https://retina.elpais.com/retina/2018/09/06/tendencias/1536224415_863634.html

Schrödinger’s cat is the most famous cat in the History of Science for a curious characteristic: tucked inside its box it can be alive and dead at the same time. I do not live or die, but alive and dead at the same time. Your state is only determined when someone opens the lid and looks inside. Then you will find the cat stiff or meowing.

This metaphor seeks to make visible one of the characteristics of quantum mechanics: quantum objects, which live in the subatomic world (cats, therefore, are not), can be in several states at once. For example, an electron can be in an overlap of two spin states at the same time (roughly, as if it were rotating in both directions at the same time). Yes, it is difficult to understand for the human mind. Based on characteristics of the nature of the microscopic world like this, quantum computing (CC) aims to make calculations and make computers faster and more efficient, “says Francisco Gálvez long-time expert at IBM dedicated to the dissemination of the CC that now faces other Projects.

**How is it computed?**

Before understanding quantum computing, it is necessary to understand classical computation. Normal computers, like the ones you and I use, work on the basis of digits 0 and 1 (hence the popular digital term). These 0 and 1 abstract, called bits, in the physical world are embodied in small electric currents that represent 0 and 1 and whose switches are a few devices called transistors, fundamental for technology. On a silicon chip there are billions of tiny transistors. With these tiny electric currents millions of mathematical operations per second are made (hence the gigahertz of the processors). When you play a videogame or write in a word processor or waste your time in a social network, what is behind, in the background, are these millions and millions of ultra-fast mathematical operations. Here is the prodigy of information technology. Some analogy can be drawn between the history of classical and quantum computing.

“This is a sweet moment for the CC,” says Miguel Ángel Martín-Delgado, Professor of Theoretical Physics at the Complutense University of Madrid (UCM). “It’s as if we were in the garage with Steve Jobs, they did not realize that there was a revolution going on with that type of garage computers, and at that moment of boiling, we are now with quantum computers.”

**The qubits**

The novelty with quantum computing is that, instead of using transistors that can generate states 0 or 1, it uses so-called qubits (quantum bits), which are not only in 0 or 1 but in an overlap of both states, such as the cat of Schrödinger, dead and alive at the same time. This superposition of states enables an exponentially greater computing capacity, faster and more efficient computers. Yes, no one “look” at the qubits, while nobody opens the cat’s box.

To make the qubits physically certain devices are used, just as classic computers have used relays, vacuum pumps and, now, millions of small transistors. The palpable base of a quantum computer can be of several types. For example, ion traps, which take atoms that are missing one or more electrons (ions) and interact with lasers, or superconducting rings at temperatures close to absolute zero. The latter is the technology most used today. Any of these devices can simulate a qubit in superposition of several states.

The advantage of qubits is that, thanks to properties such as superposition and quantum entanglement (when operating with two or more of them), they can perform many more operations at the same time as the classical bits. And that number of operations grows exponentially (2 raised to n) with the number (n) of qubits. With a qubit you can do two operations at the same time. With two qubits, four operations. With 10 qubits, 1024 operations at the same time. With 15 qubits, 32,768 operations … With this calculation power problems previously considered difficult to process are easily solvable.

The difficulties that appear on the road to quantum computing are not easy to overcome. The main ones have to do with the sensitivity of qubits to disturbances and noise. “They are the problem of decoherence, which is the destruction of quantum states by interaction with the environment, and the precision: right now the accuracy of a quantum computer can be 99.3%,” says Juan José García Ripoll, researcher of the Institute of Fundamental Physics of the Superior Council of Scientific Research (CSIC).

“Nobody is perfect”, says Professor Martín-Delgado, part of whose work in the Quantum Information and Computation Group of the UCM is to eliminate the errors of quantum computers. “These errors have their source in the environment, the qubits are coupled to the outside, to magnetic or electric fields, to thermal effects. Then when you want to do a logical operation it does not turn out perfect, “he explains. The environment acts as the observer of the Schrödinger cat, destroying the superposition of quantum states. The qubits prefer to work in isolation, far from the world’s gaze: otherwise they go on strike. In addition, the more qubits there are in a computer, the more easily decoherence can occur, it is increasingly difficult to make them work together without breaking the delicate “silence” they need.

**What is it for?**

What would (or does) serve quantum computing? At the moment it is useful in cryptography and can be used for chemical calculations, designs of new materials or to search quickly in large non-indexed databases. It can help machine learning and the development of artificial intelligence. We do not know its full potential: at the beginning of classical computing, when the ENIAC computer occupied a whole room, it was only used for complex calculations of military ballistics or other large industrial calculations and nobody imagined the endless number of applications that computer technology would reach. offer to the ordinary citizen. Or that every citizen would have a smartphone in his hand as an extension of his body.

“Quantum computing has been cooking for about twenty years,” explains Ignacio Cirac, the Spanish researcher at the Max Plack Institute. The research began in universities and research centers: they were able to build the first prototypes, which have evolved to reach 10 qubits). “What has changed a couple of years ago,” continues Cirac, “is that large technology companies have announced that they will devote great efforts to build more powerful quantum computers.” Google, IBM, Intel or Microsoft among others, invest now not a few efforts and compete to develop this technology.

For the moment they have built larger prototypes (up to more than 50 qubits), still imperfect. Therefore, in the words of Cirac, “the challenges that arise are: first, to improve current prototypes so that they have fewer errors, second, to develop applications for these prototypes, and third, much longer term, to build much larger computers ( of millions of qubits) where errors do not occur (or can be corrected) “.

There are several types of quantum computers. “Some, called optimizers, solve only optimization problems,” says García Ripoll. For example, in the D-wave optimizer (also called adiabatic computer) “the problem is reformulated as the energy of a physical system and its minimums are sought, thus throwing solutions to the problem”, says the scientist. Thus, the Volkswagen company is already working with D-Wave to optimize car routes in places like Beijing and fight against traffic jams.

Then there are the computers themselves, because they can be programmed to solve any problem. Like the machine that Alan Turing imagined and that defines what a computer is. They are those who try to develop the large companies mentioned above.

**Who will win?**

The term quantum supremacy is rejected by many scientists for its ugly connotations, and they try to change it for a more kind and precise one: the quantum advantage. This will happen when a quantum computer manages to solve the same problem as a classic computer (one of the powerful supercomputers) in a shorter time. The moment, according to the experts, is close. At Google they claim to have it at their fingertips with their quantum processor Bristlecone. Such a fact will mean a turning point in the history of technology.

The CC, in addition, is a necessary step for the technological progress: the miniaturization is coming to an end, because the transistors of the classic computers are already too small: Intel already builds them of only 10 nanometers. It is a very small device: an atom is a little less than a nanometer. And you can not build a transistor smaller than an atom, which is the brick from which things are made. The limit is near. “In addition, at those nanometric scales may appear unwanted quantum effects that make the functioning of the chips fail”, confirms Professor Martin-Delgado. The famous (empirical) Law of Moore, which says that the power of computers grows exponentially, doubling every 18 months, comes to an end by giving oneself with the very limits set by nature. The natural path to follow is that of quantum computing.

Will the arrival of quantum computers mean the disappearance of classic computers? “No,” explains Martín-Delgado, “in reality both types of computers are always going to live together. To treat the data thrown by a quantum computer, a classical computer will always be necessary. Nor will it end human creativity: when you get a piece of information you’ll have to keep thinking to see what you’ve got. ”