The electron spin of individual electrons in quantum dots could serve as the smallest information unit of a quantum computer say researchers.

Scientists from the Universities of Basel, Oxford and Lancaster have developed an algorithm that can be used to measure quantum dots automatically.

Writing in the Nature-family journal npj Quantum Information, they describe how they can speed up this hugely time-consuming process by a factor of four with the help of machine learning. Their approach to the automatic measurement and control of qubits therefore represents a key step toward their large-scale application.

According to Dr. Edward Laird from Lancaster University, “A machine is so much faster than a human at making measurement decisions that it could let us run experiments that we wouldn’t even be able to set up unaided.”

For several years, the electron spin of individual electrons in a quantum dot has been identified as an ideal candidate for the smallest information unit in a quantum computer, otherwise known as a qubit.

Controlled via applied voltages

In quantum dots made of layered semiconductor materials, individual electrons are caught in a trap, so to speak. Their spins can be determined reliably and switched quickly, with researchers keeping the electrons under control by applying voltages to the various nanostructures within the trap. Among other things, this allows them to control how many electrons enter the quantum dot from a reservoir via tunneling effects. Here, even small changes in voltage have a considerable influence on the electrons.

For each quantum dot, the applied voltages must therefore be tuned carefully in order to achieve the optimum conditions. When several quantum dots are combined to scale the device up to a large number of qubits, this tuning process becomes enormously time-consuming because the semiconductor quantum dots are not completely identical and must each be characterized individually.

Automation thanks to machine learning

Now, scientists from the Department of Physics and the Swiss Nanoscience Institute of the University of Basel, in collaboration with the University of Oxford and Lancaster University, have developed an algorithm that can help to automate this process. Their machine-learning approach reduces the measuring time and the number of measurements by a factor of approximately four in comparison with conventional data acquisition.

First, the scientists train the machine with data on the current flowing through the quantum dot at different voltages. Like facial recognition technology, the software gradually learns where further measurements are needed with a view to achieving the maximum information gain. The system then performs these measurements and repeats the process until effective characterization is achieved according to predefined criteria and the quantum dot can be used as a qubit.

“For the first time, we’ve applied machine learning to perform efficient measurements in gallium arsenide quantum dots, thereby allowing for the characterization of large arrays of quantum devices,” says Dr. Natalia Ares from the University of Oxford.

“The next step at our laboratory is now to apply the software to semiconductor quantum dots made of other materials that are better suited to the development of a quantum computer,” adds Professor Dr. Dominik Zumbühl from the University of Basel. “With this work, we’ve made a key contribution that will pave the way for large-scale qubit architectures.”