IQC media presents an introduction
to quantum key distribution
Our QKD demonstration is based at the
University of Waterloo
QKD allows us to generate a
secure key
so secret information can be sent
securely from one location to another.
It guarantees security where classical cryptography systems simple can't.
For this experiment
we use a source of entangled photons and two
receiver stations.
By conventions, the parties who want to
share information
are known as Alice and Bob. In this case
they are located
in two separate buildings with the
source position between them.
We'll start by taking a look what happens at
our source.
A crystal is used to convert laser light
into pairs of entangled photons. The entangled photons
are in a state, such that whatever
directions we measure them
the polarization of two photons are opposite.
For example horizontal and vertical
or plus and minus 45 degrees.
So, by measuring the polarization of these
photons,
Alice and Bob can obtain a shared key. The 
pairs of photon created by the crystal
are directed separately into optical
fibers. These fibers run up to the roof
of the CIT building
where we have two telescopes; one pointed
towards Alice
and the other towards Bob. The photons travel 
through free space from the source telescopes
to identical telescopes located at the
IQC headquarters
and the PI building. For now let's just
focus on Alice's photons
after they are collected by a telescope,
the photons are channeled into a receiver
box. Once in the receiver,
the photos are directed through a system
that measures the polarization
of each one. A computer then records a zero
for photons polarized horizontally
and a zero for photons polarized plus forty five degrees.
Alternatively, it records a one for
photons polarized vertically
and a one for photons polarized minus
forty five degrees.
The string of zeros
and ones that the computer has generated 
is Alice's key.
Bob, having made similar measurements on
his photons
will obtain the same sequence of bits. They therefore share an identical key.
However, the keys only agree
if the photos are left undisturbed. If an eavesdropper
disturbed the system she would create
inconsistencies between
Alice and Bob's keys. Alice and Bob would
see these errors and know that the key
has been compromised.
If no eavesdropper has been detected, Alice takes the message that she wants to send to Bob 
and converts it to binary. She then adds
the string of zeros and ones that forms the key
to the binary message, thus encrypting
the transmission.
Alice then sends the encrypted
message to Bob
who uses his copy of the key to decrypt it.
in the end, Alice and Bob have
successfully shared their message.
The goal of IQC is to develop new technologies
likes QKD that are based on quantum
mechanics.
