Speed up the discovery of new drugs, make weather forecasts ever more precise and localized, enhance artificial intelligence to previously unknown levels and more. These are the potentials attributed to quantum computers: computers no longer based on bits, but on qubits (quantum bits). These digital units exploit two known properties of quantum mechanics – the overlap e l’entanglement – to exponentially increase the calculation speed and solve problems that even for the most powerful supercomputers in the world would be impossible (or would take an unreasonably long time).
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The best-known potential application of quantum computers – theoretically demonstrated by Peter Shor in 1994 – is the ability to also decipher RSA cryptographic algorithms, used for example in the field of bitcoin, internet financial transactions and digital signatures. Such a potential has predictably attracted a lot of attention, but there is a problem: no one is still able to actually produce a quantum computer capable of doing anything of the kind and there is no way of knowing when it will be possible.
And then the era of quantum supremacy, announced by Google in 2019 when its Sycamore computer actually managed to solve in minutes a calculation that would have taken hundreds of years for a traditional computer? In reality, that was a demonstration with no practical application and designed by researchers for the sole purpose of showing the potential of Sycamore. The quantum supremacy of Sycamore therefore served to demonstrate that these computers can actually do something that traditional ones cannot, using experiments designed for exactly this purpose.
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“The most advanced quantum computers today have a few dozen physical qubits that cause decoherence (or noise),” he said. written the professor of quantum physics Sankar Das Sarma on the Mit Technology Review. “Building a quantum computer capable of deciphering RSA codes from such components would require many millions if not billions of qubits. Only tens of thousands of these would be used for computation, the so-called logical qubits; the rest would be necessary to correct the errors, compensating for the decoherence “.
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The decoherence it is the phenomenon that causes, due to the interaction between the qubits and the environment that surrounds them, the decay of quantum states before the work of the qubits is completed. This state is in fact extremely delicate: the smallest vibrations (even a collision between air particles) or the slightest changes in temperature can make the qubits exit early from the overlap they are in, providing completely random results. This is why, for example, IBM’s Q System One is protected by a hermetically sealed glass case equipped with a cooling system close to absolute zero.
In a nutshell, today’s most advanced quantum computers have a few dozen physical qubits, while to obtain the promised results – according to what Das Sarma said – millions or billions would be needed. The road to get here is still incredibly long and the advent of actual quantum computers could be much further away than is usually claimed.
However, the achievement of quantum supremacy propelled us into a new era, defined by NISQ scientists (noisy intermediate-scale quantum). With this acronym we mean the possibility of developing quantum computers of intermediate size – that is, with a still limited number of qubits – while knowing that the background noise will allow us to have only partial control over the results that can be obtained.
Not yet perfect quantum computers, but they would still be useful. Or maybe not? “The idea is that small collections of noisy physical qubits could still do something useful and better than a classic computer can,” says Das Sarma. “I’m not sure what objections to bring: how loud? How many qubits? Is it really a computer? What noteworthy problems could this Nisq machine solve? ”.
The experiments conducted so far through Nisq would not have shown substantial advantages over traditional computers or in any case – except for very specific research fields – they have no commercialization potential. “There have been proposals relating to the use of small-scale quantum computers for drug design (…) or to help in the field of finance,” concludes Das Sarma. “No technical paper has so far offered convincing demonstrations that quantum computers, let alone NISQ machines, can bring about significant optimizations.”
Although we often talk about quantum computers as if they were about to revolutionize the world, the reality is quite different: we could still be at the beginning of the road and decades away from realizing what, it seems, are still dreams.