The sender transmits the changing key sequence over a secure fiber-optic link as a stream  of polarized photons (indivisible particles of light). Because the polarization reflects the amount of electromagnetic radiation allowed to radiate at an angle to a light beam's direction, it can be considered to be a measure of the angular dependence of the light. Should an eavesdropper tap into the secure fiber-optic line, he would disrupt this stream of polarized photons by the very act of observing them and the tampering could be

Instantly detected

QC is quite different because it encodes a single bit of information onto a single photon of light. The laws of quantum physics protect this information because:

Heisenberg’s Uncertainty Principle prevents anyone directly measuring

the bit value without introducing errors that can be detected.

A single photon is indivisible which means that an eavesdropper cannot

split the quantum signal to make measurements covertly.

The quantum ‘no-cloning’ theorem means that it is not possible to

receive a single photon and copy it.

A potential snag is that in real systems, not all the photons will be received, due to inherent losses in the transmission medium. A practical QC protocol needs to incorporate some method of determining which photons have been correctly received and also of detecting any attempt by an eavesdropper to sit in the middle of the channel and act as a relay.

The first provably secure protocol for QC that resolved these problems is known as BB84, and named after its inventors Bennett and Brassard in 1984. Each bit of information is encoded as the polarization state of a single photon of light.


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