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Quantum Cryptography
and
Quantum Computing
Cryptography is about
a) manipulating information
b) transmitting information
c) storing information
Computing is about
a) manipulating information
b) transmitting information
c) storing information
What fundamental concepts of quantum physics were
explored in the quantum video?
a)
b)
c)
d)
measurement-disturbance and many worlds
wave-particle duality and the pilot wave
many worlds and the pilot wave
measurement-disturbance and wave-particle duality
Wave-particle duality means that a
quantum object can be in two places or
states at one time.
This is called a superposition and it is
essential for quantum computing.
Measurement-disturbance is essential
for quantum cryptography.
Entanglement is used for both
cryptography and computing.
Quantum Cryptography
One method of cryptography uses a long
random string of digits - a key - to
encrypt and then decrypt a message.
The key must be random, as long as the
message and it can only be used once.
Only quantum physics can
provide truly random keys.
Quantum physics can also provide a
secure way to transmit these keys.
It uses entangled polarized photons. To
understand polarized photons, you just need
to take your understanding of polarized light
and imagine what will happen if the light is
so faint, there is only one photon.
A photon has passed through a vertical polarizer. It
heads towards a second one. What are the chances
that the photon will be able to pass through it?
a) 0% if it is vertical and 100% if it is horizontal
b) 100% if it is vertical and 0% if it is horizontal
c) 50% if it is vertical and 50% if it is horizontal
d) 100% if it is vertical and 100% if it is horizontal
A photon has passed through a vertical polarizer. It
heads towards a second one. What are the chances
that the photon will be able to pass through it?
a) 0% if it is +45 and 100% if it is -45
b) 100% if it is +45 and 0% if it is -45
c) 50% if it is +45 and 50% if it is -45
d) 100% if it is +45 and 100% if it is +45
A photon has passed through a vertical polarizer. It
then passed through one at +45. What are the chances
that the photon will be able to pass a third?
a) 0% if it is vertical and 100% if it is horizontal
b) 100% if it is vertical and 0% if it is horizontal
c) 50% if it is vertical and 50% if it is horizontal
d) 100% if it is vertical and 100% if it is horizontal
Alice sends Bob a key using
polarized photons.
Animation of bb84
Eve tries to eavesdrop.
• She doesn’t know what basis to use.
• Suppose a vertical photon was sent and she chose
diagonal polarizers.
• The value she got is meaningless and she will send
Bob a diagonal photon, not a vertical one.
• If Bob uses the correct basis, he might get a
horizontal photon instead of vertical and Eve’s
presence will be detected.
Suppose Eve intercepted 16,000
photons. How many times will
she get caught?
1) 8,000
2) 4,000
3) 2,000
4) 1,000
Quantum Computers
Computers are based on quantum devices transistors - which are getting smaller and
smaller. Soon they will be so small that they
will be directly subject to quantum rules.
This is both a problem and an opportunity.
We will be looking at the opportunity.
Quantum computation is fundamentally
different from classical computation.
Our present computers store information in
bits, which can be either a 0 or a 1.
A quantum computer stores information
in qubits. These can be both a 0 and a 1
because quantum objects can be in a
superposition of two states at one time.
When the object is measured, it is
disturbed, and is always found in one
state or the other – 0 or 1.
polarization
spin
energy level
The real power comes when you have
entangled qubits. Three qubits can be in
all the possible bit combinations at once;
000, 001, 010, 011, 100, 101, 110 and 111.
That’s eight pieces of data - not three.
When measured, you will get one of them.
000 001 010 011 100 101 110 111.
How many bits can you store in 20 qubits?
a) 1,000
b) 10,000
c) 100,000
d) 1,000,000
What might a quantum computer
do that a classical computer can’t?
• Simulate quantum systems
• Make a really fast search engine
• Factor really big numbers*
* Which is why we need quantum cryptography!
To get a feeling for how quantum
computation is fundamentally different
from classical computation, we are going
to look at quantum tic tac toe.
Superposition
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Many games/calculations at a time.
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Entanglement
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Measurement-Disturbance
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Random Classical States
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Weird Quantum Result
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Weirder Quantum Result
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http://www.paradigmpuzzles.com/QT3Play.htm
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