What is Entangled photons: Definition and 47 Discussions

Quantum entanglement is a physical phenomenon that occurs when a group of particles are generated, interact, or share spatial proximity in a way such that the quantum state of each particle of the group cannot be described independently of the state of the others, including when the particles are separated by a large distance. The topic of quantum entanglement is at the heart of the disparity between classical and quantum physics: entanglement is a primary feature of quantum mechanics lacking in classical mechanics.
Measurements of physical properties such as position, momentum, spin, and polarization performed on entangled particles can, in some cases, be found to be perfectly correlated. For example, if a pair of entangled particles is generated such that their total spin is known to be zero, and one particle is found to have clockwise spin on a first axis, then the spin of the other particle, measured on the same axis, is found to be counterclockwise. However, this behavior gives rise to seemingly paradoxical effects: any measurement of a particle's properties results in an irreversible wave function collapse of that particle and changes the original quantum state. With entangled particles, such measurements affect the entangled system as a whole.
Such phenomena were the subject of a 1935 paper by Albert Einstein, Boris Podolsky, and Nathan Rosen, and several papers by Erwin Schrödinger shortly thereafter, describing what came to be known as the EPR paradox. Einstein and others considered such behavior impossible, as it violated the local realism view of causality (Einstein referring to it as "spooky action at a distance") and argued that the accepted formulation of quantum mechanics must therefore be incomplete.
Later, however, the counterintuitive predictions of quantum mechanics were verified in tests where polarization or spin of entangled particles was measured at separate locations, statistically violating Bell's inequality. In earlier tests, it couldn't be ruled out that the result at one point could have been subtly transmitted to the remote point, affecting the outcome at the second location. However, so-called "loophole-free" Bell tests have been performed where the locations were sufficiently separated that communications at the speed of light would have taken longer—in one case, 10,000 times longer—than the interval between the measurements.According to some interpretations of quantum mechanics, the effect of one measurement occurs instantly. Other interpretations which don't recognize wavefunction collapse dispute that there is any "effect" at all. However, all interpretations agree that entanglement produces correlation between the measurements and that the mutual information between the entangled particles can be exploited, but that any transmission of information at faster-than-light speeds is impossible.Quantum entanglement has been demonstrated experimentally with photons, neutrinos, electrons, molecules as large as buckyballs, and even small diamonds. The utilization of entanglement in communication, computation and quantum radar is a very active area of research and development.

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  1. Q

    A How produce twin photons in practice?

    I would be very thankful if somebody helps me with some practical advices about producing twin photons? I know that twins are produced after a laser ray is directed on a BBO crystal. After the BBO there are two rings of photons and the twins are in the intersection of the rings. 1. I wonder in...
  2. Christian Thom

    I Problem with the polarization of entangled photons

    Consider this thought experiment : we use a source of identically vertically polarized photons, such as produced by a type 0 SPDC. One beam go to Alice and the second to Bob. 1. Whatever measurement Bob makes on its beam, if Alice use a vertically polarized detector, all photons are detected...
  3. U

    I Does entanglement of photons really facilitate telepathy?

    [This](https://youtu.be/9l6VPpDublg) is a lecture by Persinger where he postulates that photon entanglements and the Earth's magnetic field can facilitate telepathy. (See from 20:00 to 30:00) Links to the study he cited...
  4. O

    I Bell's Inequality => 4 entangled Photons impossible?

    Correlation between polarization measurements of entangled photons at angles less than 45 are greater than classically statistically possible. No set of hidden variables can be preordained to explain the 75% correlation of photon measurements at 30 degrees and complete anticorrelation of...
  5. Leonardo Muzzi

    I Quantum Eraser: do entangled photons arrive at different moments?

