Cryptography Reference
In-Depth Information
light intensity
plane of light polarization
plane of filter polarization
measured
intensity
Figure 5.25: Measuring the polarization of light.
If one single polarized photon (light quantum, light wave train) hits such a
filter, the term 'intensity' loses its meaning. We have to replace it by 'proba-
bility'. If a photon polarized vertically arrives at the receiver's end with a filter
placed vertically, the photon passes this filter with a high probability. If the
receiver filter is arranged horizontally, the photon can theoretically not pass it.
However, if the receiver filter is rotated diagonally by 45 degrees, the photon
will pass it with a probability of 50 %, and the polarization is undetermined.
An eavesdropper trying to detect the polarization cannot achieve this by the
laws of quantum mechanics without changing the photon's polarization or even
absorbing it.
This means that the polarization of a photon can be determined only once at
most. This is the decisive physical principle of quantum cryptography.
Naturally, the technical details of the method are complicated. However, the
considerations above should help you to understand the following cryptographic
protocol:
1. Alice sends randomly polarized photons. Each photon is polarized hori-
zontally, vertically, or in one of the two diagonal directions.
2. Bob has a habit of setting his receiver horizontally or diagonally slanted
to the right, and arbitrarily for each photon. With each incoming pho-
ton, he can determine whether or not it passed the filter. He can decide
from the filter set horizontally whether the photon was polarized hori-
zontally or vertically. His finding will be random for photons polarized
diagonally.
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