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Quantum Entanglement Experiments Expand to Include 8 Photons

The views expressed are those of the author and are not necessarily those of Scientific American.


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Laser photo: FastLizard4/Flickr

The quantum phenomenon known as entanglement keeps spreading its arms to hold ever more particles in its spooky embrace.

Quantum entanglement is an effect through which multiple particles share correlated properties—across arbitrarily large distances—that snap into place instantaneously. For instance, a pair of entangled photons in different locations might be joined by their polarizations, a property that describes the orientation of a light wave’s oscillation. Measure one photon’s polarization, and the polarization of the other instantly assumes the same value. In other words, the photons are either both horizontally polarized or both vertically polarized, but neither assumes a definite value until one or the other is measured.

If that strikes you as more than a little counterintuitive, you’re in good company. Albert Einstein once disparaged quantum entanglement as “spooky action at a distance.” As he and his colleagues wrote in 1935, “No reasonable definition of reality could be expected to permit this.” Reasonable or no, entanglement indeed appears to be a part of reality, as numerous experiments have demonstrated.

Now, experimenters at the University of Science and Technology of China (USTC) in Shanghai have entangled not one but four pairs of photons, linking the polarizations of eight photons. The achievement, described in a study published online February 12 in Nature Photonics, extends the range of previous experiments that had entangled up to six photons. (Scientific American is part of Nature Publishing Group.) Even larger ensembles have been entangled using individual atoms as the particle of choice, but entangled photons hold much promise for quantum communication schemes, since they can carry messages across large distances.

Entanglement is a fragile state, and entangling photons with any efficiency is a major challenge; physicists generally produce a huge number of photons for every pair of successfully entangled particles. The difficulty of creating multiple pairs of entangled photons grows exponentially as more are added. Xing-Can Yao and his colleagues at USTC calculated that if they simply extended previous six-photon experiments to include another pair of entangled photons, it would take roughly 10 hours of experimental time to generate one entangled eight-photon set. (Physicists verify the presence of entanglement by running statistical tests that require large samples of photons, so an experiment that takes hours to produce a single entangled state is impractically slow.) To overcome that limitation, the researchers used an optical scheme that filters out fewer photons and hence boosts the output of entangled photons.

With a “bright” source of entangled photons, the researchers managed to generate four mutually entangled pairs with much greater frequency. They reported detecting hundreds of sets of entangled photons, at a rate of about nine per hour, which sufficed to run the kinds of statistical tests needed to verify that all eight photons were indeed linked at the quantum level.

For a lighthearted, conceptual take on entanglement, check out the recent video that I made with Scientific American‘s resident entanglement expert George Musser, along with our colleagues Mary Karmelek and Eric Olson.


About the Author: John Matson is an associate editor at Scientific American focusing on space, physics and mathematics. Follow on Twitter @jmtsn.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. And Then What? 7:23 pm 02/15/2012

    Lets assume that Entanglement is a Reality, albeit a rarity, in Nature. What does this imply? Well I guess it depends on how you choose to view the Phenomena. One way you could look at it is that Entangled Photons could be viewed as Identical Twins programmed to respond exactly the same to certain stimuli because of their identical Nature. That is to say that a Photon and its entangled Identical Twin existing within the fabric of Space-Time would react identically to “Space-Time’s reaction” to a specifically generated experimental or Nature Stimuli, and If it was Space-Time itself which was carrying the coded message to the Photon then because there is no reason not to assume that there is no upper limit on the velocity with which Space-Time Transmits messages, to itself, through itself, then indeed the entangled particles or Photons would appear to react simultaneously. That is of course if entanglement is a “Real Phenomena “ and not just a manifestation of some other unknown process, which the Theory of Entanglement seems to adequately, describe.

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  2. 2. jtdwyer 7:42 pm 02/15/2012

    As I understand, that all entangled particles are produced by effectively splitting a single originating particle is highly indicative of the physical mechanism that produces entangled pairs. In cases of larger numbers of entangled particles, they are produced by repeated splitting of already entangled particles (thus requiring the originating ‘bright source’).

    IMO, quantum entanglement should be explainable as the physical replication of quantum properties rather than any ‘spooky’ immediate (faster than light) communication of state characteristics coincident with their eventual detection.

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  3. 3. Mark Pine 5:53 pm 02/16/2012

    Matson: “Entanglement is a fragile state.”

    It is not clear that Matson is talking about entanglement. Here is the full sentence: “Entanglement is a fragile state, and entangling photons with any efficiency is a major challenge; physicists generally produce a huge number of photons for every pair of successfully entangled particles.”

    It appears that Matson is not discussing entanglement per se, but rather the difficulties that physicists have creating entangled particles and then successfully demonstrating in experiments that the particles have indeed been entangled. That seems to be a different thing.

