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Researchers Control the Flip of Electron Spin

Posted by Zonk on Sun May 29, 2005 04:27 PM
from the wheeee dept.
Technology Science
karvind writes "According to PhysOrg, physicists in Europe, California and at Ohio University now have found a way to manipulate the spin of an electron with a jolt of voltage from a battery. In this experiment voltage was applied to Indium Arsenide based quantum dot which flipped the spin of electron inside it and emitted a photon. The scientists were able to manipulate how long it would take for the electron to flip its spin and emit a photon - from one to 20 nanoseconds. This may have possible applications in optoelectronics and quantum cryptography. Results were published in the latest issue of Physics Review Letters"
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  • Election? (Score:4, Funny)

    by Anonymous Coward on Sunday May 29 2005, @04:29PM (#12672042)
    Was I the only one who thought this read "Researchers Control the Flip of Election Spin"?

  • As Usual (Score:5, Insightful)

    by superpulpsicle (533373) on Sunday May 29 2005, @04:32PM (#12672058)
    All universities new findings take 30 years before they are applied to the corporate world.

    1.) show the slashdot how electron flips
    2.) slashdot crowd say cool
    3.) show more engineers
    4.) show sponsors, marketers, businessmen
    5.) repeat step 4 for 29 years
    6.) profit!

    • Re:As Usual (Score:2, Informative)

      by whimsy (24742)
      PCR [wikipedia.org] is 20 years old and ubiquitous in industry.
    • Yeah, just like the transistor and the IC took 30 years from invention to commercial impact. It's like being an inventor that's 6 years older than before and now waiting for his Nobel prize, it may arrive tomorrow, or next year, in thirty years, or never. For the Nobel prize case, though, you might expect the actual prize to be handed over in December, in Sweden. Not generally so with corporate success.

  • 20/20 Hindsight (Score:5, Funny)

    by yotto (590067) on Sunday May 29 2005, @04:32PM (#12672061) Homepage
    *...voltage was applied to Indium Arsenide based quantum dot which flipped the spin of electron inside it and emitted a photon. The scientists were able to manipulate how long it would take for the electron to flip its spin and emit a photon - from one to 20 nanoseconds.*

    When you put it that way, I don't know why it wasn't this simple the whole time!
    • And then again, lets remove the dumbing-down and try to figure out what they *really* did...

      "...We report the observation of a spin-flip process in a quantum dot whereby a dark exciton with total angular momentum L=2 becomes a bright exciton with L=1. The spin-flip process is revealed in the decay dynamics following nongeminate excitation. We are able to control the spin-flip rate by more than an order of magnitude simply with a dc voltage. The spin-flip mechanism involves a spin exchange with the Fermi

  • Not exactly ... (Score:5, Informative)

    by maxwell demon (590494) on Sunday May 29 2005, @04:42PM (#12672105) Journal
    The title of the linked-to article in Physical Review Letters is:
    "Voltage Control of the Spin Dynamics of an Exciton in a Semiconductor Quantum Dot"
    (Emphasis by be)
    Now an exciton [wikipedia.org] is something quite different from an electron [wikipedia.org].

    • Re:Not exactly ... (Score:2, Informative)

      by Anonymous Coward
      Technically yes. But in semiconductors you cannot get isolated electron. You excite an electron from the valence band to conduction band and leave behind a hole (sorry for the technical details). The work manipulates this excited electron and flips the spin. PhysOrg is not a site for hardcore physcists.
      • Well, but excitons arent "normal" electron/hole pairs, but bound systems, which behave quite differently than the "free" positive and negatic charge carriers.

  • Mmmm... Dark exitons (Score:5, Funny)

    by gerbalblaste (882682) on Sunday May 29 2005, @04:43PM (#12672111) Journal
    "A dark exciton with total angular momentum L=2 ecomes a bright exciton with L=1."
    Finally a practical application for decay dynamics following nongeminate excitation

  • Another application I can think of (Score:3, Interesting)

    by proverbialcow (177020) on Sunday May 29 2005, @04:45PM (#12672120) Journal
    How about an ansible?

    Pair off two electrons in a shell, flip the rotation of one and you change the rotation of another - instanteously. Even if they're no longer in the same atom and millions of miles apart.
    • That is precisely what I was thinking of.

      We now have a universe wide cell phone ;)

      I was trying to dig up the article on that particular experient and see if they had found further results.

