Photo worth seeing: A chessboard made of quantum dots

Photo worth seeing: A chessboard made of quantum dots
By arranging quantum dots in a checkerboard pattern, it is possible to control multiple points via the same wires – a breakthrough in the development of quantum technologies, which are expected to require millions of dots. In the image, a checkerboard motif has been inserted into the photograph of the chip. © Marieke de Lorijn for QuTech

In today’s computer technologies, several billion transistors are controlled by a few thousand electrical paths. This is not yet possible in the field of quantum technologies. Until now, quantum dots, which contain the basic building blocks of quantum computers, so-called qubits, had to be addressed individually via their own control electronics. But now researchers at the QuTech Institute, a collaboration between Delft University of Technology and the Netherlands Organization for Applied Scientific Research, have made a breakthrough.

Inspired by the structure of a chessboard, the scientists arranged 16 quantum dots in a square so that each point can be clearly identified and controlled like in a coordinate system with the combination of a horizontal and a vertical wire. This so-called 4×4 array is currently the largest controllable arrangement of quantum dots on a chip. The picture shows a photograph of the chip with an integrated checkerboard motif.

“This new way of addressing quantum dots is advantageous for extending systems to many qubits,” explains Francesco Borsoi, lead author of the results published in the journal Nature Nanotechnology. “If each individual qubit is read from its own control path, millions of wires would be required for millions of qubits. With our checkerboard-like system, however, millions of qubits could be controlled by just a few thousand wires, with a ratio similar to that on normal computer chips.”

By reducing the number of lines in quantum dot arrays, scientists are getting much closer to producing quantum systems with a scalable number of qubits. The development is an important step on the way to quantum computers, which are expected to require millions of qubits. The QuTech researchers were only recently able to show that the germanium quantum dots they used deliver results with an accuracy previously unattainable in quantum dot systems and, when arranged as an array, can be successfully used for quantum simulations.

Last author and research leader Menno Veldhorst explains: “It is exciting to see that we have made some steps in scaling to larger systems, improved performance and opened up new possibilities for quantum computers and simulators. The question remains how big we can make these checkerboard circuits and, if a limit exists, whether we can connect multiple circuits to build even larger circuits.”

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