Quantum Teleportation

  1. Hello all,

    I went to a very interesting lecture on Quantum Teleportation, although didn't understand half of it. Anyway, the professor said that this phenomenon has been proven experimentally. The part that I understood was that it is in contradiction with Special Relativity, since the teleportation happens instantaneously. What do all the physicists make of this?
  2. jcsd
  3. jcsd

    jcsd 2,226
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    It doesn't contradict special relativity (though it appears otherwise no actual information is sent faster than c) and it's not really instantaneous as you need a classical communications channel (which cannot send information fastr than c) in order to reconstruct the state .
  4. Cryptography


    Spin orientations and "distance" of two spheres? What one rotation might instantaneously reveal of the second. Could a LIGO translator be built that would help us reconstruct? The graviton in the bulk is reduced to soliton configurations?

    Is there any consideration here that is valid?
  5. Penrose certainly has some interesting ideas, but they are controversial and go beyond the usual formalism of quantum mechanics. The good news is that they should be experimentally testable.

    Teleportation does not violate special relativity. However, if you believe the QM formalism is correct, then you can show that you need entangled states to do teleportation with any decent accuracy (technically speaking, the maximum fidelity you can get for qubit teleportation with a classically correlated state is 2/3). These are the same entangled states that violate Bell's inequality, so one can speculate that some sort of non-locality is involved in the effect.

    However, one should be cautious of drawing such conclusions from teleportation, because similar effects exist in "local hidden variable theories", i.e. theories that do not violate Bell inequalities. See:

    for a simple example and

    for a more detailed discussion.
  6. Trillion-atom triumph
    Physicists take first step towards practical teleportation.
    27 September 2001

    For the first time physicists have forged quantum entanglement between two large blobs of gas. The achievement brings closer the possibility of super-fast quantum computers and teleportation.

    Eugene Polzik and his co-workers at the University of Aarhus in Denmark have entangled about a million million caesium atoms. Four was the previous record.

    "This work should pave the way for a new generation of experiments to teleport states of matter," says Ignacio Cirac, a quantum physicist at Austria's University of Innsbruck.

    Teleportation will not involve the wholesale deconstruction and reconstruction of humans, Star Trek-style. It should allow the arrangement of one set of quantum particles to reproduce more or less instantly that of a similar collection of distant particles. In this way a message encoded in photons of light could be transmitted from one place to another without sending the photons across the intervening space.

    Entanglement also underpins attempts to perform high-speed quantum computing. It is a property without any analogue in the everyday world.

    Quantum particles such as atoms or photons can exist in distinct states, like the head or tail of a coin. Such particles can also exist in a superposition - in both states at once - comparable to a coin spinning in the air before it lands.

    If we toss two coins at once, their outcomes are independent: if one is heads, the other could be heads or tails. Two entangled quantum particles, by contrast, have interdependent fates: if one is in a 'heads' state, for instance, the other must be in a 'tails' state.

    Maintaining this kind of superposition is very difficult and for any practical applications, entanglement has to embrace thousands, or even millions, of particles. How can something so sensitive be sustained?

    Polzik and colleagues forgo full entanglement, where the state of each particle depends on the state of every other particle. Instead, they generate two loosely entangled clouds of caesium gas, one with slightly more atoms in a 'heads' state and the other with slightly more in a 'tails' state. (These two states are actually defined by the directions of the atoms' magnetic fields.)

    The interdependence of these clouds is more resilient to measurements or interactions that alter the quantum states of just a few of the constituent atoms.

    It would be impossible to maintain full entanglement of this many atoms for longer than a million-billionth of a second. Polzik's team can keep their two clouds in a loose entanglement for half a millisecond. They hope to maintain it for longer in the future, and perhaps to achieve the same thing in the solid samples needed for making quantum computers.
  7. turin

    turin 2,326
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    So what? This isn't even as interesting as a dual particle entanglement. At least with the dual particle entanglement the antistate of the correlated system isn't mere statistics. Am I missing something?
  8. It is as interesting as "dual particle entanglement" because that is exactly what it is. The only difference is that there are many particles at each location. What you can achieve operationally with this system is the same.

    It is interesting, because some physicists have speculated that "macroscopic" systems cannot be in superposition states or entangled states, at least not for "long" times.
  9. ZapperZ

    ZapperZ 29,656
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    It depends on what you mean by "macroscopic". Would 10^6 particles be considered as macroscopic? The SQUID experiment by the Stony Brook group of a few years ago involved at LEAST that many particles in a superposition state.[1] Recent experimental proposal by the Penrose group is suggesting even a larger object- MIRRORS![2]

    So there are every indications that if we can isolate the system from various degrees of decoherence (of which a superconducting superfluid can do), then there's no reason to expect we won't see superposition or entanglement for "macroscopic" systems.