      Ah, the Orson Scott Card ansible ( Ender saga) founded on junk yard parts and we didn't even have to thieve the technology from an alien race.
    • Yes, and eventually we'll figure out subspace and warp fields, too.

    • Entanglement doesn't work that way (Score:5, Informative)

      by Asparfame (96993) on Sunday May 29 2005, @05:26PM (#12672311)
      By "Pair off two electrons", I presume you mean put them in an entangled state where the spins of the two electrons are correlated? (For example, in the state |up, down> + |down, up>).

      In that case, your system won't work. Putting one of the electroncs in this spin-flipping device would destroy the fragile entanglement. In other words, flipping the spin of one would do nothing to the other.

      This is how it always is with entanglement -- entangled particles only remain entangled as long as you leave their entangled properties alone. Once you measure or modify the properties of one, the entanglement is ruined.

      • Re:Entanglement doesn't work that way (Score:5, Informative)

        by Asparfame (96993) on Sunday May 29 2005, @05:39PM (#12672375)
        Proviso: When I said that modifying the properties of one member of the pair ruins the entanglement, that was not completely correct. If you managed to come up with a scheme to flip the spin of one without measuring the spin, then entanglement would be maintained. However, this would still not flip the spin of the other electron -- the entanglement would not have a different character.

        Example: You start with the electrons having opposite (but indeterminate) spins, in the entangled state

        |down, up> + |up, down>

        (normalization constant ignored)

        Now you flip the spin of the first electron. This puts you in the entangled state

        |up, up> + |down, down>

        Entanglement is preserved, however, you have not "flipped the spin" of the second electron. You have changed the sense of the correleation though. But you still haven't transmitted any information. The spin of each individual electron was indeterminate before you meddled, and was after you meddled.

        When I said the measuring the relevant property of one of the pairs ruins the entanglement, well, that was still correct. And try as you might, there is no way to transmit classical information without performing a measurment.
        • does it mean the particle has a spin that simply has not been meassured yet?
          or does it mean the spin of the particle has not been (for lack of a better word) "set" yet?

          The Wiki authors aren't clear about it: http://en.wikipedia.org/wiki/Quantum_entanglement [wikipedia.org]

          Saying one thing: Although two entangled systems can interact across large spatial separations, ...
          and then another: ... no useful information can be transmitted in this way,

          If the particles are interacting then information is being transmi

          • Re:but what does "indeterminate" mean? (Score:4, Informative)

            by maxwell demon (590494) on Sunday May 29 2005, @07:07PM (#12672904) Journal
            It means the spin does not yet have a determined value. And this can indeed be checked. There are probability inequalities (the so-called Bell inequalities, named after Bell who found them) which must hold if the result of measurement should be pre-determined for each particle. The laws of quantum mechanics violate those inequalities, and experiments by Aspect have shown that nature obeys quantum mechanics also in this respect (the violation of the Bell inequalities has been measured).

            If those measured correlations mean interaction between those systems or not depends on which interpretation of quantum mechanics you prefer. Since there are interpretations where you don't need such an interaction, it's clear that you cannot use it to instantaneously transmit information with this effect (otherwise such interpretations couldn't possibly exist).
        • What do you mean by "occurs in a correlated manner until the end state is reached."?

          You could argue that flipping the spin of one does "affect" the other. It changes the quantum state of the pair, and the "other" is a member of the pair. But it does NOT change the state in a way that:

          a) Can be determined with measurments performed only on "the other". or
          b) Can be used to transmit information.
  • I found myself reading this article quite mindful of the frequency of stories recently that suggest the US is headed down a dangerous path of neglect and ignorance. Not only in the arena of biological research (stem cell, et al) but in technological developments as well. This is not a matter of observation but rather official administrative policy http://science.slashdot.org/article.pl?sid=05/04/0 2/183230&tid=98&tid=103&tid=190&tid=215&tid=231&ti d=14 [slashdot.org] .

    "The study was funded by EPSRC in the United Kingdom, Ohio University, Volkswagen, and the Alexander von Humboldt Foundations, with additional support by the Scottish Executive and the Royal Society of Edinburgh"

    It seems to me that this is exactly one example of the type of technology the government should be promoting, for military benefit or not. What I am not sure of is wether the researches had the option to solicit US funding or if they chose rather to not bother?

    I don't know, it struck me as a little odd considering that we're told repeatedly about how important it is to be a world leader in economy, technology, and security here is something that promotes all three and the pentagon's fat couffers are nowhere to be found. (well potentially compromises the third, but that's another story)

    • A couple of points;

      Just because you are part of America, it doesn't give you the right to discover everything in the Universe.