    Entanglement is a natural state, which may or may not be fragile (i.e., ephemeral) and rare. Perhaps it is very common and even long-lasting in some circumstances. But what Matson knows and writes about is the special case of entangled particles produced and studied by physicists in laboratories.

    So if – to speculate greatly – two brains could somehow be entangled, let’s say by exchanging information (photons, ultimately) between them – and if there were some mechanisms to keep them entangled, let’t say they somehow continued to exchange information (photons) – then perhaps entanglement might be a more common and significant state than Matson implies.

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  4. 4. Postman1 8:37 pm 02/16/2012

    A question for thought: If two entangled particles were separated, one kept in the lab and the other sent to orbit, how would the difference in time affect the two particles? Would the one in our gravity well age faster, to keep up with the one in orbit, or would the one in orbit slow down, or what? If one or the other changed its speed of passage in time, would this in effect be time travel?
    Anyone? JT?

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  5. 5. Percival 9:45 pm 02/16/2012

    I’m with you, Postman1. The fundamental issue of entanglement for me is the alleged “instantaneous” transfer of particle state information over arbitrary distances FTL. But “instantaneous” implies the ability to establish *simultaneity* between the particles (and possibly a third comparison location) over arbitrary distance, and both special and general relativity forbid that (for different reasons).

    I’m also with jtdwyer who points out that entangled particles are presumed to be created in complementary states in the first place. Why should it be considered a big deal that they retain their individual states in transit over arbitrary distance, barring interactions along the way, given conservation laws?

    In sum, it seems to me that accepting entanglement on its mathematical basis requires the disavowal of Einstein and/or Noether.

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  6. 6. jnanivn 8:26 pm 02/17/2012

    Sub:Search origins
    Cosmic Dance of Lord SIVA seems to have not been understood with due comprehension of Cosmology vedas interlinks. I welcome interaction- Cosmology Definition helpsin this direction.Necessity-Demand structure means in-depth study on East west perceptions

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  7. 7. phil23 9:47 pm 02/17/2012

    If, like most physicists, we cling to our rather primitive material realism this is all very perplexing. As “jnavin” implies there are other views of reality. I’d suggest for amusement one read or better yet listen to (he has a great voice and presentation) the 1970′s Lectures of Alan Watts, an Anglican priest and author of “The Way of Zen”. One lecture particularly discusses reality as the dream of SIVA. Don’t be narrow and prickly until you’ve given it fair consideration, it’ll cost you 30 minutes.

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  8. 8. jtdwyer 8:42 am 02/18/2012

    Postman1 & Percival – thanks:

    Firstly, I misspoke when I suggested that quantum properties are always replicated in entangled particles – their complementary state may be produced, i.e., splitting a particle with ‘up’ spin produces a complementary particle with ‘down’ spin in accordance with conservation laws.

    That being said, my assertion that the entangled state characteristic may be predetermined at the moment the particles are split still stands – how else could any conservation laws otherwise apply? In this case, while neither particles’ state may be known, once either particle’s state is detected the other’s can be determined. IMO, that the undetected particle’s state can be immediately determined once an entangled particle’s state is detected is a mathematical artifact of state predetermination rather than any actual superposition.

    As I understand, the superposition state premise implies that the particles’ entangled state characteristics indeterminably fluctuate until one particle’s state is detected. In this scenario any delay in the detection of the entangled particle’s state would seemingly allow its continued fluctuation in time.

    However, in the simpler case that state characteristics are predetermined at the moment particles are split, there is no need to consider that state characteristics are in reality fluctuating in time: while no particle’s spins state can be known by any observer, they are all physically predetermined. As Percival point out, when a particle’s spin state is detected and determined to be ‘up’, its entangled particle’s spin state may always be ‘down’ no matter when or where it is eventually detected (presuming no further particle interactions, of course).

    I don’t follow how this scenario conflicts with general relativity or supports the presence of material ether…

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  9. 9. mhenriday 3:03 pm 02/18/2012

    Jtdwyer, your explanation of the entanglement phenomenon corresponds to that favoured by Einstein, Rosen, and Polder (http://en.wikipedia.org/wiki/EPR_paradox), i e, that the entangled particles «possessed» in some hidden way their state characteristics prior to measurement, in contradiction to the theory of quantum mechanics, in which these values/characteristics are only provided by the act of measurement. Bell’s theorem (http://en.wikipedia.org/wiki/Bell%27s_theorem), however, the validity of which has now been demonstrated many times by experiment, shows that all the predictions of quantum mechanics, which likewise have been validated time and time again cannot be replicated by theories which assume local hidden variables. Thus, we seem to be left with «spukhafte Fernwirkung», which, I quite agree, is not easy to get one’s head around….

