Quantum computing: principles, applications and challenges of the new technological revolution presented at the Turin Book Fair

What is this vision?

It is a vision in which instead of considering three great mysteries that in the book I call the arcana and which are superposition, that is, Schrödinger's cat; collapse, what happens when we measure a quantum system by making it change state; and entanglement, which is what is referred to in social media as the secret of love - a link, or rather a correlation between particles, typically pairs of particles, that are spatially very distant but have interacted in the past. It is a very fragile link and one that does not allow messages to be sent between the two particles, however, in a nutshell, we can say that the state of one of the two particles is affected by the actions that are performed on the other, by measurements typically. This is what is meant bynon-locality. I believe, not just me, there is a strand of interpretation on this, that by focusing on the non-locality, which is typically related, one can have a much clearer view of the interpretation of the theory.

The non-locality Einstein did not believe in, right?

Even worse, no one believed in non-locality. Einstein is the only one who shows in 1935 that if quantum mechanics were a complete theory then reality would be non-local. He says: since reality is local quantum mechanics must be incomplete, and basically this argument of his is not answered. Bohr answers him, but this is confusing, incomprehensible.

Then it turns out...

Instead, it turns out that the model Einstein had made to show an absurdity photographs a reality that turns out to be non-local.

Is this idea of non-locality now well understood? Are there applications that rely on it, but is this done by having understood it or instead by exploiting the properties of what you have understood to exist but don't know how?

I would say more the second option: it is used. Quantum computer teleportation, all applications are based on entanglement, hence non-locality, but what is the correct description of it is unclear, there are different proposals and there is no agreement in the scientific community.

But how does the quantum computer work?

The classical computer, i.e. the one we have at home, works on the basis of the so-called bit, which is the fundamental element for calculation. The bit is a physical object that has 2 possible states, which we call 0 and 1. Typically, a microscopic circuit that can be open (0) or closed (1). In the quantum computer, instead of the bit, there is an object called a qubit. Like the bit, it has two possible states, which are 0 and 1, but according to the axioms of quantum mechanics it can also have intermediate states between 0 and 1, i.e. states that are a little bit 0 a little bit 1 like the famous shrödinger's cat, which possesses intermediate states between being alive and being dead. It also possesses states in which 0 is much more present than 1, or states in which 1 is much more present than 0: an infinity of intermediate states. The limiting thing, however, is that when the quantum computer is observed, each qubit returns either 0 or 1, i.e. the wealth of intermediate states is lost. One might therefore think that this availability of infinite intermediate states has no effect on the functioning of the computer, instead it has been demonstrated with this science called quantum information, that thanks to the availability of these intermediate states and also to entanglement, i.e. non-locality, that the quantum computer, in certain particular problems, can be faster than the classical one. Now, the question that can be asked is: physically, i.e. on the hardware level, what are these qubits? Well, over the past decades, many technologies have been developed that can provide qubits for quantum computers. For example, ion traps, superconducting circuits or quantum dots. And there are still many other proposals. some are robust in the sense that they resist interactions with the environment that destroy entanglement, while others are scalable, i.e. they make it possible to have systems with many qubits. Research is extremely active on this point. Finally, precisely because entanglement is very fragile, it must be preserved in order to perform quantum calculations. For this purpose, the quantum computer must be kept at very low temperatures, which guarantee the preservation of entanglement. Nevertheless, it is possible for errors to occur during calculation. This possibility is answered by a very rich strand of research called quantum error correction.

But why do qbits allow for infinitely more computing power than bits?

It is not immediate to describe why the quantum computer is faster than the classical one. First of all, this acceleration is only real for some specific problems, it is not universal. One of these problems is the aforementioned factorisation of large numbers. Another is the search for an object in a set without structure, the so-called 'needle in the haystack'. The best non-quantum algorithm is to proceed by 'brute force', i.e. to examine the strands of straw one by one until the needle is found. With quantum mechanics, on the other hand, it is possible to put all the needles 'in superposition', i.e. to consider them all together. Just as a qubit, once observed, returns either zero or one, so if we observe the superposition of the elements of the haystack we will find either a straw strand or the needle, and the straw strand is much more likely than the needle because there are so many straw strands and only one needle. This is where we are at the heart of quantum mechanics. To solve this problem, there are operations to be performed, to be iterated, which at each step increase the probability of finding the needle. After a given number of steps, it becomes much more likely to find the needle than a strand of straw. At that point, observation is made. It is said that one prepares a 'quantum soufflé': the computer state 'swells' around the needle and one has to pull it out at the right moment. And the right moment is after a number of steps that is equal to the square root of the number of steps needed to proceed by brute force: that's why quantum computing is faster. In this process, as I explain in detail in the book, the three 'arcana' of the theory play a crucial role: superposition, entanglement and collapse.

Discovering non-locality is a metaphysical revolution?

The debate on the structure of space is first and foremost a philosophical debate that has always been there in philosophical reflection. With the work of Einstein, the later work of John Stewart Bell, and the experiments of John Clauser, Alain Aspect and Anton Zeilinger (who were awarded the Nobel Prize in 2022) and others, this qualitative property of space was experimentally proven, and for this research the philosopher Abner Shimony coined the term experimental metaphysics.

What is it?

It is above all a slogan that was coined by Shimony, the sense is that it is a statement, a problem that has moved from the philosophical field to the experimental scientific field, this does not mean that science is gnawing away at philosophy, because many philosophers are working on the concept of non-locality as it emerges from science, I would say rather that it opens up an interaction between these two fields that have spoken little to each other in recent years.

Is the machine you have at the Politecnico di Torino still an experimental machine?

Yes, in the sense that it is a small machine, it only has 5 qbits, the one in Naples has 24, ours has a strong didactic role, on the one in Naples it is already a machine on which meaningful algorithms can be launched. But these machines work: the quantum computer theory based on non-locality is verified even by small laboratories.

I have heard that the quantum computer could have commercial applications as early as 2028, is this true?

I like to be cautious and think that the quantum computer will become an important research tool in the study of quantum systems themselves, then in the study, combined with artificial intelligence, of new molecules to be synthesised for example to absorb carbon dioxide in the atmosphere and limit global warming.

Here we talked about university research, what happens when the quantum computer is used by private individuals or governments?

It is the private individuals who have invested the most in hardware, in realisation: Google and Ibm, but also Microsoft and Amazon have the most powerful quantum computers, or even smaller companies who have only invested in this. Private individuals having the possibility to invest quickly and on a large scale are further ahead. Other entities such as the military could use the quantum computer for many purposes, for example decrypting messages from enemies, which is precisely the field in which the quantum computer should show us its strength.

Decrypting enemy messages with a previously unthinkable capability is no small feat...

Indeed, it was an algorithm made by Peter Shor in 1996 that fuelled the interest of the scientific community and beyond in these applications, because in that algorithm Shor showed that the quantum computer is much faster than the classical one at finding the prime factors of a large number. Today, much of today's cryptography is secure precisely because this problem cannot be attacked at the moment. It is a problem that is used to create cryptographic keys, and these are secure if that problem cannot be solved.

So, if from a geopolitical point of view alone with Artificial Intelligence we are already seeing the world change centre of gravity and polarise, what will happen with the quantum computer?

Those working in this field plan a future of synergy between the two technologies, clearly whoever gets there first has a very significant strategic advantage. China is investing heavily in this.