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Network Science

Chinese Physicists Achieve Quantum Teleportation Over 60 Miles 216

Posted by Soulskill
from the just-keep-the-shields-up-and-you're-fine dept.
MrSeb writes "Chinese physicists are reporting that they've successfully teleported photonic qubits (quantum bits) over a distance of 97 kilometers (60mi). This means that quantum data has been transmitted from one point to another, without passing through the intervening space. It's important to note that the Chinese researchers haven't actually made a photon disappear and reappear 97 kilometers away; rather, they've used quantum entanglement to recreate the same qubit in a new location, with the same subatomic properties as the original qubit. The previous record for transmitting entangled qubits was 16 kilometers, performed by another Chinese team back in 2010 — and perhaps most excitingly, the researchers seem confident that their system will scale up from 97km to distances capable of reaching orbital satellites, at which point we'll actually be able to build a global quantum network for all of our cryptographic needs."
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Chinese Physicists Achieve Quantum Teleportation Over 60 Miles

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  • Re:Satellites?? (Score:5, Informative)

    by GameboyRMH (1153867) <gameboyrmh AT gmail DOT com> on Friday May 11, 2012 @02:15PM (#39971199) Journal

    You can't transmit meaningful information with quantum teleportation alone, you still need a classical channel that operates by conventional means unless you want to transmit uncontrollable random garbage.

  • Re:Satellites?? (Score:5, Informative)

    by _0xd0ad (1974778) on Friday May 11, 2012 @02:16PM (#39971233) Journal

    The information contained in the qbit is transported from one entangled photon to another, but you first must get that entangled photon to the destination via more conventional means. They're doing that with a laser.

  • by Baloroth (2370816) on Friday May 11, 2012 @02:24PM (#39971363)

    This, and all other quantum "teleportation" and related entanglement phenomenon, do not allow for faster-than-light communication. The important thing to note is that the qubit is "teleported", not the photon itself: the photons are transmitted conventionally via some means (in this case, it looks like they did it through open air). Since the photons are entangled with a photon you retain, measuring one collapses the wave-function of the other and allows both parties to know what the state is simultaneously. The security ramifications are that any eavesdropper will collapse the wave-function before the receiver gets the photon, so he will not receive the photon in the same state as the receiver sent it.

    You cannot, according to what we know of physics, use quantum entanglement to send information faster than light.

  • by _0xd0ad (1974778) on Friday May 11, 2012 @02:26PM (#39971383) Journal

    Note that it's still limited by the speed of light. The key feature, however, is that it is secure: someone intercepting the photon can't copy or read its qbit state without breaking the quantum entanglement, or preventing it from reaching the destination. In either case, the receiver will immediately know that the channel has been broken. It then stops transmitting a response to the sender, and the sender perceives this as also a break in secure communications and stops transmitting. Both the sender and the receiver would then go into failure mode and send query/response polls periodically. When secure communications are re-established, they can resume transmitting data.

  • Trillions? (Score:5, Informative)

    by colinrichardday (768814) <colin.day.6@hotmail.com> on Friday May 11, 2012 @02:34PM (#39971487)

    If we have a 72-kg (158 lb.) person made mostly out of water, that's about 4,000 moles, or 2.4x10^27 molecules, which is about 7.2x10^27 atoms. The actual number might be different, but it's way more than a trillion.

  • by tstrunk (2562139) on Friday May 11, 2012 @02:35PM (#39971497)

    So it's replication, not teleportation?

    It's not replication, the quantum state of one photon is transfered from one photon to another.
    Here's an easy explanation. http://www.youtube.com/watch?v=_qmSdC7aQpY [youtube.com]

    Replication will never be possible as a quantum state cannot be copied: http://en.wikipedia.org/wiki/No-cloning_theorem [wikipedia.org]

  • Re:Lord? (Score:5, Informative)

    by FrootLoops (1817694) on Friday May 11, 2012 @04:58PM (#39973711)

    Here's a summary of the physics, including what a qubit is.

    Quantum states
    Suppose some process produces two electrons. It happens that a particular measurement of a quantum property named spin always comes out as +1/2 or -1/2 for electrons. Now suppose the process that created the electrons must obey a conservation law which forces the sum of the spins of the two electrons to be 0--say the particles that interacted to make the electrons themselves had 0 spin. One electron must then be spin +1/2 and the other must be -1/2. However, until the measurement is performed, you have no idea which is which. More is true: a fundamental part of quantum mechanics is that particle properties can be in multiple states simultaneously right up until a measurement is performed, at which point the property collapses to a single definite value which is randomly chosen based on the relative fractions (actually amplitudes) of the states it used to be in. Thus you can perform a quantum mechanical experiment exactly the same twice without getting the same outcome both times, though you can at least calculate the outcome probabilities. A qubit is simply the state of an electron's spin property before a measurement is made, which in general can be a mix of +1/2 and -1/2. This generalizes to other particles and other two-state properties. (More technically a qubit is an element of a 2-dimensional Hilbert space acting as the state space of some quantum property.)

    Entanglement
    Right after being produced, the two electrons are each in both the +1/2 and -1/2 states. They are "entagled", because if you measure the spin of one electron, from the conservation law you know what a measurement of the other electron's spin must be. Entanglement is actually a very simple consequence of the fact that quantum properties can be in multiple states simultaneously yet conservation laws still need to hold. It's an interesting exercise to try and get faster-than-light communication from this setup, though you'll be unable to. If you're familiar with the relativity of simultaneity, try to both blow up the earth and not by some set of decisions based on the entangled particles' measurements.

    Quantum teleportation
    Using a setup I will not discuss in detail, person A has a qubit encoded in the spin of an electron, and she wants to send the qubit to person B--that is, she wants person B to have an electron with the same spin property in all its mixed-up multiple-states-at-once glory. The setup requires a classical communication channel and some extra entangled particles. Using it, person A can instruct person B to prepare an electron with the same spin state as person A started with. It happens that person A's qubit is destroyed in the process, so the information "teleports", though note that it jumps from one particle to another. The information is all that's teleported, not the particle, and it's not teleported faster-than-light because of the classical communication needed.

  • by spazdor (902907) on Friday May 11, 2012 @06:08PM (#39974361)

    The information contained in those two grammes of entangled matter, isn't information that you've encoded into it. It's information which begins "existing", so to speak, when an observation is made from one end or the other.

    Hire a guy to randomly generate 65,536 sequential binary bits, written on paper, duplicated once, and then sealed in 2 envelopes. Shoot him in the face when you're done with him, to rule out information leaks. Now mail one envelope across the world to China, bearing a "don't open until x-mas" label.

    Now wait until christmas eve, then go into your 4chan folder and find your favourite 8 kB jpeg of some anonymous boobs. Open up your envelope, take that image file and, bit by bit, XOR it with the bitstream on your sheaf of paper. The resulting ciphertext is indistinguishable from random data - that is, its Shannon entropy is approximately equal to its length. Now you can call up your new Chinese penpal on the phone, read them your ciphertext, and show them some boobs which only they can decode.

    It's foolproof, except for the fact that it's pretty easy to open a postal envelope, read its contents, and re-seal it.

    Essentially what's happened here is researchers have figured out how to use particles as envelopes, which have much better sealing properties.

The use of anthropomorphic terminology when dealing with computing systems is a symptom of professional immaturity. -- Edsger Dijkstra

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