    Considering a Delayed Choice Quantum Eraser experiment such as the one in the picture above: It is said that paired photons arriving at the detectors A,B,C,or D, reach the detectors after the entangled pair has reached the interference screen. Are we actually able to measure the time interval...
  6. K

    I Steering the polarization of entangled photons

    The following is a diagram of an EPR thought experiment using photons that uses the same type of components as described in this paper http://www2.optics.rochester.edu/~stroud/cqi/rochester/UR19.pdf Would you detect any photons at the right detector? I want to be convinced one way or another...
  7. S

    I Trying to Generate Entangled Photons

    I've been trying to generate entangled photons using KTP laser pointer that normally produces green light at 532 nm. I was hoping that after removing the IR filter, I could add a reflective coating (right now the coating I have is a glittery metallic green nail polish) to reflect the green light...
  8. Joseph1785

    B Do Entangled Photons React to possible Photon-Photon Collisions?

    Summary: The limitations of engagement and possible results of the Breit-Wheeler theory collider in relation. *You will need to read the article for this to make sense Upon reading an article "Scientists discover how to turn light into matter after 80-year quest" I had a thought, How will...
  9. J

    B Entangled Photons: Adam Becker's QM Book "What is Real?

    I'm reading Adam Becker's QM book "What is Real?" At one point he says: "The single wave function shared by the two entangled photons guarantees that they will always behave in the same manner when encountering two polarizers with parallel axes. It does not specify what they will do. But merely...
  10. B

    I Phase shift after double-slit for entangled photons

    Hi all, I'm trying to understand how to describe the quantum state of entangled photons, including their phase, if one of them encounters a double-slit. Here's a simple example: Suppose you have two polarization-entangled photons A and B in the following Bell state: \begin{equation}...
  11. B

    Quantum state of entangled photons

    Homework Statement Suppose two polarization-entangled photons A and B in the following Bell state: \begin{equation} \Phi=\frac{1}{\sqrt{2}}\bigl(\left|H_{A},H_{B}\right\rangle + \left| V_{A},V_{B}\right\rangle\bigr) \end{equation} 1. What is the state if the photon A passes through a...
  12. C

    I Entangled Photons in Opposite-Configured Mach–Zehnder Interferometers

    So, I have a pretty basic (for this forum, not for me) question: how will two entangled photons behave in the opposite-configured Mach–Zehnder interferometer systems? One of the photon goes through the closed system while the other goes through the open one. Do they simply lose their...
  13. K

    B Why 50% of entangled photons pass through the polarizer?

    Hi. If we have 1 million entangled photons separated from their "entangled partner". We send all those photons (without their "entangled partners") through a polarizer. Each photon has has 50% chance of passing through the polarizer. So 50% of the photons will pass through the polarizer and...
  14. K

    B Exactly why FTLC is impossible with entangled photons?

    It’s written almost all physics books and courses that entangled photons can’t be used for faster than light communication. The question about used entangled photons to communicate faster than light has been discussed before, but I didn’t get exactly why faster than light communication with...
  15. C

    A Simple way to create entangled photon pairs?

    Two photons arrive at a hypothetical 50:50 Beam-Splitter with no phase shift between reflected and transmitted modes. One enters the Left side and the other the Bottom side of the BS as shown in Fig.1 of the link below: https://drive.google.com/open?id=0B5JsDLKoUSA5emk5Qk9nUHVIelE Each photon...
  16. C

    Can entangled photons be used to explore black holes?

    Gabriela Lemos and her team successfully entangled photons. Would it be possible to explore the interior of a black hole by letting one of the entangled photons enter beyond the event horizon and observe the impact on the other?
  17. N

    I Are two entangled photons described by the same wavefunction?

    Are two entangled photons described by the same wave function or wave function shape? Heres an example... Say for example, we have a laser in TEM01 mode that is shooting individual photons (this mode as two distinct maxima). Then the individual photons are going through a BBO crystal to become a...
  18. J

    B Entangled photons thought experiment

    Following scenario: One million entangled photon pairs are created and are sent to Alice's and Bob's laboratory such that Alice receives 1 million photons(or any other large number) of which each is entangled to one of the million photons Bob receives, their laboratories being at a distance to...
  19. yeezyseason3

    Asynchronous Entangled Photon pairs

    Is it possible to produce an entangled photon pair and be able to delay the reception times by introducing something like a fiber bragg grating which has multiple indices of refraction which propagate wavelengths at different frequencies? Or will, because of their entangled state, do some...
  20. entropy1

    Probability of entangled photons passing filter independent?

