A new approach to tackling quantum dots offers the prospect of increasing the number of qubits in quantum systems and represents a breakthrough for quantum computing.
Researchers have developed a way to handle many quantum dots with just a few control lines using a checkerboard-like method. This enabled the operation of the largest gate-defined quantum dot system ever built. Their result represents an important step in the development of scalable quantum systems for practical quantum technology.
Quantum dots can be used to hold qubits, the building blocks of a quantum computer. Currently, each qubit requires its own address line and dedicated control electronics. This is highly impractical and in stark contrast to today’s computer technology where billions of transistors are handled with only a few thousand lines.
Addressing like a chessboard
Researchers in the QuTecha collaboration between Delft University of Technology (TU Delft) and TNO have developed a similar method for tackling quantum dots. Just as chess pieces’ positions are indicated using a combination of letters (A to H) and numbers (1 to 8), their quantum dots can be addressed using a combination of horizontal and vertical lines. Any point on a board can be defined and addressed using a specific combination of a letter and number. Their approach takes the state of the art to the next level and enables the operation of a system of 16 quantum dots in an array of 44.
Lead author Francesco Borsoi explains: This new way of approaching quantum dots is beneficial for reaching many qubits. If a single qubit is controlled and read using a single wire, millions of qubits will require millions of control lines. This approach doesn’t fit very well. However, if qubits could be controlled using our checkerboard system, millions of qubits could be addressed using just thousands of lines, corresponding to a ratio very similar to that of computer chips. This line reduction offers the possibility of increasing the number of qubits and represents a breakthrough for quantum computers, which will eventually require millions of qubits.
Improving quantity and quality
Not only will quantum computers require millions of qubits, but the quality of the qubits will also be extremely important. Last author and principal investigator Menno Veldhorst: Just recently, we demonstrated that these types of qubits can be used with 99.992% fidelity. This is the highest for any quantum dot system and means an average error of less than 1 in 10,000 operations. These advances have become possible by developing sophisticated control methods and using germanium as a host material, which has many properties favorable for quantum functioning.
Early applications in quantum simulation
Given that quantum computing is in an early stage of development, it is important to consider the fastest path to a practical quantum advantage. In other words: when will a quantum computer be better than a conventional supercomputer? An obvious advantage may be the simulation of quantum physics, since the interaction of quantum dots is based on the principles of quantum mechanics. It turns out that quantum dot systems can be very effective for quantum simulation.
Veldhorst: In another recent publication, we show that an array of germanium quantum dots can be used for quantum simulation. This work is the first quantum coherent simulation using standard semiconductor manufacturing materials. Veldhorst: We are able to perform rudimentary simulations of resonant valence bonds. While this experiment was based on just a small device, running such simulations on a large system could solve longstanding questions in physics.
Veldhorst concludes: It is exciting to see that we have made several strides in moving to larger systems, improving performance and gaining opportunities in quantum computing and simulations. An open question remains: how big can we make these checkerboard circuits and, if there is a limit, if we can interconnect many of them using quantum links to build even bigger circuits.
Reference: Shared control of a 16semiconductor 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, Nanotechnology of nature.
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