Myths and Facts about Quantum Computers
In October 2019, quantum computing dominated news headlines around the world for several days. A team of researchers from the tech giant Google had managed to achieve quantum supremacy, beating the largest supercomputers on the planet with a quantum computer.
Not only that, but the time difference was simply staggering: a few minutes versus the thousands of years required to perform the same calculation on a traditional computer.
Dozens of articles and reports in the press, radio and television echoed this historic milestone and tried to explain to the non-specialized public what Google’s achievement really consisted of and what those mysterious quantum computers that had been used to achieve it were.
No magic or fantastic superpowers
In popular articles on quantum computing it is common to find a series of recurring analogies and images that do not correspond to reality and that contribute to creating false myths around the true capabilities of quantum computers.
One of the most repeated is that “a quantum computer finds the solution to a problem by simultaneously testing all possible options”.
This explanation does not oversimplify the operation of quantum computers. Rather it seems to endow them with fantastic superpowers whereby completing any calculation is a matter of pressing a button and waiting a few seconds.
But then, isn’t it true that a quantum computer uses massive parallelism to explore all the solutions to a problem at the same time?
Quantum computers are not the solution to solve all kinds of calculations.
As in many things that have to do with the quantum world, the answer is both yes and no. It is true that one of the main properties on which quantum algorithms rely is superposition, that mysterious tendency of certain physical systems to find themselves in a combination of several different states. But that is only a part, and a rather small one, of the whole story.
mathematical choreography
We could define quantum computing as the discipline that studies the use of the properties of subatomic particles to perform calculations. Among these properties is, yes, overlap, but also entanglement and interference.
In a way, we could say that a quantum algorithm first creates a superposition of many possibilities to explore, then intertwines these possibilities with their results, and finally makes the bad solutions interfere with each other so that only those that interest us survive.
This phase of annihilating unfavorable options is the most difficult and delicate part of the whole process. It is a kind of complex mathematical choreography, to use the words of Scott Aaronson and Zach Weinersmith, that we only know how to carry out in some specific problems.
Moreover, it has long been shown that in certain tasks it is not possible to take advantage of quantum computing to speed up calculations with respect to traditional computers.
A quantum computer is not, therefore, that magical device capable of instantly solving any problem that the tabloid press sometimes wants to sell us. But neither is it simply a faster computer.
Another of the fallacies that is common to find in popular articles on quantum computers is the reduction of all their capabilities to a mere increase in speed.
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