Quantum Cryptography Slowed by "Dead Times"

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."

Comments

I have no clue what this is about

[Wannabe Code Monkey]

I read the summary and didn't understand a single part of it, but it sounded interesting and I though, "The article must explain things better." But after reading the article I still have no idea what is going on. Is there someone else that could maybe help explain what this story is all about?

Re:I have no clue what this is about

[Anonymous Coward]

hardware which may or may not exist yet is going to run slowly unless said hardware is improved at some time in the future when it may or may not exist.

Re:I have no clue what this is about

[kmac06]

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.

Re:I have no clue what this is about

[TheRaven64]

Einstein explained radio like this:

You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? And radio operates exactly the same way: you send signals here, they receive them there. The only difference is that there is no cat. Quantum cryptography is the same, the only difference is that the absence of cat may or may not be alive.

meta

[TheSHAD0W]

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

Re:meta

[itsdapead]

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...

Organization often confussed with...

[Nezer]

It should be noted that the Joint Quantum Institute does *entirely different* research than the Quantum Joint Institute located in Amsterdam.

Quantum encrypted messages is nonsense

[temcat]

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

Amazing!

[neiko]

Quantum transmission speeds will be slow until someone figures out how to speed it up.

Dead time in scientific instruments

[confused_demon]

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.

Re:How can you have security issues?

[ScrewMaster]

but they use a quantum *handwaving* thing to ensure the bits sent traditionally haven't been messed with.

It's a Force thing. The quantum circuits say, "you don't need to see his information" and anyone trying to listen in simply waves the information on its way.

Tell God you support Quantum Neutrality

[Anonymous Coward]

That fucker is just trying to throttle bandwidth unless the matter-energy providers and Waveform Collapsing Union pay his exorbitant Higgs access fees.

store it

[Arthur B.]

If you plan to be sending encrypted data to someone, you can exchange a one time pad the size of the data through the quantum channel. When you need to communicate, just use the one time pad at a speed not limited by quantum cryptography.

You can continuously refill this one time pad thus the real limitation is
  - your average rate of encrypted data over the year
  - disk space (but that's very cheap)

peak speed of encrypted data transmission is not constrained

We demand an immediate release of the cat

[140Mandak262Jamuna]

The Society for the prevention of cruelty to Animals vehemantly condemns subjecting animals to needless cruelty in the name of scientific experiments. Release Schrodinger's Cat Now!

Re:Use a cat!

[91degrees]

Ah. In that case I put it down to a reflex action. Or maybe slashcode has been altered so that it always mods down the first post as redundant as a matter of principle.

Re:How can you have security issues?

[SocratesJedi]

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.