A snapshot of the quantum chip contains an array of 16 quantum dots, seamlessly integrated with the checkerboard texture. Each quantum dot, like a pawn on a chessboard, is uniquely identifiable and controllable using a coordinate system of letters and numbers. Image source: Marieke de Lorijn for QuTech. Credit: Marieke de Lorijn for QuTech
The new approach to solving quantum dots offers the prospect of expanding the number of qubits in quantum systems and represents a breakthrough in Quantum Computation.
Researchers have developed a way to solve many quantum dots with just a few control lines using a checkerboard-like method. This enables the operation of the largest-ever gate-defined quantum dot system. Their results represent an important step in the development of scalable quantum systems for practical quantum technology.
Quantum dots can be used to contain qubits, the fundamental building blocks of quantum computers. Currently, each qubit requires its own address line and dedicated control electronics. This is very impractical and is in stark contrast to today’s computer technology, where billions of transistors are operated with only a few thousand lines.
How to address like a chessboard
Researchers at QuTech—a collaboration between Delft University of Technology (TU Delft) and TNO—have developed a similar method for solving quantum dots. Just as the position of a chess piece is determined using a combination of letters (A to H) and numbers (1 to 8), their quantum dots can be determined using a combination of horizontal and vertical lines. Any point on the chessboard can be identified and dealt with using a specific combination of a letter and a number. Their approach takes cutting-edge technology to the next level and enables the operation of a system of 16 quantum dots in a 4×4 array.
First author Francesco Borsoi explains: “This new way of treating quantum dots is very advantageous for scaling to many qubits. If a single qubit were to be controlled and read out by a single wire, then millions of qubits would require millions of control lines. This approach doesn’t scale very well. However, if qubits could be controlled with our checkerboard-like system, then millions of qubits could be processed by “just” thousands of lines, corresponding to a very similar ratio rate in computer chips. This reduction in the number of lines offers the prospect of expanding the number of qubits and represents a breakthrough for quantum computers, which will eventually require millions of qubits.”
Improve quantity and quality
Quantum computers will not only require millions of qubits, but the quality of the qubits will also be extremely important. Final author and principal investigator Menno Veldhorst: “We recently demonstrated that these types of qubits can work with 99.992% accuracy. That’s the highest for any quantum dot system, and means an average error of less than 1 in 10,000 operations. These advances have been made possible by the development of sophisticated control methods and by using germanium as the host material, which has many properties favorable for quantum behavior.”
Early applications in quantum simulation
With quantum computing in its early stages of development, it is essential to consider the fastest path towards a real quantum advantage. In other words: when will quantum computers be “better” than conventional supercomputers? One obvious advantage could be to simulate quantum physics, since the interaction of quantum dots is based on quantum mechanical principles. It turns out that quantum dot systems can be highly efficient for quantum simulation.
Veldhorst: “In another recent publication, we demonstrate that a sequence of germanium quantum dots can be used for quantum simulation.” This work is the first coherent quantum simulation using standard semiconductor manufacturing materials. Veldhorst: “We were able to do rudimentary simulations of resonant covalent bonding.” While this experiment is based on a small device, performing such simulations on a large system could solve longstanding questions in physics.
Future job
Veldhorst concludes: “It is exciting to see that we have taken some steps in scaling up to larger systems, improving performance and gaining opportunities in simulation and quantum computing. . An open question remains how large can we make these checkerboard circuits and in limited case, can we connect many of them using quantum bonding to build even bigger circuits.”
Reference: “Common control of a 16-semiconductor quantum dot crossbar array” by Francesco Borsoi, Nico W. Hendrickx, Valentin John, Marcel Meyer, Sayr Motz, Floor van Riggelen, Amir Sammak, Sander L. de Snoo, Giordano Scappucci and Menno Veldhorst, August 28, 2023, Natural nanotechnology.
DOI: 10.1038/s41565-023-01491-3
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