by Science Buff
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Why Do We Need Quantum Computing

Chapter 1 of 4

Computing Power Is Reaching Its Limits
Scientists are attempting to build a new kind of computer called a Quantum Computer because it promises tremendous computing power enough to help us solve some really tough mathematical problems that are holding back our progress in a number of fields. In this beginners guide we try to understand how the computers we use today have reached their limit of computation.

So let's dive in! 

Chapter 2 of 4

Computers Have Gotten Really Small
Well, you are probably reading this guide on a laptop computer or a desktop computer or perhaps your mobile phone. Your computer device can now fit into a kids school bag or into your pocket if its a mobile phone. You can watch YouTube videos, browse your Facebook, visit a number of websites and so much more on these devices. 

You might take the small size and power of these devices for granted.

But there was a time long long ago when these computers were the size of an entire room but could hardly do the calculations that you do on a simple calculator app on your phone today. Look at this one below from the 1940s. 

Chapter 3 of 4

How Did Computers Become So Small
Well, to understand that we need to know what makes up a computer. A computer is basically an electronic circuit made up of connecting wires and a number of switches that can be switched ON or OFF. You can see an example of a switch in the diagram below. This switch is however a mechanical switch that you would have to push with your finger. In a computer's electronic circuit, switches can be opened and closed electrically. They are special switches called transistors.
By placing switches (transistors) at the right places in a circuit, we can start and stop the flow of electric current in different places in the circuit. This helps us build decision logic (condition) into the circuits. That is the circuit now has the ability to decide the flow of current based on a certain condition. Such a decision logic is called a Logic Gate. Given below is the diagram of an OR Gate that allows current to the bulb when either switch A or switch B is ON  (closed). 
We can add one or more of such logic gates to make more complicated conditions, which are nothing but computations, and you have just built yourself a computer.

The room-sized computer shown in the previous slide, was huge, because it had a large number of such gates, made of switches (transistors) and they needed space. But over time, we found new manufacturing techniques to create smaller transistors and fit them in smaller spaces, like circuit boards. As the transistors got smaller, the circuit boards became smaller which we started calling 'chips'. 

We have made these transistors so small that we can now fit nearly 4.3 billion of them on a chip as small as the tip of our fingers. Using so many transistors we can create a large number of logic gates and do highly complex computations, thanks to which our mobile phones can stream YouTube videos in HD. 


Chapter 4 of 4

Can We Go Any Smaller?
We are already reaching the physical limits of making the switches (transistors) smaller, and have created a switch as small as 1 nanometer . This is in the atomic range. The size of atoms lies in the range of 0.1 to 0.5 nanometers. 

But at such small sizes, particles like electrons start behaving weirdly. They stop obeying some of the laws of physics that we normally apply to large objects (like a basketball); instead they operate on a completely new set of laws. Scientists have a name for this weird world of small particles; they call it the "Quantum World" and the study of these weird new laws is called "Quantum Physics"

Difficult To Create Switches In The Quantum World

A charged particle can be repelled (or slowed down) by an opposing electric field. In a transistor (switch), we create a wall of charged atoms in the middle of the transistor material. These charged atoms produce an Electric field along the axis of the current flow. If we create an Electric field that opposes the negatively charged electrons, the electrons slow down to zero speed ; as a result stopping the current. Thus the wall of charged atoms acts as a barrier to the flow of current. 

Here is where the weird Quantum properties come into play. Due to a weird phenomenon called "Quantum Tunnelling", some electrons making up the current may just disappear from one side of the wall and magically appear on the other side of the wall,  thus leading to some current even when there is an opposing electric field. Thus the switch doesn't behave perfectly; it doesn't stop all the current. We need complete ON or OFF states to rely on our switch for computations. 

You must be puzzled what Quantum Tunnelling is all about. Without going into too many details, it is a phenomenon observed in the really small particles (in the quantum realm), where they have the probability of passing through a potential barrier, because in the quantum world particles behave as a wave of probabilities.

So we can't go any smaller, because of which we have reached our limit of computing power.
Since we cannot have a perfect transistor switch at such small sizes, we reach a final limit of how many transistors we can fit in a small area, keeping the transistors just big enough to avoid the Quantum Tunnelling effect. This in turn holds us back from increasing the computation power of our computers. 

This is why scientists are working hard to tame the different quantum phenomenon itself to their advantage in computing. This has led them into developing an entire field of quantum computing research which uses new methods of making calculations. 

If you enjoyed reading this guide, check out my guidebook on Wave Particle Duality and Superposition Explained  where we discuss the important theory of 'Superposition' in quantum physics that forms the basis of quantum computing.
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