The quantum computer prototypes developed by IBM, Google, Intel and Honeywell, among other companies, have few qubits. Its complexity has continued to increase over the last five years, but even so, their qubits are not enough so that we can stop considering them prototypes and can begin to face a truly significant range of problems. Your ability to right your own mistakes is at stake.
The companies that I mentioned in the previous paragraph and some others are making efforts to develop technologies that allow them to increase the scalability of their quantum chips, but it is not easy to get them ready. And it is not because it is crucial not only to have high quality qubits; It is also essential to have a precise control system. This quote from the Spanish physicist Ignacio Cirac, who is one of the founding fathers of quantum computing, is taken from the conversation we had with him in June 2021 and expresses very clearly why it is so important to have many qubits:
“The number of qubits will depend on the type of problems we want to solve with quantum computers. To tackle symbolic problems we will need to have several million qubits. Probably even hundreds of millions of qubits. Right now we are talking about a hundred qubits, so “There is a long way to go. There are people who say that with 100,000 qubits maybe a specific problem can be solved, but really a lot of qubits are needed.”
This experiment draws a horizon in which chips with millions of qubits appear
The quest for the high scalability that will presumably make fully functional quantum computers possible can be approached using very different strategies. One of them is to refine the silicon transistor manufacturing technology currently used by integrated circuit manufacturers to make it possible to produce a chip capable of agglutinating many qubits. A group of researchers at the University of Basel in Switzerland has taken this approach in an extraordinarily promising experiment.
Spin is an intrinsic property of elementary particles, like electric charge, derived from their angular rotation moment.
And they have managed to develop a two-qubit logic gate inside a conventional silicon transistor. Their strategy is to turn to a type of qubit that uses the spin of an electron, or the spin of a hole (a hole essentially identifies the absence of an electron in a semiconductor). Spin is an intrinsic property of elementary particles, as well as the electric charge, derived from their angular rotation moment. The first experimental evidence supporting its existence came in 1922 thanks to the experiments of German physicists Otto Stern and Walther Gerlach.
The reason why it is not easy to understand precisely what spin is is because it is a quantum phenomenon, so it is not entirely correct to describe it as a conventional rotational motion in space. Even so, the description that I have proposed in the previous paragraph is usually used for didactic purposes because it helps us to intuit without too much effort what we are talking about. In any case, the quantum nature of this property tells us something important: measuring it is difficult.
Both electrons and holes have spin, so this property can take on one of two possible states: up or down. The analogy with the bits of classical computers, which can also adopt one of two possible values (0 or 1) is obvious. However, the spin of a hole has an important advantage over the spin of an electron when used to implement a qubit: can be completely electrically controlled without the need to resort to additional elements on the chip, such as, for example, micromagnets.
To focus the shot a little more, what we are interested in knowing is that physicists at the University of Basel have shown that it is possible to trap and use the spin of a hole in a semiconductor to make a qubit. Said like that it doesn’t seem like a big deal, but it is a very important milestone. And it is because it opens wide the door to the possibility of using current semiconductor manufacturing technology, which has indisputable maturity, to produce integrated circuits capable of agglutinating millions of qubits. For now we only have this experiment, but there is a possibility that this strategy will prosper and in the medium term it will give us a very pleasant surprise. Hopefully.
Imagen | IBM
More information | University of Basel
In WorldOfSoftware | China has reached one of the holy grails of quantum physics. So says Peter Zoller, father of quantum computers