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Technology Science

Researchers Control the Flip of Electron Spin 157

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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Researchers Control the Flip of Electron Spin

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  • Election? (Score:4, Funny)

    by Anonymous Coward on Sunday May 29, 2005 @05:29PM (#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 @05:32PM (#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 [] 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.
    • You sir, are talking out your ass:

      Giant magnetoresistance was first discovered in pure crystal layers in 1988 by Peter Grünberg of the Jülich Research Centre and Albert Fert of the University of Paris-Sud [...] IBM produced the first commercial device based on this effect in December 1997.

      Oh, btw, it's used in every hard drive produced today and is one of the reasons HD prices have been falling even steeper than RAM and CPU.

  • by yotto ( 590067 ) on Sunday May 29, 2005 @05:32PM (#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!
    • 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

      Update: researchers are now stunned to discover they can manipulate the spin with a jolt of voltage of electricity from any source.

      Not just batteries.
    • 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

  • Does this development get us any closer to the development of a quantum computer? I don't even remember the last I heard about that branch of science...
    • Really? You haven't heard anything? That's strange, 'cause it's all over the news, especially in the context of cryptography... q=quantum+computing&btnG=Search+News [] 140 google news stories.
    • 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
      • Also, I'll have to look back into the time-dependent Schrodinger equation to remind myself how this plays in quantum mechanics.

        Actually, the single-particle three-dimensional time-dependent Schrödinger equation does not account for spin or relativistic effects. If anything, you could use the configuration interaction method to analyze spin in a nonrelativistic Schrödinger equation.
    • by heelios ( 887437 ) on Sunday May 29, 2005 @05:56PM (#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.

      • by jfern ( 115937 )
        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).
      • by kmac06 ( 608921 )
        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.

          • Grandparent was referring to the fact that reading the charge on the tiny capacitors in DRAM destroys the charge. Afterwards, the hardware then needs to recharge the tiny capacitors in DRAM that it read. Despite this flaw DRAM is still used, so perhaps quantum computers could recreate quantum qubits after reading their states.
            • I realize what he's talking about, but he's not quite on the ball. Reading the charge doesn't obliterate the state, and the caps naturally decay which is what refresh is all about anyway. And as I stated, while not an expert at quantum physics, I don't believe you can refresh quantum bits the same way that you can refresh decaying charge in a cap.

              • You're right, it's a different principle, and the analogy wasn't correct. However, you work with the limitations you have. You can work with memory cells that lose the information when you read them, and you can work with qubits that lose their information when you read them. In fact, since this is a crucial part of the theory, this is something that is assumed from the start and is able to be worked around.
        • And don't think of that 'other world' explanation. It's not really what is happening according to current theory, just a convenient/spooky way to explain it to some lay people. And yes, IAAP

          Isn't that a rather religious statement? I mean, the many world interpretation fits the data just as does the Copenhagen interpretation (which I was taught). I personally see both interpretations as "useful" in that you can choose the most convenient one to think about the problem at hand.

          As for the spookiness, I d

    • by jfern ( 115937 )
      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.
  • It seems like this is just a small-scale example of effects like this [], where the whole planet does a whoopsie.
    Seems like a step in the direction of understanding gravity.
    Let's fund this kind of research, shall we?
    • 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

      • I've wondered a little, in the NMR case, if it would be possible to induce nuclear fission or at least some kind of nuclear decay through heavy RF excitation of nuclei. No.

        At least not with static magnetic fields realizable on earth - you might be able to get some interesting stuff happening with magnetar level fields.

        OTOH, there is some interesting stuff that happens with nuclear quadrupole moments and gamma ray emissions. The gamma ray emissions show some directionality when the thermal energy drops be

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

    by maxwell demon ( 590494 ) on Sunday May 29, 2005 @05:42PM (#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 [] is something quite different from an electron [].
    • 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.
  • by gerbalblaste ( 882682 ) on Sunday May 29, 2005 @05:43PM (#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
  • by proverbialcow ( 177020 ) on Sunday May 29, 2005 @05:45PM (#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.
    • by Asparfame ( 96993 ) on Sunday May 29, 2005 @06:26PM (#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.
      • by Asparfame ( 96993 ) on Sunday May 29, 2005 @06:39PM (#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.
        • Could you not then use statistics as a means of gathering some information?

          Say that you create one million pairs of entangled particles with random spin. You set the spin of one hundred of the particles and then measure the other one hundred paired particle on the other end. You can then discard those one hundred pairs and repeat the process with another hundred. This would mean setting once (while in a indeterminate state) then observing once.

          This seems to fit with your explaination, but I am probably
        • 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: []

          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

          • by maxwell demon ( 590494 ) on Sunday May 29, 2005 @08:07PM (#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).
          • Whether it well and truly is indeterminate is a question that has tormented quite a few physicists. The idea that there isn't indeterminacy but instead there are 'hidden variables' that control it is appealing. However the greatest minds that have ever lived have word for decades on this, and have found no satisfactory model for this idea.

            These days scientists just all accept that it really well and truly is not set by the universe - it truly is in a quantum state, existing as a probability.


            http:/ []
  • by 1nt3lx ( 124618 ) on Sunday May 29, 2005 @05:46PM (#12672128) Homepage Journal
    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 2/183230&tid=98&tid=103&tid=190&tid=215&tid=231&ti d=14 [] .

