In this lesson, we introduce one-time pad.

This is scheme considered to be the strongest encryption message ever.

First, we choose a long random bit sequence as key,

which is as long as the bit length of your plaintext.

Then we use XOR operations on plaintext and the key to generate the ciphertext.

See here on the second row.

It was sent over to the insecure channel and it's all saved in the storage.

If we try to decrypt with a different key like a bit pad to here,

the row number four,

we will it yield plaintext two which is completely different from the origin text.

Fiber network has been used for

a long time in distant telecommunication and data networks.

The same fiber connection can also be used to distribute

one-time pad call Quantum Key Distribution, QKT for short.

This BB84 protocol was first proposed in 1984

by Charles H. Bennet and Gilles Brassard.

It utilized fiber channel and polarization of the photon and quantum characteristics.

Besides sending the photon in the Fiber Channel represent zero and one,

the photon can also be polarized using the filter very similar to the sunglass.

The photon, after going through the first polarized filter,

will be polarized in the direction of the filter axes.

It can be vertical,

horizontal or it can be diagonal.

If the beam goes through

the second filters and the two filter are perpendicular,

then no photon can get through.

If the light intensity after the second filter is proportion to

the square of the cosine of the angle between the two fields axes.

Lets look at an example of the Quantum Key Distributions.

Here, Alice tried to send one-time pad to Bob even though they recognize hacker

Trudy maybe observing the opposite Fiber Channel in the fiber connections.

Alice and Bob each has two sets of filters rectilinear basis,

which means horizontal and vertical and diagonal basis filter.

Let Alice assign vertical as zero, horizontally as one,

lower left to the upper right polarization as zero,

and upper left to lower right as one.

Given that pattern and he will try to send these kind of

assignment length alphabet as a bit stream I should say in plaintext to Bob.

Alice picks one-time pad with information 1 0 0 1

1 1 0 0 1 0 1 0 0 1 1 0 to Bob.

Transfer them bit by bit to Bob using one of the two bases at random.

Bob does not know which bases Alice use,

so he randomly pick one.

If he pick his right,

he gets correct bits,

if not he get random bits.

Bob tell Alice afterward after the transmission that the filter type he

used is a R-type OD type receiving the plaintext.

Alice tell him which of them are right,

which of them are wrong,

and based on that now they have the same understanding which

are the correct bit to use as a one-time pad.

Here is a picture of what actually happened based on that simple scenario.

Alice pick a one-time pad 1 0 0 1 1 1 0 01 0 1 0 0 1 1 0.

Alice randomly pick the filter type to send the signal.

See row A here.

The first signal is sent with diagonal filter.

Upper left to lower right direction is one.

The second using rectilinear filter.

Therefore after polarization is zero bit.

Same for the third bit and so on.

Transfer this bit by bit to Bob using one of those two bases at random.

Bob does not know which bases to use.

He randomly pick one.

Bob choose the pattern of r r t d d r r as a filter pattern to receive.

See row B, the second row in the set up.

If he picks it right,

he gets correct bit;

if not, he gets random bit.

Photon filter with 45 degree to

photon polarization will randomly jump

the axis either filter or perpendicular to that.

This is how the Quantum Uncertainty Principle characteristics is utilized here.

Row C show what Bob received.

Bob he then tell Alice the polarization filter axis he used to receive in plaintext.

Alice tell him which bits are right and wrong in plaintext

is show in row D for the result here.

Now both of them understand they have to correct bit string of

0 1 0 1 1 0 0 1 show in Row E here.

Trudy on the other hand use a filter pattern in row F. However,

the a's bit 11's bit,

12 and 15 bits he gets it wrong and the random nature

of the quantum phenomena leading to unpredicable bit pattern of those bit.

Therefore, he didn't get the one-time pad.

Here we show the real-world example where the Quantum Key Distribution is used.

On March 2007, Los Alamos National Lab and NIST carryout experiments.

They demonstrate QKD can be transmit over

the fiber with the length of 148.7 Kilometer.

October 21st 2007, QKD developed by

ID Quantique Company in Europe is used to

secure the transmission of vote in Swiss election.

2015 Battelle, one of the industry company

using KQD to secure connect Columbus Ohio headquarter,

their headquarter to their production facility in Dublin,

Ohio to make sure all the communication is secret.

August 2015, University of Geneva and a cloning company demonstrate,

they use KQD on

307-kilometer fiber connections and was able to

generate secret key at the rate of 12.7 kilobits per second.

At the end of last year 2016,

China complete 2,000-kilometer Quantum link between Beijing and Shanghai.

It is reported in IEEE spectrum October 2016 issue,

this Beijing/Shanghai project will form

the backbone of China's Quantum Communication Networks.