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Home Experiments Bell's Inequality Test Type I Downconversion Beta-Barium-Borate (BBO) Crystal Array for diy Entangled Photon Source
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Type I Downconversion Beta-Barium-Borate (BBO) Crystal Array for diy Entangled Photon Source

Photop BBO crystal array for diy entangled photon sourceOur diy entangled-photon source, shown in the book’s Figure 142,  uses two BBO crystals that support type I down-conversion that are mounted according to a design by Paul Kwiat and his colleagues at the Los Alamos National Laboratory.

The nonlinear crystal in our photon entangler comprises two 5 mm x 5 mm x 0.1 mm BBO crystals mounted face-to-face at an angle of 90 degrees to each other. As shown in the book’s Figure 140, pump photons polarized at 45 degrees produce two cones of entangled down-converted photons.

Photop BBO crystal array for diy entangled photon source

 

Photop Certificate of Compliance for Beta Barium Borate BBO crystal array for diy entangled photon source

 
 

Please visit www.prutchi.com and www.diyPhysics.com for other cutting-edge d.i.y. projects, and remember to check out our new d.i.y. Quantum Physics book:

 
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5 Comments  comments 

5 Responses

  1. Christian Buhl Sørensen

    I truly LOVE your book.

    But what was the price of your BBO-crystal? As an elective in my 5 semester, I am doing a project about single photon interferometry, and i have a hunch, that a setup with a crystal like this would greatly improve my data.

    It’s really your book that inspired me to do it, so thank you very much!

    • Hi Christian,
      Thank you very much for your kind words. It is very encouraging for us to learn that our book is having a positive impact with young scientists and engineers like you.
      Regarding the BBO crystal that we used, it cost around $1,400. Unfortunately, many of the components in Chapter 8 have not yet been integrated into consumer products, so their price is still high. However, we fully expect to see prices drop very drastically in the next few years as these components become part of mass-produced gadgets.
      Again, thank you very much for your nice words about our book.
      Cheers,
      David

  2. [...] gamma-ray photons. A small disk of radioactive sodium-22 runs $80, compared to $1,400 for a source of entangled optical photons. When the sodium atoms decay, they give off antielectrons, or positrons, which in turn annihilate [...]

  3. Erik Ayer

    I also found your book highly inspiring, and in fact it was the push I needed to start working on an experiment based on Dr. John Cramer’s work in nonloclity.

    What I desperately need to know is the rate that entangled pairs are generated with BBO cristals, particularly with two mounted at 90 degrees. Somewhere, I ran across a page that said about 1 in 1E10 photons are downconverted. Another page, somewhere, mentioned that as many as 40% of downconverted photons are entangled. Using a 300mW blu-ray laser diode, this would result in ~68k/sec entangled pairs, which is definitely above the dark count specifications for PMT or APDs.

    Is this anywhere near accurate? More importantly, are the sources of information on the web for downconversion and entanglement rates? I’ve had a very hard time finding anything.

    Thank you!

    • Hi Erik,
      Thank you very much for your kind comments.
      Regarding the yield, in page 213 we mention that a rough estimate for the number of entangled photons per second produced by the Photop two-crystal stack is approximately pump power [mW] x 6 x 10^4, but there are some additional constraints that you have to keep in mind which take the number of detectable photons down to the region where only SPADs are really effective:
      1. You want to work only with degenerate photons (those that fall within the cone where each photon in the entangled pair carries half the energy)
      2. Photons get absorbed along the way by the polarizers and collection optics
      3. Detecting IR photons with PMTs – even those sensitive to IR – usually requires chilling because of the high dark counts in the low energy side of the spectrum. Besides, PMTs have horrible quantum efficiency, especially in the IR.
      4. With two detectors you collect only a small aperture of the cone, and you do want to keep the aperture very tight to ensure that you count only entangled pairs
      Bottom line is that with *VERY CAREFUL* alignment, and using good optics you can expect around 100 to 200 coincidence counts (detected entangled photons) per second with a >100mW pumping and with the irises closed enough to ensure sufficient purity.
      Best Regards,
      David

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