      Perhaps I am reading into your post too much, but it implies that because of a lack of funding, it should have been an US group who headed this discovery?

      Not any one group, organization or country can push forward the bounds of humanity on their own.

      I for one relish any discovery that is made. I also realize that their is a political element to everything, especially scientific
  • As I recall, there was an episode of Deep Space Nine that played with this idea about controlling an electron's spin. There was a device which would randomly make an unusual proportion of electrons spin in one direction, the the result was that a person's luck would be changed to either unusually good or unusually bad. I thought it was an intersting idea on what might happen if you change one of the fundamental aspects of matter in that spins are always balanced, some kind of quantum conservation of momen
  • Cryptography is in a desperate state right now. Virtually every product that needs to include it has in implemented in such a way that it's basically useless. And so quantum crypto is rolling in more and more these days with newer and better discoveries (like the one here) coming out periodically. However, yeah, it's great, w00t, applications for quantum crypto, etc, but that doesn't really mean much. We already have messages that are unbreakable through brute-force. All that needs improvement through our c
  • as I understand it, the entanglement effect only concerns the yet undecided spin before/during the first measurement which leads to the spin of the *other* entangled electron being determined at the same time. ("same time" can be tricky...)

    An explanation for this interaction taking place has been to say that the two not connected objects are actually still connected... just not connected in space but some sort of "phase-space" ...

    My question now is: are they still connected afterwards! They should be,

    • My question now is: are they still connected afterwards!

      No. The measurement destroys the entanglement.
      • No. The measurement destroys the entanglement.

        what about flipping the spin before measuring?

        That may seem screwball, but doesn't simply flipping the spin effect the other entangled particle? Would it also flip? If so, would it give off any measurable signal (a photon)?

        I don't care about determining the spins of the particles (and hope they stay undeterminable), I just care about making them repeatedly give of signals at selected time intervalls...

        (I suppose that would mean that the spin shouldn't

        • what about flipping the spin before measuring?

          Flipping the spin is measuring in the quantum mechanical sense. Perhaps you should think 'interaction' instead of 'measurement'.

          Spin is angular momentum. Angular momentum is conserved. Thus, to change the angular momentum of something means interacting with it.

          I don't care about determining the spins of the particles

          It doesn't matter if you care or not. It doesn't matter if you look at the results or not. It doesn't matter if it's you even have a result.
          • What you said it mostly correct. However, it is theoretically possible (and has been done in practice as well, in fact) to flip a spin without performing a measurment. Flipping a spin is not always a measurment.

            If you start in the state (normalization constants ignored):

            2 * |up, down> + |down, up>

            where the first electron is more likely to be measured in the up state, and you apply a "coherent spin flip" (ie a flip performed WITHOUT measuring the spin) to the first electon, you end up in the state
          • in a way you're repeating the last sentence of my previous post:

            I suppose that would mean that the spin shouldn't be determinable from the emitted photon, otherwise that would equal a measurement...

            I understand that the manipulation doesn't get to lead to a way of deducing the spin. Of course "cares" don't influence reality, it was a figure of speach. cool down.

            I was assuming that the manipulation would not equal a meassurement, and it seems there are ways to do so as the my "neighbour-poster" expla

    • Entangled quantum things aren't connected in any way except mathematically. It's nothing more than saying that if you have two electrons entangled with opposite spin, then measuring one of them tells you the spin of the other, without you having to measure it. When you measure the spin of the first one, you've disturbed it as a result, and it no longer has any relationship to the other electron at all. However, the cool and useful thing is that you have gained information about another electron without m
      • other. Google search "Quantum Entanglement" yielded this: (from http://plato.stanford.edu/entries/qt-entangle/ [stanford.edu])
        In the second part of the paper, Schrödinger showed that, in general, a sophisticated experimenter can, by a suitable choice of operations carried out on one system, steer the second system into any 'mixture' of quantum states he chooses, i.e., not steer the system into any one particular state, but constrain the state into which the system evolves to lie in a given set, and at the same time
      • so what you're saying is the "indeterminate state" in the case of the entangled photons means that their states had not been "set" yet?

        I have always been boggled how "indeterminate state" was to be understood.

        If it meant the particle has a state that simply has not been meassured yet?
        Or if it meant the state of the particle has not been (for lack of a better word) "set" yet?

        • Closer to "not yet set". But even that is misleading.