    Henri

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  10. 10. iWind 8:47 pm 02/18/2012

    What jtdwyer is proposing is called a (local) “hidden variable theory,” and they do not match experiments.

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  11. 11. jack.123 4:44 am 02/19/2012

    Imangine this looking at a pair of galaxys that have been lensed by an object.Would the photons be entangled?Now lets not look at anything but wave function drop as a method of transfering information.If someone else somewhere in the univerise is looking at the same galaxys the same distance away or before we look.Could we see the drop of wave function of them looking and viceversa?Now do the same thing with starlight put through a spliter.Could there be signals in starlight that we don’t know is there,because we are not looking for them? If entaglement is a action of space-time.It would not violate general relativty and would have no speed limit.

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  12. 12. freeed 2:07 pm 02/19/2012

    May I remind all that the night of February 19, 2012 through February 20 is Shivaratri, The Night of Shiva?
    IMHO the cosmic dance of Shiva that began with the Big Bang probably created uncounted entangled particles that even now are spooky linked. “Thoughts have wings” so I wish all glories to all who read this, notably jnanivn and phil23. One might also consider reading Guru Geetaa Likeeta, song to Shiva the guru, whose Sanskrit syllables were designed to produce cordant resonant frequencies in the body and all space.
    BTW, whatever became of TRANSVERSE polarized light and could that be experimentally studied for entanglement? This may be the bridge between “religion” and science as certain yogis may produce these polarities via kundalini yoga. :)

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  13. 13. profchuck 12:10 pm 02/20/2012

    It would seem that a seminal question regarding entangled particles is “can the phenomena be used to communicate information at faster than light velocities?” It is my understanding that the answer is probably “no”. Lets assume that an entangled particle pair consists of one particle in laboratory A and its entangled counterpart is located in laboratory B. (Never mind how the particles were created or came to be located in two separate spots) Further assume that the distance separating A and B is arbitrarily large.
    The process of entanglement states that if we measure the state of the particle located at A we then unambiguously know the state of the particle located at B. I think I see how such a phenomena could be used as an unbreakable encryption system and maybe even as part of a quantum teleportation process but it does not imply some form of instantaneous communication between the two particles.

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  14. 14. iwikler 4:42 pm 02/20/2012

    Hey, you QM guys, help me out here with a little history lesson:
    Is non-locality now regarded as an accepted part of the entanglement concept and is the entanglement concept itself regarded as an accepted part of quantum mechanics, or is there still a group of quantum physicists who holds to the “ERS view” and thinks that Bhor’s theory is “incomplete” in that ( e.g., non-locality) lacks observable “realism” and is not complete, or, at the very least, that non-local events cannot be transmitted faster than the speed of light (which is the really “spooky” part of non-locality)?

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  15. 15. Iggle 6:49 am 02/21/2012

    Jtdwyer, Einstein suggested the “pre-programming” you mention. However, experiments have shown that it is not the case. I believe it was a physicist named Bell who proved this.
    The article mentions that entanglement could be used in communication. But, it is my understanding that no actual information is transferred. The first particle’s spin (or whatever) is completely random. And, while the second particle will match that spin, there is no useful information generated by that. You can’t control the first particle’s spin, so you can’t say something like “UP” equals true and “DOWN” equals false for communication purposes.

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  16. 16. Tucker M 1:38 pm 02/21/2012

    Iggle is correct. As intuitively appealing as JTDwyer’s suggestion is (Einstein thought the same!), it has been experimentally falsified; there’s tons available about this on Wikipedia and elsewhere. And to answer iwikler, I don’t believe this is even controversial anymore, no. While there are still interpretations of quantum mechanics that avoid non-locality, they’re even less intuitively satisfying (read: believable) than non-locality is (example: superdeterminism). See for example “Loopholes in Bell test experiments” on Wikipedia.

    As for the use of entanglement in communication, profchuck is correct that entanglement cannot be used for faster-than-light communication; I believe this has been formally proven as well. But contrary to Iggle, that doesn’t mean entanglement is useless for communication. To the contrary, entangled particles transmitted at light or sub-light speeds can (in principle) be used for purposes of unbreakable encryption.

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  17. 17. Dr. Strangelove 3:49 am 02/22/2012

    Gentlemen

    Einstein had good reason to challenge quantum entanglement since this “spooky action at a distance” appears to be a form of faster-than-light communication. He devised the EPR paradox to disprove it. But contrary to his expectation, EPR paradox experiments confirmed it.

    I hold that it is indeed a form of superluminal communication as Einstein feared because it is synchonized instantenous action of particles regardless of spatial separation.

