Researchers find best position for atomic qubits in silicon

Researchers from the Center of Excellence for Quantum Computing and Communication Technologies (CQC²T) working with Silicon Quantum Computing (SQC) have located the “sweet spot” for the positioning of qubits in silicon to scale atom-based quantum processors.

Creating quantum bits, or qubits, by precisely placing phosphorus atoms in silicon – the method developed by CQC²T Director, Professor Michelle Simmons – is a cutting edge approach to the development of a quantum silicon computer.

In the team’s research, published today in Nature communications, precision placement has been found to be essential for developing robust interactions – or coupling – between qubits.

“We have located the optimal position to create reproducible, strong and rapid interactions between the qubits,” says Professor Sven Rogge, who led the research.

“We need these robust interactions to design a multi-qubit processor and ultimately a useful quantum computer.

Two-qubit gates – the central building block of a quantum computer – use interactions between pairs of qubits to perform quantum operations. For atomic qubits in silicon, previous research has suggested that for certain positions in the silicon crystal, interactions between qubits contain an oscillatory component that could slow gate operations and make them difficult to control.

“For nearly two decades, the potential oscillatory nature of interactions has been predicted as a challenge for scaling,” says Professor Rogge.

“Now, through new measurements of qubit interactions, we have developed a deep understanding of the nature of these oscillations and propose a precision placement strategy to make the interaction between qubits robust. This is an outcome that many believed to be impossible. “

Finding the “ sweet spot ” in crystal symmetries

The researchers say they’ve now discovered that exactly where you place the qubits is key to creating strong, consistent interactions. This crucial idea has significant implications for the design of large-scale processors.

“Silicon is an anisotropic crystal, which means that the direction in which atoms are placed can significantly influence the interactions between them,” explains Dr. Benoit Voisin, lead author of the research.

“Although we already knew about this anisotropy, no one had explored in detail how it could actually be used to dampen the oscillating interaction force.”

“We have found that there is a special angle, or sweet spot, in a particular plane of the silicon crystal where the interaction between the qubits is most resilient. It is important to note that this sweet spot is achievable using the Existing lithography techniques developed under a scanning tunneling microscope (STM). at UNSW. “

“At the end of the day, the problem and its solution come directly from crystal symmetries, so it’s a nice twist.”

Using an STM, the team is able to map the wave function of atoms in 2D images and identify their exact spatial location in the silicon crystal – first demonstrated in 2014 with research published in Materials from nature and advanced in a 2016 Nanotechnology of nature paper.

In the latest research, the team used the same STM technique to observe atomic-scale details of interactions between qubits of coupled atoms.

“Using our quantum state imaging technique, we were able to observe for the first time both the anisotropy of the wave function and the interference effect directly in the plane – that was the point to understand how this problem plays out, ”says Dr. Voisin.

“We figured out that we first needed to determine the impact of each of these two ingredients separately, before looking at the full picture to resolve the issue – that’s how we might find that sweet spot, which is easily compatible. with the precision of atomic placement, offered by our STM lithography technique. “

Building a quantum computer in silicon atom by atom

UNSW scientists at CQC²T are leading the world in the race to build quantum computers based on silicon atoms. CQC researchers²T, and its associated marketing company SQC, are the only team in the world with the ability to see the exact position of their solid state qubits.

In 2019, the Simmons Group took a milestone in their precision placement approach: the team first built the fastest two-qubit gate in silicon by placing two qubits of atoms close together, then observing and by controllably measuring their spin states in real time. . The research was published in Nature.

Now, with the latest advances from the Rogge team, researchers at CQC²T and SQC are positioned to use these interactions in larger scale systems for scalable processors.

“Being able to observe and precisely place the atoms in our silicon chips continues to provide a competitive advantage for manufacturing silicon quantum computers,” says Professor Simmons.

The combined teams of Simmons, Rogge and Rahman are working with SQC to build the first useful commercial quantum computer in silicon. Co-located with CQC²T at the UNSW Sydney campus, SQC’s goal is to build the highest quality and most stable quantum processor.