    Consider two polarisation-entangled photons A and B fired at two polarisationfilters that are at a certain angle α. Are the probability that A is passing its filter and the probability that B is passing its filter indepedent probabilities? I am aware that is probably an incredibly stupid...
  21. sciencejournalist00

    Can calcium ions be used as a source of entangled photons

    By heating calcium in a tantalum oven and subjecting the calcium vapor stream to blue laser, calcium atoms absorb the blue photons and emit two photons of lower energy as they decay twice. Once they decay from the highest excited state to an intermediate excited state, and then from the...
  22. M

    Entangled Photons: What Happens to Photon B? Can Two Combine?

    If a photon A is entangled with photon B and one somehow destroys photon A, what will happen to photon B? Will it also get destroyed? And can two entangled photons combine into one?
  23. DrChinese

    A Another loophole-free test of Bell's theorem

    This just showed up from a team led by Zeilinger, for those interested in loophole-free Bell tests: http://arxiv.org/abs/1511.03190 A significant-loophole-free test of Bell's theorem with entangled photons Marissa Giustina, Marijn A. M. Versteegh, Soeren Wengerowsky, Johannes Handsteiner...
  24. J

    Entangled photons and their Parents

    I am curious if photons are quantized because their 'parent' particles are quantized, and the photons are entangled with their parents. In other words, if you were to have two entangled particles emit a photon(s) that is entangled in energy somehow, do they emit a single photon, two photons...
  25. S

    Measuring properties of entangled photons

    I am not clear on the below, so any additional information would also be good. Let's say a pair of photons is entangled on 2 properties. When we measure on property A -- the values for property A (for both the entangled photons) becomes determinate/locked/fixed when we measure on property B...
  26. P

    Creating radiowave entangled photons

    I would like to create entangled photons at radiowave frequencies. To do this I thought it might help to understand as much details as possible how entangled photons are created by parametric down-conversion. Since the down-conversion doesn't happen often, what are the special conditions? Are...
  27. X

    Question about SPDC (Making entangled photons)

    Hello everyone, I have a question about the process of spontaneous parametric down conversion (or SPDC). This process is used to create two entangled photons in the visible regime by shining a laser through a beta-barium borate (BBO) nonlinear crystal. My question is, can this process be...
  28. N

    Coherence time of parametrically downconverted entangled photons

    What is the coherence time for a pair of entangled photons produced in a nonlinear crystal? Is it related to coherence time of the pump photon? Also, if we say that the coherence time of the two photons is T, then does it mean that the two photons can interfere with each other even if their...
  29. 0

    Entangled photons in double slit experiment

    Hi, imagine setup with source of entangled photons (A and B). Photon A travels to double slit so that either wave or particle pattern can be observed, photon B hits detector either before (setup 1) or after (setup 2) photon A has hit screen. Question: Will setup 1 show interference pattern...
  30. Dadface

    Entangled photons and polarisation.

    I have been looking up the practical details of certain quantum eraser type experiments and got stuck on a couple of points. I refer to a particular experiment which can be found by googling: "A DOUBLE SLIT QUANTUM ERASER EXPERIMENT" (Sorry I don't know how to include the address) In...
  31. G

    Why does a BBO crystal create entangled photons?

    Hi, If we pump a beta barium borate (BBO) crystal, we get one circle of vertically polarized photons ##\left|V\right\rangle## and an intersecting circle of horizontally polarized ones ##\left|H\right\rangle##: http://quantum.ustc.edu.cn/old/img/image002.gif At the intersection points of the...
  32. Dadface

    Detecting Entangled Photons: Principles and Experimental Methods

    Hello all, I have been searching and trying to find out how entanglement is detected experimentally, but I have a few sticking points. Initially I just want to know the principles behind the experiments. In summary this is how I understand it so far. 1. Entangled photons produced eg in pairs...
  33. R

