Quantum cryptography: The BB 84 protocol (U3-02-03) - 雙語字幕
In order to achieve data transmission that is absolutely safe from eavesdropping,
we need a secret random pattern, the so-called one-time pad that only Alice and Bob know.
Here, we show how we can transmit the one-time pad using polarized photons.
The of the photon can be represented on the block sphere too.
An arrow represents horizontal polarization.
A downward arrow, vertical polarization.
In this space, horizontal or vertical polarized photons are eigenstates with a 100% predictable outcome.
If the photons are however polarized in plus or minus 45 degree direction,
we obtain a random result in the horizontal and vertical base, as can be seen here.
Generating a random pattern is thus very easy in quantum physics.
However, how can we securely transmit a given random pattern as a one-time pad so that only Alice and Bob can share it?
Alice could transmit horizontal or vertical polarized photons,
and Bob could measure them in the same base, and would with 100% probability obtain the same result.
If there were a spy who measured the photons in the same base,
and then, depending on the result, re-transmitted the same polarization, the spy would have won.
Yet, the spy does not actually know the measuring base.
In the wrong base, that is, in this case, plus or minus 45 degrees, he would obtain an entirely different random pattern.
And as well,
for the spy,
would in fact then no longer transmit horizontally or vertically polarized photons to Bob, but photons polarized in plus or minus 45 degrees.
In addition, Alice and Bob would notice that somebody had eavesdropped on them, because their random patterns would be completely different from one another.
If Alice and Bob were always to transmit and receive in the same bag, base.
The spy would only have to keep on trying to crack the code for sufficiently long enough until he received the message without changing it.
Once the spy had found the right base, Alice and Bob would no longer notice that somebody had eavesdropped on them.
Alice and Bob can however prevent that,
using for example the so-called BB-84 protocol,
which we are now going to explain in more Alice would like to send her key to Barb.
For that purpose, she sometimes transmits photons polarized in the horizontal vertical base, sometimes in the plus-minus base.
Barb is likewise constantly randomly changing the measuring base, without knowing the base used by Alice.
In this way, Bob receives his measuring results, which of them can be used as a one-time pad then.
This is where Alice can help.
She now discloses only the respective measuring base, but not the polarization state of the photon.
Next, Bob compares his respective base with that of Alice.
Only the approximately 50%
of cases where both have been Taking the same basis,
give rise to identical data, which Bob can use as a key, that is, as a one-time pad.
There is, however, no absolute certainty.
Because if the key is stored digitally on Bob's PC, the Quantum channel no longer helps.
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