    I believe quantum entanglement can be used for superluminal communication. The transmitter can be a series of light pulses emitted at opposite directions. Two receivers can be lightyears away equidistant to the transmitter. The two receivers can communicate instantenously by passing the light pulses through a polarizer at one of the receivers.

    The polarizer’s angle can be adjusted rapidly. Each angle corresponds to an alpha-numeric symbol. The other receiver can decode the symbols (messages) by observing the sequence of polarization of the light pulses.

    This is better than science fiction. The technology is simple.

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  18. 18. Tucker M 9:52 am 02/22/2012

    Dr. Strangelove,

    It’s a nice thought, but it doesn’t work. Don’t take my word for it; read up on it. There is no way to use entanglement for faster-than-light communication. If quantum physics ever gets proven to be wrong or incomplete, that would be another matter; but as currently formulated, the theory just doesn’t allow for it. Even general relativity allows for faster-than-light communication in theory, given sufficient warping of spacetime. But quantum entanglement just doesn’t.

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  19. 19. Dr. Strangelove 8:47 pm 02/22/2012

    Tucker,

    Yes a nice thought indeed and it should work unless Einstein is correct that the “spooky action at a distance” is just a figment of Bohr et al’s imagination. That’s the whole point of the EPR paradox. Instantaneous communication (synchronization) between entangled particles is impossible.

    Don’t take my word for it; read up on it. I recommend the books “In Search of Schrodinger’s Cat” by John Gribbin and “Physics of the Impossible” by Michio Kaku. Gribbin described the same set up of polarizer that I proposed here. Kaku had a whole chapter on quantum teleportation – real experiments on instantaneous transmission of information.

    It is your misconception that quantum physics does not allow faster-than-light communication.

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  20. 20. Dr. Strangelove 9:52 pm 02/23/2012

    My proposed polarizer set up with multiple variable angles is too complicated. So I will follow Gribbin’s original set up of polarizers, which is simpler and works in principle.

    Take three polarizers in series. The first two are oriented at 90 degrees (vertical). The third is slanted at 45 degrees. Take light photons at quantum state (undefined polarization). When the photons pass through the first polarizer, their quantum state collapses into classical state (random polarization).

    When they hit the second polarizer, some photons pass through and become polarized light (90 degree polarization). When they hit the third polarizer, since it is misaligned (45 degrees) the photons are blocked. No photons pass through the third polarizer.

    Now, change the experiment. Remove the first polarizer. So photons at quantum state pass through the second polarizer and collapse into classical state. When they hit the third polarizer, since they have random polarization, some will pass through.

    Hence, by changing the experiment, the outcome has changed. First experiment has zero probability of photons passing the third polarizer. Second experiment has nonzero probability (some will pass through) Two possible outcomes: zero and nonzero probability. We can use this to transmit information. Zero is assigned as dot. Nonzero is dash. This is Morse code.

    Take entangled photons at quantum state moving in opposite directions. Two receivers lightyears apart will receive the photons in each direction. Perform the above experiment. Place the first polarizer at receiver A. Second and third polarizers are at receiver B.

    Receiver A can instantaneously send a message to receiver B by placing and removing the first polarizer. When A places polarizer, B observes zero probability (no photons passing third polarizer). When A removes polarizer, B observes nonzero probability (some photons passing third polarizer)

    This is faster-than-light Morse code communication.

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  21. 21. wpatten333 4:54 am 02/27/2012

    I believe scientists will soon find that entanglement works to control in every aspect of a living cell. The actions of the atoms in the different parts of the cells to individual molecules, and to groups of molecules all entangled with sigle atoms and molecules of the DNA strands that act like radios sending instant information, thus giving us life. This is real its really how the universe works. spooky! but true. Soon they will simulate simple structures from the DNA to do “simple” entanglement experiments, leading the way for the worlds first super computer. And it doesn’t stop there. artificial life maybe, instantaniouse calculations, This is just the key. Entanglement!Reply to me if anyone agrees or disagrees, or knows if experiments of this sort have already been done. Thank you

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  22. 22. buczo 12:30 pm 03/9/2012

    Chaps,

    have your red that ? http://www.bottomlayer.com/bottom/argument/Argument4.html
    do you know it ? entanglement as a result of non-local calculation performed in universe’s CPU ? corpuscles are simply pixels ? where are those pixels ? are they where we see them they are ? or is it only our mind’s projection that they’re there ?

    sorry if I’m talking bullshit .. I’m new in here .. and I’m an IT guy … so my way of thinking might be hugely skewed :) .

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  23. 23. Montesaga 4:47 pm 09/3/2012

    The photons are not that far apart. I would say they are next to each other still. If you go far enough out into space you sill see that everything on earth is on the same little spot.

    Link to this

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