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 WorldA 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.