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Comment by dchichkov | Hacker News Reader

"""

  One advantage of the technique is that the two photons
  need not be of the same energy, Zeilinger says, meaning
  that the light that touches the object can be of a
  different colour than the light that is detected. For
  example, a quantum imager could probe delicate biological
  samples by sending low-energy photons through them while
  building up the image using visible-range photons and a
  conventional camera. The work is published in the August
  28 issue of Nature.

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Would this mean then that I could "image" something with a wavelength that makes things like walls, clothes or skin transparent and then see them with visible wavelengths? For example, use entangled infrared photons to see through water vapor (clouds) but end up with visible wavelength images of my subject?

My thinking are implications in satellite imaging, security, medicine, etc.

(I have basically a caveman understanding of this topic, so clarification would be helpful)

This is by far the most interesting part. I was always taught that to get the interference you had to have identical particles -- and I was surprised when I heard this is how we know to treat e.g. two different photons the same, because two different photons in the same location etc. can interfere with each others' probability. But if there is an extremely clear difference between particles that can still generate these patterns, it means things like hidden variable theories have a much better ring to them.

I've only skimmed the arXiv link briefly, but the method does not rely on interference between photons of two different frequencies. In optical experiments, entangled photons are frequently generated through a nonlinear process in a crystal. High energy photons go in and with some probability lead to the production of two lower energy photons. Energy and momentum conservation lead to constraints on what these can be, such that if you've measured one photon you know all about the other. This is how they are entangled.

In the setup of this experiment (see Fig. 1 of the arXiv link), there are two conversion crystals labeled NL1 and NL2. These convert the pump laser into two longer wavelengths, referred to as the signal wavelength and idler wavelength. The object is probed at the idler wavelength, but the interference depends only on photons at the signal wavelength.

it is already possible with existing devices - like night vision googles for example, where one kind of photons hit some electronic photon capturing device, CCD sensor for example, and the CCD would generate electric current which can be used to generate another kind of photons on the screen.

Similar, in principle, information transfer is happening in this experiment too - see my other comment https://news.ycombinator.com/item?id=8235764 - image carrying beam physically affects crystal (it is IR after all, and it does heat up the crystal and thus the distribution of the heating up of the crystal is also affected by the object being imaged) where photons used to generate final image are formed.

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