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Quantum Cryptography Slowed by "Dead Times"

Posted by CmdrTaco on from the what-i-call-the-time-before-my-first-cup-of-coffee dept.

coondoggie writes "Researchers at the National Institute of Standards and Technology and the Joint Quantum Institute said today that technological and security issues will stall maximum transmission rates at levels comparable to that of a single broadband connection, such as a cable modem, unless researchers reduce "dead times" in the detectors that receive quantum-encrypted messages."

7 of 75 comments (clear)

  1. Re:I have no clue what this is about by Anonymous Coward · · Score: 0, Informative

    The problem comes when researchers attempt to observe the time their predictions are based on. As the waveform collapses from a simultaneous dead and non-dead state to reveal its true underlying characteristic, novel techniques created by S. King et al. become necessary to fill the volume of space between the two sides.

  2. Is this really a problem? by Anonymous Coward · · Score: 1, Informative

    Most of these implementations (like http://www.idquantique.com/products/vectis.htm) use quantum mechanics only for key exchange and not for generating a one time pad.

  3. Quantum encrypted messages is nonsense by temcat · · Score: 2, Informative

    The only quantum thing in quantum cryptography is key distribution, or key generation, to be more precise.

  4. Dead time in scientific instruments by confused_demon · · Score: 5, Informative
    I've got some clue what's meant by dead time for scientific measurements. Basically, dead time is the amount of time after an event is measured that it takes for the detector system to reset. For example, if you're using a Spectroscopy Amplifier (or another shaping amplifier) dead time comes from two signals coming too close to each other. Shaping amplviers are setup so that a square wave (or a step) would produce a characteristic shape (usually a flat-topped Gaussian shape, or a trapazoid). The purpose of the shaping is to allow for the imperfections in detector to be integrated into a single measurement which is easier to process (signal being generated slowly or slowly getting through prior parts of the circuit). If a second signal occurs while the shaping is still taking place it will be integrated into the output of the shaping amplifier resulting in a garbled output for both inputs.

    The net result is that as you send more and more signals to a spectroscopy system, the dead time increases and eventually you get no output because the electronics are constantly saturated. A well put together system will include a measurement of dead time so you know how many signals you're loosing.

  5. Re:I have no clue what this is about by kmac06 · · Score: 3, Informative

    Most single photon detectors are a reverse biased photodiode. When a photon strikes it, it creates an electron-hole pair, which then collide with other electrons creating more pairs, making an avalanche effect that results in a pulse, indicating a photon. After this pulse, there is some "dead time" before everything is settled down back to its original state. During this dead time, if a photon hits the detector it will not be detected. Typical dead time is about ~50 ns, limiting the device to about 20M counts/second.

  6. Re:meta by itsdapead · · Score: 2, Informative

    So use the quantum cryptography to exchange a large classic private key.

    AFAIK that's basically how it works - the quantum link can't transmit any actual "information" - it just allows Alice and Bob to exchange a big random number in a way that allows them to detect whether Eve is listening in. Even that requires a "conventional" information link and several rounds of back-and-forth commuinication to "agree" on the key.

    I guess the other problem is that to be 100% guanranteed uncrackable the key needs to be the same length as the plaintext - "cycling" the key introduces redundancy that could be open to a "brute force" attack, and part of the motivation behind quantum cryptography is that the guys in the next lab are trying to build quantum computers that could eat that sort of calculation for breakfast...

    --
    In a survey of 100 programmers, 111111 thought that duck-typing was a good idea.
  7. Re:How can you have security issues? by SocratesJedi · · Score: 2, Informative

    My current understanding of quantum tech is the data still goes by traditional means but they use a quantum *handwaving* thing to ensure the bits sent traditionally haven't been messed with. It works under a simple principle: Bits coming down a traditional wire from Alice to Bob can be intercepted by Eve, read and then re-generated down the wire so that neither Alice nor Bob know that Eve has read the message. Quantum cryptography exploits a property of a quantum system that says that if you measure a system: (a) you change the system and (b) you can't get all of the original information about the system back out [think Heisenberg Uncertainty Principle: if you know position accurately, you don't know velocity accurately; there are other pairs of physical quantities that have this relationship too]. (a) and (b) together imply that Eve can't measure a signal and then re-create it and send it on it's way without Alice and Bob noticing a problem with the statistics they expect to observe. The benefit of quantum cryptography is that Alice and Bob know that there was an interceptor if there was one and can react accordingly. Because it is relatively low-throughput, usually only some garbage data like a key is exchanged and then more traditional means of cryptography can be used.

    IAAUP (I am an undergraduate physicist), but if YAAP and know better, please correct me if I'm at all wrong.