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Entangled Photons Make a Picture from a Paradox
- DanGPhoton 12y agoAbout 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-measurements.asp http://physics.illinois.edu/people/kwiat/interaction-free-me... ) I could never figure out why this did not get more attention at the time
- pyre 12y agoIs 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).
- dang 12y agoAlso https://medium.com/the-physics-arxiv-blog/entangled-photons-produce-quantum-images-of-invisible-targets-they-never-hit-97ea2d275eba https://medium.com/the-physics-arxiv-blog/entangled-photons-... and http://spectrum.ieee.org/tech-talk/semiconductors/devices/quantum-entanglement-camera http://spectrum.ieee.org/tech-talk/semiconductors/devices/qu..., via https://news.ycombinator.com/item?id=8234014 https://news.ycombinator.com/item?id=8234014 and https://news.ycombinator.com/item?id=8234548 https://news.ycombinator.com/item?id=8234548 respectively.
- dchichkov 12y ago""" 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.
- hawkice 12y agoThis 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.
- bkcooper 12y agoI'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.
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- bane 12y agoWould 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)
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- probablyfiction 12y ago"Spooky action" indeed
- lisper 12y agoIt 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: http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf for a more detailed explanation.
- ClayFerguson 12y agolisper, 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.
- waqf 12y agoYou didn't read lisper's link, did you?
- ClayFerguson 12y agoYes I read his comment. Both entanglement and measurement apply to individual particles, so the opposite of what he said is true.
- lisper 12y agoYou 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.
- tucif 12y agoThe preprint of the paper is avalable on the arxiv, in case you don't have a Nature subscription: http://arxiv.org/abs/1401.4318 http://arxiv.org/abs/1401.4318
- mabbo 12y agoThis 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?
- Strilanc 12y agoThe 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 http://arxiv.org/abs/quant-ph/9907007
- debt 12y agoI believe this was posted to HN recently: http://phys.org/news/2013-10-quantum-trajectory-superconducting-bit.html http://phys.org/news/2013-10-quantum-trajectory-superconduct... Or something similar to it. There are few techniques to observe quantum state.
- ClayFerguson 12y agoAs 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.
- debt 12y agoThe 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.
- michael_nielsen 12y agoPlease 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.)
- debt 12y agoWhy is it false then?
- jmaygarden 12y agoBecause 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.
- sologoub 12y agoThat'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!
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- bjornsing 12y agoEh? As I read it it means you can "probe" the film with higher/different energy particles than you probe the sample...
- ClayFerguson 12y agoIt 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.
- shadowmint 12y agoIf 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.
- teekert 12y agoBut 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-from-a-paradox-1.15781 http://www.nature.com/news/entangled-photons-make-a-picture-...)
- axilmar 12y agoThe 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.
- JoeAltmaier 12y agoAre 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.