Even as early as the late Information Age, some applications required even higher security than that provided by public key cryptography. In these cases a secret key can be transmitted via a quantum channel. Such a system makes use of quantum complementarity in the measurement of the polarizations of photons. If a measurement is made to determine if the photons are polarized up-down or left-right then all information about polarizations in the exactly intermediate directions is completely lost. A channel capable of producing and detecting individual polarized photons can therefore be made completely secure. In practice the security is not absolute, but repeatedly using summation parity checks on random subsets of the bits sent can reduce the chance of undetected eavesdropping to a vanishingly small level.
The use of single-photon quantum channels is limited to distances of about 100 km, reducing their value for communicating over large distances without the use of repeater stations. Signals transmitted using brighter pulses of light (i.e. by sending more photons) can be transmitted over much longer distances, but with a greatly increased chance of undetected eavesdropping.