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

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: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.