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.
My thinking are implications in satellite imaging, security, medicine, etc.
(I have basically a caveman understanding of this topic, so clarification would be helpful)
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.
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.
"Physicists have devised a way to take pictures using light that has not interacted with the object being photographed."
Then in another paragraph it says:
"In the first path, one photon in the pair passes through the object to be imaged"
So the light actually has interacted with the object to be photographed. The opening paragraph is obviously wrong.
Furthermore, another paragraph says:
"In ghost imaging, even though only one photon interacts with the object, both photons need to be collected to reconstruct the image, whereas in the Vienna team's work only one photon needs to be detected"
Then another one says:
"The remaining photon from the second path is also reunited with itself from the first path and directed towards a camera"
So again, the article contains a huge ambiguity, because in one paragraph it says that only one photon is needed for the photograph and then it says that one photon needs to be reunited with another photon (even if they are entangled, they are still different photons), so two photos are needed for the photograph.
Or is this not the way this works? Are the entangled pairs not actually used to build the image? I find it difficult to understand what they do exactly although the Nature site seems pretty clear:
"This form of imaging uses pairs of photons, twins that are ‘entangled’ in such a way that the quantum state of one is inextricably linked to the other. While one photon has the potential to travel through the subject of a photo and then be lost, the other goes to a detector but nonetheless 'knows' about its twin’s life and can be used to build up an image."
> We show that "interaction-free" measurements can be regarded as counterfactual computations, and our results then imply that N [the number of times that the computer is not run] must be large if the probability of interaction is to be close to zero. Finally, we consider some ways in which our formulation of counterfactual computation can be generalised.
Or something similar to it. There are few techniques to observe quantum state.
That would be a really exciting application for imaging in noisy envrionments.
for a more detailed explanation.
> actually measurement is the act of collapsing the wave, and entanglement only exists before the wave function collapse. It's possible to never measure something, and it could be entangled forever.
Entangled particles continue to be entangled even after they are "measured". If this were not true, faster-than-light communication would be possible. To understand why, read the paper.
The only way to "undo" an entanglement is to time-reverse the process that created the entangled pair to begin with, i.e. to bring the members of the entangled pair back together.
one can't wonder why for example the beam L4'-L5 isn't filtered out - if this beam does carry information about the image then we would have FTL communication possible.
Another question is why L3-L3' red beam - the image info carrying beam - is fed into NL2 crystal thus providing a chance to physically affect formation of NL2-D5-L6 yellow beam, ie. to exchange information. Some of the red beam photons entering NL2 would be consumed and re-emitted by the NL2 crystal's atoms (and the location of these atoms will be affected by the red beam internal density ie. by the image of the object "O") and thus would physically affect the yellow beam generation, i.e. passing some image info into the yellow beam. Thus no entanglement or other "spooky" QM features necessary for the NL2-D5-L6 yellow beam to become carrying some image info.
See Michael Nielsen's answer elsewhere in the comments for why this is not the case.
Another question is why L3-L3' red beam - the image info carrying beam - is fed into NL2 crystal thus providing a chance to physically affect formation of NL2-D5-L6 yellow beam, ie. to exchange information.
The interference requires us to not know which path the photon took way back at BS1, in that diagram. The probability of pair formation in the nonlinear crystals is quite low, so with very high likelihood, only one of the crystals will be responsible for producing a pair of photons (including the yellow signal photon.)
Suppose you did not route the imaging photon through NL2. Then observing an output photon at D3 would tell you that the pair production happened at NL2 (since there is no way for the imaging photon to get there.) This will destroy the interference pattern of the "yellow" photons. That is why the beam is sent through NL2.
why do you need interference pattern in situation when the imaging beam is sent through NL2? Sending the imaging beam through NL2 causes it to transfer the image info into the yellow beam formed in NL2 (the red beam heats the crystal and excites the crystal's atoms which affects yellow beam formation) and thus makes all the discussion of interference and entanglement moot and unnecessary. The other yellow beam can be just discarded as the yellow beam from NL2 alone is enough to produce the picture.
(For this kind of thing, stating credentials seems useful: I was a professional quantum physicist for 13 years, much of that time spent working on the theory of entanglement, and entanglement-based effects such as quantum teleportation.)
If you're asking in general, the very brief summary is this:
(1) In both relativistic and non-relativistic formulations of quantum mechanics, it's possible to prove general no-go theorems which show that measurement of entangled states convey no information whatsoever.
(2) In non-relativistic quantum mechanics, the Schroedinger equation has solutions which can propagate information faster than light. However, those solutions disappear in proper relativistic formulations of quantum mechanics. Physicists regard the non-relativistic solutions as mathematical artifacts: mathematically interesting, but not of any direct physical interest.
(3) People have constructed non-linear variants of quantum mechanics in which it is possible to send information faster-than-light using entanglement. These are toy models - a little like saying "But what if I had a box that could solve the halting problem" in computer science - and, to put it mildly, they are not regarded as promising candidates for physical theories. Not just on account of the FTL problems, either, but also problems like violation of energy conservation, and violations of the second law of thermodynamics. Fun toy theories, but not serious proposals.
"The remaining photon from the second path is also reunited with itself from the first path and directed towards a camera, where it is used to build the image, despite having never interacted with the object."
So, they need both the particles together. It doesn't work if one is near Alpha Centauri and cannot be combined with the one on Earth. I'm a complete layman, but this is my understanding of it.
Apparently, the breakthrough here is being able to use different wavelength and intensity of light to interact with an object than that desired for imaging. No information is actually transferred through entanglement.
Entangled particles start off in a superposition of possible states, when we measure the state of one particle, we can deduce the state of the other particle because we know that the superposition collapses for both particles when the state of either particle is measured, however, no classical information is actually communicated between the particles at the time of measurement.
Just like how the the collapsing wave function in the double-slit experiment appears to "reach back in time" to destroy the interference pattern, a similar effect occurs when the superposition of our entangled particles collapse. Scientists have ruled out the idea that a "hidden variable" secretly stores the true state of the entangled particles prior to measurement, but for the purposes of communicating classical information, this might as well be the case.
Now you give one of the books to a space traveller, who gets on a rocket that will take him 10 light years away from Earth. You also agree, that once he is there, both you and him will look at your respective books and look at the same time and it's value. So he goes away, and then when the time comes, you open your book, look at the time and read it's value - it's 1, so that means that his book must say 0. Now, there is no "transfer" of information involved - you have not communicated with someone 10 light years away. The information was encoded in the book when it was created, not when you read the information. The same with quantum entanglement - it's already decided when the particle is created,not when you look at it - so it doesn't matter how far the particles are apart, you are just reading pre-defined information.
Edit: no, I am not a physicist either, but I read this explanation somewhere and I think it explains it fairly well.
Some theoretical answers have been given already.
From a practical perspective: In order to transmit any meaningful data through a quantum-entangled channel, you always need an additional classic channel as well. QE alone cannot transmit any information. The only reason then to use QE for data transmission is that it makes the transmission tamper-proof.