          We can take electrons and put them in a known "indeterminate" state, such as |up> + |down>. While you might say the spin is not "set", in fact the QUANTUM state of the spin is precisely set.

          And if we know what the quantum state of the particle is, we can put it into a determinate state even without measuring it.

          Thought experiment.

          In principle, according to quantum mechanics, one can construct a device that will perform the following transformati
      • Perhaps you don't realize that you've made an extremely controversial statement. What you claim this army physicist did is contrary to accepted quantum mechanics since circa 1930. Do you have any evidence to back up this claim?

        According to quantum mechanics, the interaction of the "remote photon" can not produce a measurable change on the "local photon" in the way you have described.
    • No, I don't think the two effects are related at all...
    • No, the Earth magnetic spin is a non-quantum effect, that is, the explanation is probably modelled by classical mechanics, just assuming that certain things are surrounded by magnetic fields. This is quite different, there's a single particle that you can get to enter different energy configurations by exposing it to energy. It's more related to nuclear-magnetic spin excitation, which is employed in magnetic radiology and also in "pure" NMR chemical applications, than the planet magnet.

      In addition, gravity

    • It could definitely have implications in quantum computing. It is interesting that TFA didn't mention any application here, though. Also, I'll have to look back into the time-dependent Schrodinger equation to remind myself how this plays in quantum mechanics. Seems that the precise timing of the electron spin flip could have some interesting quantum effects... -F

    • Re:Quantum computing? (Score:5, Informative)

      by heelios (887437) on Sunday May 29 2005, @04:56PM (#12672183) Journal

      Not exactly I am afraid. There are still huge issues to quantum computing. Namely isolation and data retrieval.

      A quantum computer (or at least it's processor) needs to be totally shielded to the outside world while it operates as any interraction or mesurement from the outside world will break the theory. Also, at this moment, you cannot retrieve the processed data without interfering, right? So as soon as you get the data from one of the virtual processors working in 'other worlds', the thing breaks and you can't get anything anymore from it. So it's in fact pretty useless I'm afraid.

      I don't think we're going to see a quantum computer in the years to come, and much less under our desks. Even if they were invented I believe our governments will keep them away from us as they could be quite mean to encryption.

      • Quantum error correction can be used to fix quantum errors. If the errors are independent, local, then there is a fault tolerant threshold, below which you will be able to correct enough errors to do arbitrary quantum computation. Error correction will typically add polylog (polynomial in log n) overhead in the number of qubits and the running time. So Shor's factoring algorithm becomes O(n^3 log^a n) instead of O(n^3).
      • No, it doesn't need to be completely sheilded. As far as 'reading' the information simultaneously detroying the quantum state, that is true...just like reading a memory cell that uses a capacitor destroys the information, yet we miraculously still build computers with them :P. Also, the application of quantum computers is quite limited (factoring numbers is one thing they can do well), don't expect to be playing Doom 6 on them.

        And don't think of that 'other world' explanation. It's not really what is happe

        • Uh... sensing the charge on a capacitor doesn't destroy the charge on that capacitor. Yes, there will be some (increased) leakage by the sense amps but you're clearly stretching things here.

          While my understanding of quantum theory is not as extensive as my electrical knowlege, I'm under the impression that making an observation of a quantum bit destroys its state. The two don't seem to be anything similar.

    • This advance deals with manipulating the spin of a single electron (a single qubit) The hard part of quantum computing is reliably maniuplating two qubits. With single qubit operations and measurements, and a two qubit CNOT, one can perform arbitrary quantum computation.
      • The great Richard Fenyman said that "anyone who was NOT confused by and annoyed at quantum mechanics did not understand it"!

        • Re:very interesting (Score:2, Informative)

          by Anonymous Coward
          I don't think Einstein wasted his life on arguing against QM. If it was not him, the subtleness of QM wouldn't have been exposed. And the issue raised by him isn't resolved completely. You can refer to J Bell's paper to understand why it is not a trivial problem. Currently QM is accepted because it works and there had been various non-intutive way to explain them (hidden variables, parallel universe etc). And again if you think nothing useful came out of GUT, you are only reading popular science articles an
    • What if the unsuspecting electron is one of a correlated pair? When the flip occurs, does the sibling electron (perhaps a galaxy away) simultaneously flip

      No. Flipping the spin is performing a measurement.
    • The excited scientists set up a new experiment to show this, but were perplexed when they started getting photons flying out of the test chamber.

      Curiously they formed a pattern:

      ALL U BASES ARE BELONG TO US! W00T!

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