Quantum computers are a peculiar world. On the one hand there are billions in investments, and resounding promises of a quantum revolution now just around the corner. On the other, tangible results that are struggling to arrive, and deep skepticism – at least in part of the scientific community – that these can ever prove truly useful, or sustainable. Google is among the main players in the market, and has just announced a milestone that could shift the needle in favor of quantum chips: in a study on Naturethe new Willow chip would demonstrate for the first time a verifiable quantum advantage on hardware, using an algorithm called “Quantum Echoes”. What does it mean, and what consequences could it have?
Quantum supremacy
To understand Google’s announcement, we need to remember what quantum computers are. Basically, these are chips that go beyond the binary logic of traditional computers, bits made of 0 or 1, on or off, instead using qubits that can be 0, 1, or a combination of the two states at the same time. Their theoretical operation is based on two concepts that come from quantum physics: that of quantum superposition, the principle according to which a particle can exist in two states simultaneously, until it is measured and collapses into one or the other; and that of entanglement, that “spooky action at a distance” that worried Einstein so much, for which two particles can constitute a system whose quantum state is in superposition, and in which the state of one of the two particles influences that of the other, regardless of the distance that separates them.
In a quantum chip the qubits can therefore theoretically be superimposed, and simultaneously represent the 0 and 1 of the classical bits. And also entangled, in which case they share the same quantum fate, and by measuring one you automatically know the state of the other. Essentially, therefore, these are computers with a very different architecture from the traditional one, which could make it possible to perform some types of calculations much more quickly than is the case with even the most powerful latest generation supercomputers. This at least in theory: in practice, however, the advantages are yet to be demonstrated. And this is what the Google Quantum Lab has been trying to do for several years now, aiming to demonstrate quantum supremacy: that is, to prove that a quantum computer can perform a calculation that is impossible for traditional ones, or which would require an infinitely longer time even with the most powerful supercomputers.
Big G’s ads
Theoretically, Google’s quantum computer has already achieved the goal: in 2019 the company announced that it had carried out a calculation called Random Circuit Sampling (RCS) in 200 seconds, which would take millennia to be solved by the most powerful current supercomputers. Issue resolved? Not really, because despite the claims of Google researchers, we are faced with a problem known as the “quantum supremacy verification dilemma”: how do we know if the results of the quantum computer are correct, given that it is not possible to verify them using any traditional computer?
The test used on that occasion is also a calculation specifically designed to exploit the strengths of quantum computers, but without any possible practical application. Even accepting that the Google chip actually beat traditional computers, therefore, it was impossible to say with certainty that this would ever translate into any tangible benefit. In the new study, however, Google ensures that it has dispelled both doubts.
The new algorithm
To solve the problem, the company has developed a new chip, Willow, which is more powerful and much less prone to errors, one of the critical points of current quantum chips. And he devised an algorithm that provides verifiable results, and deals with a concrete physical problem. According to Google, the technique works like an extremely advanced form of echo: you send a signal into the quantum system (the qubits on the chip), perturb one of the qubits, and then reverse the evolution of the signal to see which “echo” it returns.
The technical specifications are too complicated for non-experts, but taking what is reported in the Nature study at face value, Willow succeeded in the task 13 thousand times faster than the most powerful supercomputer would do using the best known classical resolution algorithm. The type of problem addressed in this case concerns an observable physical measurement (the diffusion of quantum information in a system), and is verifiable because it can be repeated on other supercomputers of comparable power, and at least in part, also with traditional methods.
Google researchers also claim to have used their Quantum Echoes algorithm in a real experiment (which has not yet been published in a scientific journal), in collaboration with the University of California at Berkeley, in which it demonstrated that it can study the structure of two molecules with the same precision as traditional methods (nuclear magnetic resonance), also revealing information that is usually not available. In short, according to Google, quantum computers have finally demonstrated their potential.
The limits of the research
For now, however, it’s best to take Big G’s claims with a grain of salt, and a generous pinch of skepticism. In fact, for years the company has accustomed us to announcements of revolutionary successes in this field, which have still never led to any concrete progress. The fact is that this is a truly niche area of science, where a handful of experts set goals, and then claim to have achieved them. However, without providing any proof that their quantum computers will ever really be useful for anything: even if quantum supremacy were unequivocally demonstrated, in fact, it would only be the first step; the final goal would be to achieve an effective “quantum advantage”, that is, to have important and useful calculations that quantum computers can perform much faster than traditional ones.
At that point, it would actually be possible to imagine a true quantum revolution in digital technologies. Or at least in some fields, such as cryptography, climate modeling, or the study of drugs and proteins. How long it might take, however, is impossible to say: Google assures that the revolution is around the corner; other experts are much more cautious, and there is no shortage of those who believe that quantum computers are ultimately destined to turn out to be a splash in the water.