    Possibility of million entangled photons

    Entangled photon-pairs are being produced in labs with 'Spontaneous Parametric Down Conversion' apparatus. While in the core of our sun gamma photons are being produced through nuclear fusion which then down-converted into myriad of low-energy photons. Is it possible that those myriad of...
  34. DrChinese

    Double-slit interference pattern measurement with entangled photons

    Although it is well known that a series of individual photons will build up an interference pattern in a double slit setup, there have been few (if any) experiments where such a pattern is built up by sending photons through one at a time. Here is a great experiment from one of the top quantum...
  35. C

    Measuring the polariztion of a pair of entangled photons

    Hi, I'm quite new to quantum mechanics, learning about it in my free time in a life-long learning fashion :) I've been trying to find a solution to a problem for some time, and the results I included below appear to be OK, but I have doubts about the method I used, so any help or guidance...
  36. I

    Entangled photons in bell experiment: transfer phase or angular momentum?

    Regarding the polarization correlation studies generated using parametric down conversion. All the studies appear to be done correlating the polarization of linearly polarized photons. Has any experiment been done showing the same effect with circularly polarized light? 1) If this...
  37. StevieTNZ

    Two Groups of Entangled Photons

    If we have two groups of photons; each group consisting of two entangled photons. We allow one of the photons from each group to interact with another object. If we perform a polarisation measurement on each photon of one group, will the photons in the other group, independent of whether...
  38. StevieTNZ

    Entangled Photons: V/H & 45/135 Polarisations

    When you create a pair of entangled photons, are they simultaneously in a superposition of V and H, and 45 and 135 polarisations?
  39. thenewmans

    Opposite spin of entangled photons

    Quick question: Do entangled photons have opposite spin? If so, how do we know? I mean I know that they should because of conservation of angular momentum. But Cos(0)^2 = Cos(180)^2. So it looks like they’re the same. Or am I missing something.
  40. DrChinese

    What happens to Entangled Photons after a polarizing beamsplitter (PBS)?

    SpectraCat and I have been discussing what happens to entangled photons after encountering a polarizing beamsplitter (PBS). A beer rides on the outcome (in addition to one I already won), so you know I can't let this go. SpectraCats says: collapse of the wave function occurs at the PBS. I...
  41. N

    Alain Aspect's Experiment: Investigating Non-locality and Entangled Photons

    Alain Aspect's experiment: In the Alain Aspect experiment, the two entangled photons will indeed both go through their respective polarizers. I'm hip with that. I'm also hip with Aspect's diligence in making it a "two-channel" testing apparatus. What I don't understand is why should...
  42. C

    A very simple question re: entangled photons

    Hi I've been struggling to understand entanglement for some time now, and find that most explanations assume some basic knowledge that I don't have. Please could somebody help clear up a very basic query for me? If two photons are entangled, and we have no knowledge of their state of...
  43. F

    Are Entangled Photons the Same Particle?

    Has it or can it be proven whether a "pair" of entangled photons are really the same photon (or not)? Thanks, FRQ
  44. A

    Generation of polarization entangled photons

    Hi, What crystal do I need to generate polarization entangled photons? I read BBO (beta-BaB2O4) crystal are needed but when I call the engineers at commercial companies and mention the term "entangled photons" it goes way over their heads and they just say they don't make this crystal. I...
  45. DrChinese

    Entangled Photons with no shared Past

    OK, here is some pretty wild stuff about entangled photons. In an experiment reported in the Preprint Archives last week, a team of distinguished physicists are continuing to push the bounds of what we can understand as "entanglement". They demonstrate entanglement of 2 photons which have...
  46. B

    Does a beam of entangled photons create interference fringes?

    Does a single beam of entangled photons create interference? Hi, This question has come up in an FTL thread going on right now. I've seen two different answers to this question, and I'd be interested if anyone knows the real answer. Here's the setip: shine a laser beam on a crystal which...
  47. Z

    What are Quantum Entangled Photons and how are they produced?

    What are Quantum Entangled Photons and how are they produced?