Researchers at QuTech in the Netherlands have developed a way of controlling a large array of quantum dots with a relatively small number of control lines. The technique is an important step towards the development of scalable quantum systems for quantum computing and other quantum technologies.
Quantum dots are nanoscale collections of atoms that can store quantum information in the form of quantum bits, or qubits, which form the basis for quantum computers. At present, however, each qubit requires its own control line, or electrostatic gate, to manipulate its quantum state. Since a fully functional quantum computer will require millions of qubits to work, this implies the need for millions of control lines. This is not very practical and is one of the stumbling blocks to scaling up quantum technologies.
The QuTech researchers, led by Menno Veldhorst, adopted a “shared-control” approach inspired by classical random-access computing architectures in which millions of transistors are operated with a just a few thousand lines. In their technique, they made a quantum chip hosting a 16-quantum-dot system in a 4×4 chessboard-like array. “The quantum dots of the array are addressed collectively using a few shared control voltages and allow us to confine unpaired (hole) spins in each site,” explains Francesco Borsoi, a postdoctoral researcher at QuTech and the first author of a study in Nature Nanotechnology on the work.
A ratio similar to those in conventional computer chips
“In this way, the scaling of the control lines with the quantum dot number is sublinear, obeying a ‘Rent rule” with an exponent of 0.5,” Borsoi continues, citing a power-law pattern observed by the IBM scientist E F Rent for classical computing in the 1960s. “In other words, and by stretching the concept further, we can imagine controlling one million qubits with only around one thousand control lines.”
Although much more work needs to be done before this number can be reached, this figure would correspond to a ratio similar to those in conventional computer chips, he says.
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“Our architecture has the advantage of being scalable as defined by a Rent’s factor that has proven to be scalable in classical technology,” he tells Physics World. “Crossbar arrays of this type could thus perhaps be employed as unit cells of larger structures and connected to form a network of quantum computing registers.”
The researchers now plan to focus on ways of tuning such large quantum dot arrays in a reliable fashion. This may involve machine learning methods that could enable scalable and autonomous tuning of the quantum dots and their interactions. “We also plan to investigate how to perform selective quantum operations in such arrays while minimizing signal crosstalk and develop very uniform material platforms that facilitate all the above challenges,” Borsoi says.