“Reaching this goal of controlling the single electrons in a 2D array of quantum dots was very important”

If the majority of the calculations

performed in other transistors point to 1 and not 0, then 1 is chosen as the result. This is not possible in a quantum computer since you cannot make an exact copy of a qubit, so quantum error correction works in another way: State-of-the-art physical qubits do not have low error rate yet, but if enough of them are combined in the 2D array, they can keep each other in check, so to speak. This is another advantage of the now-realised 2D array. The result realised at the Niels Bohr

from the EU, helping us to slowly move from the level of a single quantum dot with a single electron, to having two electrons and now moving on to the two-dimensional arrays. Two-dimensional arrays are a really big goal, because that’s beginning to look like something you absolutely need to build a quantum computer. So Leti has been involved with a series of projects over the years, which have all contributed to this result.’

The development has been gradual. In

2015 researchers in Grenoble succeeded in making the first spin qubit, but this was based on holes, not electrons. Back then, the performance of the devices was not optimal, and the technology has advanced so that the devices now at NBI can have 2D arrays in the single electron regime. The progress is threefold, the

researchers explain: ‘First, producing the devices in an industrial foundry is a necessity. The scalability of a modern, industrial process is essential as we start to make bigger arrays, for example for small quantum simulators. Second, when | @scwmagazine

making a quantum computer, you need an array in two dimensions, and you need a way of connecting the external world to each qubit. If you have four or five connections for each qubit, you quickly end up with an unrealistic number of wires going out of the low-temperature setup. But what we have managed to show is that we can have one gate per electron, and you can read and control with the same gate. And lastly, using these tools we were able to move and swap single electrons in a controlled way around the array. A challenge in itself.’

Two-dimensional arrays can reduce errors The computers we use today produce plenty of errors, but they are corrected through what is called the repetition code. In a conventional computer, you can have information in either a 0 or a 1. To be sure that the outcome of a calculation is correct, the computer repeats the calculation and if one transistor makes an error, it is corrected through a simple majority.

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 will be to look for spins – spin signatures – in the presence of a magnetic field. This will be essential to implementing single and two-qubit gates 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 enough to enable universal quantum computation. The paper also notes the importance of this research for the development of fault-tolerant quantum computers. ‘The development of 2 × N qubit arrays may prove useful for the realisation of fault- tolerant spin-qubit quantum computers, trading topological constraints against lower error thresholds. The systematic loading of such extended arrays with individual electrons, as well as the controlled movement of electrons along the array, can be facilitated by virtual control channels similar to those used in linear arrays. Recent experiments even suggest that the capacitive coupling of multiple 2×N arrays on one chip may be possible, opening further opportunities for functionalisation and extensions.

Winter 2021 Scientific Computing World

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