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Entangled Photons Make a Picture from a Paradox (scientificamerican.com)
100 points by foolrush on Aug 27, 2014 | hide | past | web | favorite | 44 comments


  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.

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.

The preprint of the paper is avalable on the arxiv, in case you don't have a Nature subscription: http://arxiv.org/abs/1401.4318

The first paragraph of the article says:

"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.

Are they entangled, or just potential photons? When splitting light, photons go 'both ways' potentially even though is a classical sense there is only 1 photon that will ever be detected. Recombining them is still meaningful in a wave sense, because each potential photon path interacts with the other when the paths are reunited.

But what if I keep the entangled photon around for very long while separating the two by a large distance. Then I image the cat using the photons I kept around which forms a cat image at the very large distance. Very spooky action.

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." (http://www.nature.com/news/entangled-photons-make-a-picture-...)

About 15 years ago I saw Paul Kwiat at UIUC give a talk on "Interaction Free Measurements" which seems to be effectively the same thing as here. (cf http://physics.illinois.edu/people/kwiat/interaction-free-me... ) I could never figure out why this did not get more attention at the time

Is this really "Interaction Free"? You're hitting it with photons. They comment on using low-energy photons, but that just minimizes the interaction, while still producing something useful (visible spectrum image).

This being hacker news, I can't help but wonder about the implications for quantum computing. Maybe this is already an answered question, but can one read a Q-bit without interaction that might break it's entanglement?

As far as I have ever heard, you must collapse the wave function to measure a Q-bit, and that is when entanglement always stops. However if we can create systems where a collapse can always 'retrigger' a new entanglement we could theoretically create computers that run at the Planc Time as the clock rate, but we don't know how to do that yet. That's decades off, or centuries.

The term you're looking for is counterfactual computation [1]:

> 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.

1: http://arxiv.org/abs/quant-ph/9907007

I believe this was posted to HN recently:


Or something similar to it. There are few techniques to observe quantum state.

If this works as described I wonder if it could be used to distinguish incoming scattered entangled photons from other incoming 'noise' photons.

That would be a really exciting application for imaging in noisy envrionments.


Eh? As I read it it means you can "probe" the film with higher/different energy particles than you probe the sample...

It implies that now we can do detection experiments at higher frequencies (higher energy) which therefore means more precision than before. To probe things with higher precision normally means you need higher energy/frequency, but this experiment implies now we can detect at theoretically infinitely high frequencies? Something is off. There has to still be a dependency with frequency of light in any experiment. However I think Time itself is quantized (no I can't substantiate that), and therefore the Planc Time is the limit of the highest frequency measurable, so that frequency is the limiting factor for both energies and frequencies.

"Spooky action" indeed

It actually isn't spooky at all once you understand it properly (but it is almost never explained properly, especially in the popular press). Measurement and entanglement are really the same phenomenon. The process of "measuring" photon A was actually begun when it became entangled with photon B (and vice versa). We call it a "measurement" when a large number of particles (like a measurement apparatus or a brain) become mutually entangled with each other. See:


for a more detailed explanation.

lisper, 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.

You didn't read lisper's link, did you?

Yes I read his comment. Both entanglement and measurement apply to individual particles, so the opposite of what he said is true.

You didn't read waqf's comment, did you? You may have read my comment but you clearly didn't read my link.

> 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.

You are correct, I didn't read your link. Why? Because every word of what you said in your comment was wrong. So why would I click the link?

I dunno, because you might learn something?

it is actually pretty spooky setup. May be there is a Nobel Prize lurking there, yet looking at the Figure 2 of their arxiv paper http://arxiv.org/ftp/arxiv/papers/1401/1401.4318.pdf

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.

if this beam does carry information about the image then we would have FTL communication possible.

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.

>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.

The implications of faster than light communication should not be understated. Using quantum entanglement, you could "send"(whatever that means) a text to someone near Alpha Centari faster than sending a text to someone in Zimbabwe.

Please don't repeat this. It is false.

(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.)

Why is it false then?

If you're speaking specifically of this experiment, nowhere in the article does it say that it's using entanglement to do faster-than-light communication. So I don't think there's anything to explain.

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.

Because both photons in the pair must be correlated. From the article:

"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.

That's how I read this as well. That said, the medical applications are amazing - they might be able to do MRI-like (or better?!) imaging without the harmful effects!

MRI imaging is effectively harmless (unlike, say, CT scans, which might cause harm).

I'm not a physicist, but as far as I understand, it just doesn't work like that.

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.

Think about it this way. You create two books with a table - in one column, the time, in another column, a value, either 0 or 1. They are identical, except that the 0s and 1s are switched in both books.

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.

> Why is it false then?

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.

This doc is great. Thanks for taking the time to write it out.

My pleasure. Glad it was useful.

I will just call you a troll right off the bat. Entanglement & FTL is such a well known meme, and you make such an obviously false statement. :)

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