heat from infrared light from the heat of red light.

20. Jan 18, 2016

### sciencejournalist00

Even if I get down-converted light, I cannot entangle matter qubits with it because the other non-converted photons will create quantum decoherence and destroy the original entanglement.

However, if I excite matter qubits with lasers and wait for fluorescence light to be emitted and then superpose fluorescence light from multiple matter qubits on a beam splitter and use detectors to herald the outputs, I may get entangled matter.