    [1] Friedman, J. R., Patel, V., Chen, W., Tolpygo, S. K. & Lukens, J. E. Nature 406, 43-46 (2000).
    [2] Marshall et al., PRL 91, 130401 (2003).
  10. I found this a while back and basically its saying tht the quantum effects extends quite a bit into the macro universe

    Molecules make quantum waves

    THE WEIRDNESS of the quantum world has taken another step towards the everyday world as a result of experiments which show iodine molecules behaving as waves in interference experiments. This brings properties often regarded as unique to subatomic particles out into the open on much larger scales.
    It is a fundamental feature of quantum mechanics that entities described by the quantum equations are not simply particles or waves, but exhibit a mixture of wave and particle properties. Light, for example, will behave as a wave in interference experiments, with two sets of waves interacting with one another to form a new pattern, just as ripples on a pond (or in your bath) interact with one another. On the other hand, in other experiments light will behave as a stream of tiny particles, called photons.
    Wave-particle duality was first discovered to be a feature of light in the early part of this century (Albert Einstein's Nobel Prize was awarded for his proof that photons exist). In the 1920s, researchers found that electrons, traditionally regarded as particles, could behave as waves in experiments where an electron beam is diffracted from a crystal lattice -- indeed, in one of the nicest examples of wave- particle duality, the physicist J. J. Thomson {ED: NB always "J J", never referred to by name} received a Nobel Prize for discovering that the electron is a particle, while his son George received a Nobel Prize for proving that the electron is a wave.
    Moving up the mass scale, first neutrons (each nearly 2,000 times the mass of an electron) and then beams of atoms and molecules were shown to diffract like waves when passed through small apertures. Over the past ten years or so, the wave-particle duality has been demonstrated ever more clearly. Not just diffraction (in which one beam, or wave, bends as it passes an obstruction) but interference (in which two beams or waves interact with one another) has been demonstrated both for electrons and atoms. Now, a team of researchers at the University of Paris-North, at Villetaneuse in France, has done the trick with molecules.
    In the traditional version of the interference experiment with light, two beams of light are generated by passing light from a single source through two slits in a screen. Then, the two beams are allowed to interfere, producing a characteristic stripey pattern of light and shade. The new experiment is conceptually similar, but instead of passing through holes in a screen the iodine molecules (I2, which each have a mass about 254 times that of a neutron) interact with laser beams. The first interaction, with a pair of laser beams, puts each molecule into what is known as a "superposition of states", effectively two wave packets marching side by side. A second pair of laser beams recombines the wave packets to make "particles". At least, that is the theory. What happens in practice? After they have passed through the laser beams, the iodine molecules arrive at a detector. The distribution of the molecules arriving at the detector does not resemble the pattern you would expect if they were a stream of particles travelling through the experiment, but exactly matches the stripey pattern of peaks and troughs corresponding to interference by waves (Physics Letters A, vol 188 p 187). These are the heaviest "particles" which have ever demonstrated their wave "character" directly in experiments.
  11. ZapperZ

    ZapperZ 29,656
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    Hum... you may want to update your database a bit. :)

    Matter waves interference has been demonstrated for molecules as large as a C60 and C70 buckyballs. This is a gazillion times larger than an electron, and more than 200 times heavier than a proton.[1,2] Both of these came from the Anton Zeilinger's group. I think there's something even larger than these two, but for the life of me, I can't find the reference at the moment.


    1. M. Arndt et al., Nature 401, 680 (1999).
    2. O. Nairz, et al., Phys. Rev. Lett. 87, 160401 (2001), or read here: http://xxx.lanl.gov/abs/quant-ph/0110012

    P.S. I found it! It was another demonstration of interference, but this time, using biomolecules! These molecules are at least twice as big as C60 and C70.[3] The results also came from the Zeilinger's group. And yes, he has been very productive, what with all the entanglement experiments and all, which is why he is one of my leading candidates for the Nobel Prize some time soon.

    [3] L Hackerm├╝ller et al. Phys. Rev. Lett. 91, 090408 (2003), or read here: http://physicsweb.org/article/news/7/9/4/1
    Last edited: May 21, 2004

  12. Thanks for updating me I forgot about the buckyballs
  13. It does depend on what you mean by "macroscopic" and also what you mean by "long", which is why I put them in quotes. Some spontaneous collapse models can get around the SQUID result by saying that the number of "degrees of freedom" that are in superposition has to be large rather than the number of particles. Some also say that there has to be superposition over two macroscopically distinct locations.
  14. ZapperZ

    ZapperZ 29,656
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    You certainly brought up an important point, that there is still this issue of making a clear definition of "macroscopic", or what is often referred to as "macrorealism". In case you haven't come across this, I would highly recommend Tony Leggett's paper on this very issue - the testing of the limits of QM as the various parameters become larger.[1] Here, he attempted to clearly define what is "macroscopic", and what are the criteria of distinguishing macrorealism ("What is the correct measure of Schrodinger's-cattiness?").

    I like the paper a lot, and trying very hard not to be completely seduced by it. But each day I find that I'm slowly being dragged into the dark side of the force. :)


    [1] A.J. Leggett, J. Phys. Condens. Matt. v.14, p.415 (2002).
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