One of the obstacles to progress in the search for a quantum computer has been the fact that devices, qubits, so far by universities and in small numbers have entered a quantum computer and made real calculations. But in recent years, a pan-European partnership with CEA-Leti, head of French microelectronics, has been exploring everyday transistors – which are in the billions of all our mobile phones – for use as qubits.

The French company Leti makes giant leaves full of devices, and after measuring them, researchers at the Niels Bohr Institute, University of Copenhagen, have seen that these industry-produced devices are suitable as a qubit platform capable of moving into the second dimension. a step towards a working quantum computer. The result is now published *Nature Communications*.

#### The quantum dot in a two-dimensional matrix is a jump

One of the key features of the device is the two-dimensional matrix of the quantum dot. Or more precisely, two points on two quantum networks. “What we’ve shown is that we can do only one electron control at all of these quantum dots. That’s very important for the development of a qubit, because one of the possible ways to make qubits is to use a single electron rotation. So controlling electrons and doing 2D quantum dots was very important for us. “, says PhD student Fabio Ansalon, currently at the NBI Center for Quantum Devices.

The use of electron rotations has been shown to be an advantage in setting qubits. In fact, their nature makes them “quiet” when the tour interacts with the noisy environment, which is an important condition for achieving high-performance qubits.

Extending quantum computer processors to the second dimension has been shown to be essential for more efficient implementation of quantum error correction routines. The correction of quantum errors will allow future quantum computers to suffer errors during calculations in the face of individual qubit failures.

#### The importance of industry scale production

Anasua Chatterjee (NBI), an assistant professor at the Center for Quantum Devices, added: “The original idea was to make a series of spin qubits that could be lowered into a single electron and able to control and move. In this sense, it was great to deliver samples that Leti used. A lot of credit from the pan-European project consortium and generous EU funding will help us slowly move from a single quantum point to a single electron, and now we can move to two-dimensional matrices. it seems like something you absolutely need. So Leti has been involved with a number of projects over the years, which have helped that result. “

#### The credit for getting here goes to many projects across Europe

Development has been gradual. In 2015, researchers in Grenoble managed to make the first spin qubit, but that was based on holes, not electrons. Back then, the performance of devices made in the “hole regime” was not optimal, and as technology has advanced, devices now in the NBI can have two-dimensional arrays in a single electron regime. The researchers explain that the progress is threefold: “First, it is necessary to produce the devices in an industrial foundry. The scalability of a modern industrial process is essential when we start making larger arrays, for example for small quantum simulators. Second, when making a quantum computer, you need a two-dimensional matrix, and you need a way to connect the outside world with each qubit. If you have 4-5 connections per qubit, you will quickly end up with an unreal number of wires coming out of the low temperature configuration. What we’ve got to show is that we can have one door for each electron, and you can read and control it with the same door. And finally, using these tools, we moved and exchanged electrons individually in a controlled manner in the matrix, a challenge in itself. “

#### Two-dimensional matrices can control errors

Controlling device errors is a chapter in itself. The computers we use today cause many errors, but they are corrected through what is called a repetition code. On a conventional computer, you can have the number 0 or 1 to ensure that the information is the correct result of the calculation, the computer repeats the calculation, and if a transistor makes a mistake, it is corrected by a simple majority. . If most calculations performed on other transistors point to 1 and not 0, then 1 will be chosen as the result. Since this is not possible to make an exact copy of a qubit on a quantum computer, quantum error correction works in a different way: state-of-the-art physical qubits do not have a low error rate yet, but if they are combined enough in the 2D matrix, they can keep each other in control. so to speak. This is another advantage of the 2D array now implemented.

#### The next step from this milestone

The results obtained at the Niels Bohr Institute show that it is now possible to control a single electron, and to perform the experiment in the absence of a magnetic field. So the next step will be to look for turns – rotation signatures – in front of a magnetic field. This will be essential to implement a single qubit and a two-door qubit between single qubits in the array. The theory has shown that some single-port and two-qubit gates, called the whole set of quantum gates, are sufficient to allow universal quantum calculation.

Reference: Fabio Ansaloni, Anasua Chatterjee, Heorhii Bohuslavskyi, Benoit Bertrand, Louis Hutin, Maud Vinet and Ferdinand Kuemmeth, “Single Electron Operations in Quantum Arrays Manufactured in Quantum Point Casting”, December 16, 2020, *Nature Communications*.

DOI: 10.1038 / s41467-020-20280-3