What are quantum computers? How are they different from classical computers? Will quantum computers replace classical computers?

In the recent studies,  Google quantum machine performed a target computation in          200  seconds, as per the calculations they determined that it would take world's
supercomputer roughly 10,000 years to perform same computation.

Quantum computers can crack our encrypted data/ passwords/ bank details in seconds which can took classical computers many years to break.

Many studies also says that we can also unlock encrypted message of aliens from quantum computers.
It has very wide use in space computation for fast computation.

What makes these quantum computers so powerful ?

First of all try to understand how a normal (i.e classical computer) works.

• We all know computer works on binary language i.e (1 or 0), (True or False), (On or Off).

• Computer is made up of billions of transistors. That can be switched on or off means 1 or 0.
  

• Computer stores information in the form of 0 or 1 in bits. i.e these bits can be turned on or off.

• At a time these bits can have only one value i.e (1 or 0),  (on or off), (have current or not have current).

• Now, coming to that transistor. These are used to make logic gates like -
  NOT, OR, AND, NAND, NOR, XOR etc.
  
  
• These gates are used to perform logic operations like adding two nos. , multiplying two nos. , and complex arrangement to see videos, play games etc.

This is arrangement of these gates to add two numbers.

In the above example it takes two numbers in 3 bits and gives the output results of 4 bits.

So all the computation we perform like seeing videos, photos, playing games are the clever arrangement of these gates.

Quantum Computers -

Now you have basic knowledge of working of classical computer so we are discussing quantum computers. For this you should have some knowledge of quantum mechanics.

As we have discussed bits and logic gates in classical computers, quantum computer consist of qubits (quantum bits), and logic gates similar but not same as classical logic gates.

• So, we are going in the structure of atom, atoms have fixed shells have electrons in different spin states (i.e spin angular momentum +1/2 or -1/2).

• So we denote +1/2 as binary 1 and -1/2 as binary 0 in electron spin which acts as our qubit (which were simply bit in classical computers made up of whole atom instead of electron)
• At a particular instant qubit can have either 0 or 1 or both at the same time .
• As we know in quantum mechanics we cannot determine exact spin of electron, we can only determine it's probability of having +1/2 or -1/2 at a particular instant. So, particle have to spins at same time also referred to as Entanglement.
  
• So, 1 qubit is equivalent to 2 classical bits.
  2 qubit is equivalent to 4 classical bits.
  And, N qubit is equivalent to 2n classical bits.
  It rises exponentially.
  
  For example- If you have 300 quibts in fully entangled state. So you have 2 to the power 300 classical bits which are many particles are in our universe
• Our classical computer performs n operations at a time using n bits where as quantum computer performs 2 to power n operations in n qubits in same time. So, in same time quantum computers can do more work and it increases exponentially.

Quantum logic gates -

Unlike or classical logic gates quantum mechanics have different but similar logic gates.

1- Measurement gate - Measures the inputted entangled spin state.

Electron in entangled state i.e it have different probabilities of 0 and 1 like coin have head and tail and can end up with any one 0 or 1.



Giving output 1 it can also be 0 like- head and tail in coin.

2-Swap gate - Swaps the input of two states.

Initially

Finally

3-Pauli-X gate - Like classical not gate.

Initially


Finally


There are many more gates like control gate etc. which i have not mentioned.

So using these quantum gates we make quantum computers.

Requirement for quantum computers to work - 

• Electron in an atom should be thermally stable for which we use very low temperature near about 0.02 K which is about absolute 0 k.

• In contrast, each comparable quantum computer using 25 kilowatts of power costs about $25,000 per unit per year to run.

Is quantum computer replacement for classical computers - 

No, quantum computer can only do parallelism computation (as electron exist in entangled state) in small period of time but take more time for 1 operation and can only do many operation in less time. 

If you want to see videos, play games, writing documents classical will perform well.

But for cracking encryption that is composed of multiple of two prime numbers or searching a record in database we use quantum computer for fast computation. 

So, they can only replace classical computers in only some fields.

Feel free to ask any doubt.