Important milestone in creating a quantum computer

Quantum computer: one of the obstacles to progress in the search for a functional quantum computer is that the functional devices that go to a quantum computer and perform the actual calculations, the qubits, until now were made by universities and in small numbers. But in recent years, a pan-European collaboration, in partnership with the French leader in microelectronics CEA-Leti, has been exploring day-to-day transistors – which are present in billions on all of our cell phones – for use as qubits . French company Leti makes giant device-filled wafers and, after measuring, researchers at the Niels Bohr Institute at the University of Copenhagen found that these industrially produced devices are suitable as a qubit platform capable of moving to the second dimension, a step towards a functional quantum computer. The result is now published in Nature Communications.

Quantum dots in two-dimensional matrix is ​​a leap forward

One of the main characteristics of the devices is the two-dimensional matrix of quantum dots. Or more precisely, a two-by-two network of quantum dots. “What we have shown is that we can control a single electron at each of these quantum dots. This is very important for the development of a qubit, because one of the possible ways to make qubits is to use the spin of a single electron. So , achieving this goal of controlling individual electrons and doing it in a 2-D matrix of quantum dots was very important for us “, says Fabio Ansaloni, former Ph.D. student, now postdoctoral at the Quantum Devices center, NBI.

The use of electron spins has been shown to be advantageous for the implementation of qubits. In fact, their “silent” nature makes spins weakly interacting with the noisy environment, an important requirement for high performance qubits.

Extending quantum computer processors to the second dimension proved to be essential for a more efficient implementation of quantum error correction routines. The correction of quantum errors will allow future quantum computers to be fault tolerant against individual qubit failures during calculations.

The importance of industrial scale production

Assistant Professor at the Center for Quantum Devices, NBI, Anasua Chatterjee adds: “The original idea was to make a series of spin qubits, reduce electrons to a single electron and be able to control and move them. It’s really great that Leti was able to deliver the samples we used, which in turn allowed us to achieve this result. Much credit goes to the consortium of pan-European projects and generous EU funding, helping us to slowly advance the level of a single quantum dot with a single electron to have two electrons, and now moving on to two-dimensional matrices. Two-dimensional matrices are a really big goal, because it’s starting to look like something you absolutely need to build a quantum computer in. So Leti involved with a series of projects over the years, which contributed to this result. “

Credit for getting this far belongs to many projects across Europe

The development was gradual. In 2015, researchers at Grenoble were able to make the first spin qubit, but this was based on holes, not electrons. At that time, the performance of devices made in the “hole regime” was not ideal, and technology has advanced so that devices now in the NBI can have two-dimensional arrangements in the regime of a single electron. Progress is threefold, the researchers explain: “First, producing devices in an industrial foundry is a necessity. The scalability of a modern industrial process is essential as we begin to make larger matrices, for example, for small quantum simulators. , when making a quantum computer, you need a matrix in two dimensions and a way to connect the outside world to each qubit. If you have 4-5 connections for each qubit, you will quickly end up with an unrealistic number of wires coming out of the low temperature configuration. But what we can show is that we can have one door per electron, and you can read and control with the same door. And finally, using these tools, we can move and exchange electrons in a controlled manner around the matrix, a challenge in itself. “

Two-dimensional arrays can handle errors

The control of errors that occur on devices is a separate chapter. The computers we use today produce many errors, but they are corrected through what is called a repetition code. On a conventional computer, you can have the information at 0 or 1. To make sure that the result of a calculation is correct, the computer repeats the calculation and if a transistor makes an error, it is corrected by a simple majority. If most calculations performed on other transistors point to 1 and not 0, then 1 is chosen as the result. This is not possible on a quantum computer, as you cannot make an exact copy of a qubit, so quantum error correction works another way: state-of-the-art physical qubits still don’t have a low error rate, but if enough are combined in the 2-D matrix, they can keep each other under control, so to speak. This is another advantage of the 2-D array now realized.

The next stage of this milestone

The result at the Niels Bohr Institute shows that it is now possible to control single electrons and perform the experiment in the absence of a magnetic field. Therefore, the next step will be to look for spins – spin signatures – in the presence of a magnetic field. This will be essential to implement single and two qubit ports between the single qubits in the array. The theory has shown that a handful of single and two qubit gates, called a complete set of quantum gates, are sufficient to allow for universal quantum computing.