    "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
      • I did not mean to give the impression that I felt the US should have its hand in every scientific discovery in the universe. I meant it to be a critique of our current administration's scientific policy.

        It was supposed to be a nose thumbing at the president's agenda and rhetoric.

        I am well aware that the sun will one day set on the American empire, otherwise I don't think I would have left that comment at all.

        Your points are all valid, and I agree with them.

        Whether has always been a problem word for me.
    • They are funding it. I know, because my graduate advisor is working on it (here in the US, supported by DARPA). In fact, a lot of money in the US is going into this...
  • 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
    • Ok, which way must I hold my ferromagnet to get good luck? Because a ferromagnet actually works through a strong bias of spins in one direction ...
    • it wasn't with electrons it was with neutrinos. and it wasn't exactly to one person, just a field around that person, which is why when the stations computers/replicators were used to make a larger variation of the device the larger ones caused problems all over the station with their larger field.
  • quantum crypto (Score:2, Informative)

    by cryptoz ( 878581 )
    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
    • Well, you could probably say that about dozens of past inventions and developments in science. Other people have probably done it many times in the past.

      I don't have examples handy and I'm too tired to find them, but they have probably even said that about things which we use directly or indirectly everyday in our lives nowadays.

      Almost every non-ridiculous science is useful! And even some of the seemingly ridiculous science is useful too (and I'm not one to judge what's ridiculous or not...).
  • 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,

    • by maxwell demon ( 590494 ) on Sunday May 29, 2005 @06:04PM (#12672225) Journal
      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
            • But that's not flipping a spin. It's flipping two spins, which is different because the total angular momentum is the same before and after the flipping.

              • My notation was |spin of first electrion, spin of second electron>.

                Initially, I was in a superposition of |up, down> and |down, up>.

                Then, I performed an operation that flips the spin of only the first electron, leaving me in state |down, down> + |up, up>

                I have not flipped two spins. And what does angular momentum conservation have to do with it? In this case, the expectation value of total angular momentum is conserved, but there's no need for that to be true in general of a unitary operat
          • 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 [])
        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
    • instantaneous transmission of data...
      are they still connected afterwards! They should be, shouldn't they??

      I think they are. I also think you make me think of Homeopathy [] and stuff like that. Interesting!

    • While electron spin entaglement is quite difficult, entangled photon polarization (as an information "protocol") is quite straightforward.

      An army physicist I worked with as an undergrad used entagled photon polarization as a method to make remote measurements. Because particular compounds can specifically polarize light, polarization of one photon will cause the immediate polarization of its entangled sister photon. (This means that you could, in theory, shine a laser beam at a chemical vapor a mile away,

      • 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.
        • Sure do. Here is a diagram as well (original paper by Dik Bouwmeester and friends is referenced at the bottom):


          • This is a quantum teleportation experiement. Reading from the link:

            "Also, quantum teleportation does not allow for faster-than-light communication. Although the teleported particle attains the polarization value instantly, the people at the sending station must convey the fact that teleportation was successful by making a phone call or using some other light speed or sub-light-speed means of communication."

            This statement, from the link you quote, contradicts your implication that quantum entanglement ca
  • ...flipped the spin of electron inside it and emitted a photon...

    (IANA quantum physicist, but...) 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... and maybe squirt a photon to dazzle some Arcturians?

    • 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:


  • absolutely riveting.
  • Must be something very wrong with my faith in politics and politicians, since I've misread the title of the article as:

    Researchers control the flip of the election spin

    It took me a while to find out what is the idiom I don't know in this sentence, before I carefully read it again ;)))

    (of course non-native English user)
  • Isn't this the beginning steps towards the interstellar travel mechanism used in the James Blish series "Cities in Flight"? at amazon [] which was affectionately nicknamed the "spindizzy"?
  • These appear to be the same type of quantum dots used to create artificial atoms [].

    Artificial atoms, as I understand it anyway, are exhibited when these wells/quantum dots are crafted to store electrons in certain configurations (modeling "orbital shells"/energy levels?). The quantum dot has then been shown to exhibit some properties of an element corresponding to that electron configuration.

    Is there a connection somewhere here with the electron spin "flip" (whatever that means)? If you take an ordinary a

    If you can flip the spin of an electron, wouldn't that be a condensing of our current storage technology?

    However, after doing a Google for the size of an electron as compared to an atom, this road bump chucked my question out the window. 66.htm []
    • The question of electron vs. atom "size" isn't really relevant to storage technology, even with a suitable definition of size because you'll always be using electrons attached to atoms. (Unattached electrons would repel each other, making it hard to get the same density.).

      So, an atom may hold 1-100 electrons, but no more than 2 or 3 would have the "freedom of movement" to use for bits. In practice you'd do very good to have one bit pr. atom, and for redundancy at least ten atoms pr. bit sounds like a very

      • good answer.

        Of course, retail will continue to suck the general public dry with SDRAM prices.
        Even with new technology, we can expect prices to be inflated beyond reasonable.

        i.e. 128Mb USB RAM should not cost $50

        Can't wait till we get that storage to within humane levels.
  • Call me back when they finally get the transporter working, so they can BEAM ME UP!!

Our business in life is not to succeed but to continue to fail in high spirits. -- Robert Louis Stevenson