 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Before we talk about Gunho’s misconception and truth about Quantum computer.
We should know what is quantum computers.
Let’s start with the definition of Quantum Computer.
This is the definition.
It is not easy for you
Because these three concepts, Superposition, Entanglement, Quibt are not familiar with you
But, you have to know it. Then, let’s start
The first is Superposition
It is due to wave property of Quantum state
Similar with sound or light, Quantum state can be added together.
So, let’s think about spinning example
If we have electron spinning like this, is it up? or Down?
We can express it like 50 % up plus 50 % down
Next is Entanglement.
It is due to the conservation of angular momentum.
You can think two quantum material is connected
Thus, if we know about one side’s information
we know the other side automatically
For spinning example, if one is upper spin then entangled the other spin is down
A quantum bit is the basic unit of quantum information
It means the quantum version of classical bit. 0 or 1
A quantum bit can exist in superposition, probability
Let’s see example in bloch’s sphere.
The red one is up, meaning 0.
The blue one is down meaning 1.
Then the purple one, psi, is it one? Zero? up? Down?
It is both. So, we should use superposition
Is it clear?
If it is not clear for you, let’s toss the coin
Classical bit.  the result should be head or tail.
 
But for qubit, it can behave like this
Is it head?tail?
We call both. So we should use probability
Now, understand it?
But it’s time to add entanglement
Let’s assume the two coins are entangled in opposite direction.
The toss it.
We don’t know the result.
But, if we know the one is head?
The other one should be tail.
Then, what is quantum computer again.
Comparing classical computer and quantum computer will help you to imagine.
Classical computer is based on classical physics
And quantum is based on quantum physic
The electric circuit in classical computer can express only one state,
but quantum phenomenon which is for quantum computer can express more than one state.
And information storage is different.
voltage is used for classical computer
Which low is 0 and High is 1
But for quantum computer we use probability.
As mentioned in previous slide.
Up is 0, down is 1.
But superposition it means 0,1, both.
So, do you guys know how Quantum Computer became famous?
Before we get to this question let’s talk about this problem
There is a circuit that takes in two inputs (which is X and Y)
and spits out two outputs.
In this problem the upper direction is 0 and down direction is 1.
input x always passes right through this circuit unaffected.
We don’t know what this circuit does to input y. But there are only four possible cases.
Case A: it does absolutely nothing to it
Case B: it flips it,
These two cases are called laid back since it is independent of x
Case C: it flips it like before, but only if x is a 1.
Case D: it flips it only if x is a 0.
These two cases are called Needy since it is dependent of x
So, with that in mind, here is the circuit
can you figure out if it’s a laid-back case, or a needy case?
You can only see the outcomes not what is happening in circuit
and you can use circuit just once.
So, when we use classical computer to solve this problem. result is shown like this.
We can not solve this problem since we can not distinguish Laid back case and needy case
So classical computer needs at least two chances to solve this problem.
But by using Deutsch’s algorithm and Quantum computer we can solve this problem
H, Handamard gate map the basis state 0 to one over root 2 and one over root two,
and map 1 to over root 2 and minus one over root two.
By using the Deutsch’s algorithm we can get each cases outcome like this
Then we can distinguish the Laid back case and needy case
Problem solved!
After Deutsch’s Approach, other quantum computer algorithms are discovered
their fast operation and usability make quantum computer becomes a very popular and topical topic
Shor's algorithm is a quantum computer algorithm for integer factorization.
Informally, it solves the problem like Given an integer N, find its prime factors
Grover's algorithm is a quantum algorithm that finds with high probability of the unique input of a black box function
that produces a particular output value, using just evaluations of the function,
where N is the size of the function's domain.
Finally let’s talk about the Misconception and Truth.
First, Misconception is the statement that quantum computer is always fast.
This is portionaly true like the deutsch’s algorithm and shor’s Grover’s
Quantum computer is fast at approximation, simulation such as traffic jam.
However, Since Quantum computing is related to matrix.
It is slow if target function is not one to one and don’t have inverse matrix
Let’s see second misconception
This misconception is parts are right and parts are wrong.
As we mentioned earlier in Misconception 1, quantum computers can play a big role in solving non-deterministic problems,
but there are some cases which it is better to use classical computers.
At the present level, even if quantum computers have commercialized,
they are used only for simple mathematical calculations, not for games and Windows.
And replacing all the functions of a classical computer with quantum computers shows very low efficiency, such as virtual machines.
So, development Goal in these days is not complete replacement.
It is highly likely that an era in which quantum computers and classical computers will complement each other in the future.
Third misconception is quantum computer has been developed to perfection.
Consider the currently called a quantum computer such as D-Wave.
D-Wave is the commercially available quantum computer that D-WAVE System made.
But we called D-Wave as a Half quantum computer.
It is because D-wave has a structure that classic computer receives and uses the result calculated in quantum CPU.
It's like having a quantum CPU on a classic computer.
So, where the quantum computer such as D-Wave is used?
NASA and Google use D-Wave for research quantum annealing method and co established Quantum Artificial Intelligence Lab.
Quantum Artificial Intelligence Lab research quantum simulation,
quantum neural networks,
quantum assisted optimization,
qubit metrology and superconducting qubit processors.
In case of IBM, one of the leading companies of quantum computer technology,
announced IBM Q System One, which is the world's first quantum computing system with a 20qubit system for commercialization
In addition, they unveiled IBM 'Quantum Experience' for accessing, programming and performing various tests by connecting to 5 qubit quantum processors in the cloud.
Now it can be freely use to program and do various test about quantum processor.
They also developed Qiskit, an open source framework for quantum computing.
And Let’s see quantum computer’s limitation is like this
First, quantum state is unstable
quantum computer can only function if the coherence of the qubit ensures that the quantum information remains intact
The researchers have now discovered that the coherence spontaneously disappears over the course of time and with the stored information as well.
Second is managing sensitive processor and maintaining superconductivity is very expensive and take a lot of space.
This is the end of our UCC, I hope Gunho will never make mistake like this.
