Some applications require 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 polarisations of photons. If a measurement is made to determine if the photons are polarised up-down or left-right then all information about polarisations in the exactly intermediate directions is completely lost. A channel capable of producing and detecting individual polarised 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 quantum channels is limited to distances of several tens of kilometres, reducing their value for communicating over large distances. 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.
The future of Ad Astra