Engineers and physicists at UCL have developed a new fabrication process for building quantum computers that achieves an almost zero failure rate and shows strong potential for scalability, according to new research.

A study published in Advanced Materials reports the first reliable method for precisely arranging individual atoms in a grid, an achievement 25 years in the making. The technique offers near-perfect accuracy and can be scaled up, marking a significant step toward building practical quantum computers. However, major engineering hurdles remain before this vision can become reality.

Quantum computers, in theory, could solve problems that are beyond the reach of traditional, transistor-based computers. One promising approach involves using single atoms in silicon as quantum bits, or qubits. These atoms are cooled to extremely low temperatures to preserve their fragile quantum states, and they can be controlled using electrical and magnetic fields. This allows them to process information similarly to how classical transistors switch between binary states of 0 and 1, except with vastly more complex potential.

Unlocking the Power of Quantum Mechanics

This allows the computer to harness the power of quantum mechanics, the deep laws of physics that determine how the universe works. This includes phenomena such as superposition, or the ability of qubits to be in many different arrangements at the same time, and quantum entanglement, which is the ability of qubits to be inextricably linked.

These features mean complex problems can be represented in new ways. For a problem with an exceptionally large number of possible outcomes, a quantum computer is able to consider the possibilities simultaneously, rather than one at a time like a normal computer would  which would take today’s best supercomputer millions of years to process.

Various approaches to building a quantum computer are underway, but none have yet managed to reach the scale and low error rates required.

One approach to building a quantum computer is to precisely position individual ‘impurity’ atoms in a silicon crystal, which allows manipulation of their quantum properties to form qubits. One of the benefits of this approach is that it has inherently low qubit error rates and is underpinned by scalable silicon microelectronics technologies. The standard approach uses phosphorus as the impurity atom, but because single phosphorus atoms can only be positioned with a 70% success rate, this system remains some way off from the near-zero failure rate that is required to build a quantum computer.

In this study, researchers at UCL hypothesized that arsenic might be a better material than phosphorus to achieve the low failure rate needed to build a quantum computer.

They used a microscope capable of identifying and manipulating single atoms, similar to the needle on a vinyl record player, to precisely insert arsenic atoms into a silicon crystal. They then repeated this process to build a 2×2 array of single arsenic atoms, ready to become qubits.
A Step Toward Scalable Quantum Devices

Dr Taylor Stock, first author of the study from UCL Electronic & Electrical Engineering, said: “The most advanced quantum computing systems in development are still grappling with the twin problems of how to mitigate qubit error rates and how to scale up the number of qubits.

“Reliable, atomically precise fabrication could be used to build a scalable quantum computer in silicon. The prevailing view was that single-atom fabrication using arsenic would suffer the same problems as phosphorus. But based on our calculations, we realized that single arsenic atoms might be placed more reliably than phosphorus, and we’ve been able to do this successfully. We’ve been conservative in estimating that we can place atoms with 97% accuracy, but we are confident that this can be increased to 100% in the near future.”

At the moment, the method developed in the study requires each atom to be positioned by hand one at a time, which takes several minutes. Theoretically, this process can be repeated indefinitely, but in practical terms, it will be necessary to automate and industrialize the process in order to build a universal quantum computer – which means creating arrays of millions, tens of millions, or even billions of qubits.
Industry Collaboration and Future Outlook

The authors say that the silicon semiconductor industry, currently worth around $550 billion, should be able to contribute to advancing the field, as arsenic and silicon are both commonly used in the construction of semiconductors for classical computing. The approach developed in this study is expected to be highly compatible with current semiconductor processing and could hopefully be integrated once engineering challenges have been addressed.

Professor Neil Curson, senior author of the study from UCL Electronic & Electrical Engineering, said: “The ability to place atoms in silicon with near perfect precision and in a way that we can scale up is a huge milestone for the field of quantum computing, the first time that we’ve demonstrated a way of achieving the accuracy and scale required.

“We now have a huge engineering challenge ahead to be able to do this more quickly and easily – but this is the first time that I’ve felt certain that a universal quantum computer can be built.”