Detail of Quantum Protocol BB84 : Part 2, Defeat of Eve
Now, let move on to the case in which there is Eve in between Alice and Bob eavesdropping the communication.
* * * Alice * * *
| Orig Key |
0
|
0
|
1
|
1
|
1
|
0
|
1
|
0
|
1
|
0
|
1
|
0
|
1
|
| Mode |
0
|
1
|
0
|
0
|
0
|
1
|
0
|
1
|
1
|
0
|
1
|
1
|
0
|
| Crea St |
u
|
r
|
d
|
d
|
d
|
r
|
d
|
r
|
l
|
u
|
l
|
r
|
d
|
* * * Eve * * *
| Mode |
1
|
0
|
1
|
0
|
0
|
1
|
1
|
1
|
0
|
1
|
1
|
1
|
0
|
| Obs St |
r
|
u
|
l
|
d
|
d
|
r
|
l
|
r
|
d
|
l
|
l
|
r
|
d
|
* * * Bob * * *
| Mode |
0
|
0
|
0
|
1
|
0
|
0
|
1
|
1
|
0
|
0
|
0
|
1
|
0
|
| Obs St |
d
|
u
|
u
|
l
|
d
|
d
|
l
|
r
|
d
|
d
|
u
|
r
|
d
|
| Recn Key |
1
|
0
|
0
|
1
|
1
|
1
|
1
|
0
|
1
|
1
|
0
|
0
|
1
|
When Eve is eavedropping, there is 50% chance of Eve mispredicting the correct mode of trnslation, and she sends out the spin states to Bob in wrong mode. Even if Bob and Alice pick out the data where their mode coincided, there is 25% chance of finding disagreements in encripted original key sequence and dreconstructed key sequence.
Redundancy of BB84 Protocole, E91 and B92 Protocoles, Quantum Roman Empire
Ecologically inclined among you might have founded this whole business rather too cumbrsom. Why do we need three sets of random number just to share one sequence? Can we not be more efficient user of resources. In fact, Bennnet, one of the founder of BB84, later came up with a refined version of the protocole using just one random sequence and having more straightforward scheme. This is called B92. That was preceeded by different protocol of quantum random number sharing called E91, which was discovered by a Polish Physicist Artur Ekert. He has utilized the entangled pair of quantum state which we shall cover soon in quantum computing. Later, it was proved that BB84 and E91 are mathematically equivalent. To this date, B92 is considered to be the most efficient quantum code sharing protocol. In B92, Bob has to have extra high-tech device called "quantum state filter". The coding is done such that "0" is translated into "up" state and "1" into "right" state which are not diagonal to each other! There is no need to throw a dice for mode selection. There is good reason for us to be so averse to using dice. Think of Caesar's crossing Rubicon after thouing one. We surely do not wish to see history decided by chance many more times.
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