Important milestone in the creation of a quantum computer

Quantum Computer: One of the obstacles to progress in the search for a working quantum computer has been that the work tools that go into a quantum computer and the actual calculations, the kwbits, have so far been made by universities and in small numbers. But over the past few years, a pan-European collaboration, in partnership with French microelectronic leader CEA-Leti, has been investigating the everyday transistors – which are present in billions in all our cell phones – for their use as quits. The French company Leti fills giant wafers with devices, and after the measurement, researchers at the Niels Bohr Institute, University of Copenhagen, found that these industrially manufactured devices are suitable as a qubit platform that can move to the second dimension, ‘ an important step for a working quantum computer. The result is now in Nature communication.

Quantum points in two-dimensional array are a leap forward

One of the most important features of the devices is the two-dimensional array of quantum points. Or more precisely, a two-by-two grid of quantum points. “What we have shown is that we can realize single electron control in each of these quantum points. This is very important for the development of a kwbit, because one of the possible ways to make kwbits is to turn a kwbit. “Achieving this goal of controlling the single electrons and doing so in a 2-D series of quantum dots was therefore very important to us,” said Fabio Ansaloni, former Ph.D. student, now postdoc at the Center for Quantum Devices, NBI.

The use of electron spins is beneficial for the implementation of qubits. In fact, their ‘quiet’ nature makes turns poorly interact with the noisy environment, an important requirement to obtain high-performance qubits.

It has been proven that the expansion of quantum computer processors to the second dimension is essential for a more efficient implementation of quantum error correction routines. Quantum error correction will enable future quantum computers to be error tolerant against individual errors during the calculation.

The importance of production in industry

Assistant Professor at the Center for Quantum Devices, NBI, Anasua Chatterjee, adds: “The original idea was to make a variety of spin qubits, go to single electrons and be able to control and spin them around In that sense, it’s really great that Leti was able to deliver the samples we used, which in turn made it possible for us to achieve this result. A lot of credit goes to the pan-European project consortium and generous EU funding. , which helps us to move slowly from the level of a single quantum point with a single electron to two electrons, and now move on to the two-dimensional arrays. Two-dimensional arrays are a very big goal because they start to look like something you absolutely needed to build a quantum Leti has therefore been involved over the years in a series of projects that have all contributed to this result. ‘

The honor of getting this far belongs to many projects in Europe

The development was gradual. In 2015, researchers in Grenoble managed to make the first rotating qubit, but it was based on holes, not electrons. At the time, the performance of the devices manufactured in the ‘hole regime’ was not optimal, and technology advanced so that the devices now available at NBI could have two-dimensional arrays in the single electron regime. The progress is threefold, the researchers explain: “Firstly, the manufacture of devices in an industrial foundry is a necessity. The scalability of a modern, industrial process is essential because we are starting to make larger arrays, for example for small quantum simulators. Secondly , if you are making a quantum computer, you need a two-dimensional array and you need to connect the external world with each kwbit.If you have 4-5 connections for each kwbit, you will quickly come up with an unrealistic number of wires from the low temperature setup.But what we managed to show is that we can have one gate per electron, and that you can read and control with the same gate.And lastly, using these tools, we could only electrons move in a controlled way around the array, a challenge in itself. ‘

Two-dimensional arrays can control errors

Controlling errors that occur on the devices is a chapter in itself. The computers we use today produce many errors, but these are corrected by the repetition code. In a conventional computer you can have information in a 0 or a 1. To make sure that the result of a calculation is correct, the computer repeats the calculation and if one transistor makes a mistake, it is corrected by a simple majority. . If the majority of the calculations performed in other transistors indicate 1 and not 0, 1 is selected as the result. This is not possible on a quantum computer, as you can not make an exact copy of a qubit, so quantum error correction may work differently: the latest physical qubits do not yet have a low error rate, but if it’s enough to be combined in the 2-D array, they can keep each other in check, so to speak. This is another advantage of the now realized 2-D array.

The next step from this milestone

The result realized 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. So the next step would be to look for spin – signatures – in the presence of a magnetic field. It is essential to implement single and two qubit gates between the single qubits in the array. Theory has shown that a handful of single and two kwbit gates, called a complete set of quantum gates, are sufficient to enable universal quantum calculations.