What about transistors? The smaller they become, the better our beloved technological devices become, but in 2021 transistors are predicted to reach their physical limit. Fortunately, quantum computers might come into help.
Moore's Law
All electronic devices that surround us are made of tiny little things called transistors, semiconductor devices that either serve as amplifiers or switches. Their sizes have shrunk considerably since the time they were invented; in 1907 they were in the order of millimeters, whereas nowadays their sizes vary in the range of nanometers. If you bring many of them together in a chip, you create the so-called integrated circuit chips, which enable our devices to operate.
"Approximately every two years the number of transistors on integrated circuits doubles", Moore's Law says. This relation affects the processor's speed and the price of electronic devices. The figure below shows the increase in the number of transistors in an electronic device every two years. In order to fit so many transistors in one electronic device, their size has to decrease. But can we make them as small as we want to? Quantum mechanics prohibits us from doing so.
Size limitation and quantum weirdness
When a transistor operates as a switch, what it basically does is that it either lets the electrons (current) flow, or it blocks them. It either outputs a logical 1, or a 0 (elementary computational logic). Transistors today achieve extremely small sizes, such as 14 nm, or 1/500 of a red blood cell, thus converging to the size of few atoms. At this scale, a weird quantum mechanical effect will start to appear, the quantum tunneling. When an electronic wavepacket hits a potential barrier, most of it will be reflected, but a fraction will still propagate through the potential barrier (a phenomenon that we do not see in the macroscopic world if the energy of the potential barrier is larger than the energy of the incoming wavepacket). This poses a big problem since the current will flow through the transistor even when it is not supposed to do so, leading to false logical values. The image below shows the phenomena of quantum tunneling.
Facing the problem of quantum tunneling, it appears that next year we will reach the physical limit of the physical gate length of transistors, as the following graph shows. From that point on, we cannot make transistors smaller.
So, what's next?
Since we cannot make transistors infinitely small, thus producing faster and more powerful devices regularly, we need to find a new way to carry on technological progress. It appears that on one hand quantum mechanics can be quite mean and strict by throwing at us limiting phenomena such as the quantum tunneling that we mentioned earlier, but on the other hand it can be quite generous and helpful, by offering us new possibilities through two quantum phenomena:
quantum superposition
quantum entanglement.
These two fundamental properties of quantum mechanics enable quantum computers to perform in few seconds calculations that would take classical computers the age of the Universe to complete.
In the next articles, we will inspect these two phenomena in detail, as well as the concept of "qubit", the smallest unit of information in quantum computers, and the advantages that quantum computers are expected to bring, along with their limitations.
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