Reaching the quantum “sweet spot”: Researchers find the best location for atomic qubits in silicon

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

Creating quantum bits, or qubits, by precisely positioning phosphorus atoms in silicon: the method introduced by CQC²T Director Professor Michelle Simmons – is a world-leading approach in the development of a silicon quantum computer.

In the team’s research, published today in Nature Communications, precision positioning proved essential for developing robust interactions – or coupling – between qubits.

‘We identified the optimal location to create reproducible, strong and fast interactions between the qubits,’ says Professor Sven Rogge, who led the research.

“We need these solid 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 silicon qubits, previous research has suggested that for certain positions in the silicon crystal, the interactions between the 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 is expected to be a challenge for scale-up,” says prof. Rogge.

“Now, through new measurements of the qubit interactions, we have developed a deep understanding of the nature of these oscillations and propose a precision positioning strategy to make the interaction between the qubits robust. This is a result that many believed was not possible “.

Finding the “weak point” in the crystalline symmetries

Atomic scale image of two interacting donors in silicon. Image: CQC2T

The researchers say they have found that exactly where the qubits place themselves is essential for creating strong and consistent interactions. This crucial insight has significant implications for large-scale processor design.

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

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

“We found that there is a special angle, or weak point, within a particular plane of the silicon crystal where the interaction between the qubits is strongest. Importantly, this weakness is achievable using existing tunneling microscope (STM) lithography techniques developed at UNSW.

“In the end, both the problem and its solution originate directly from the symmetries of the crystals, so this is 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 position in the silicon crystal – first demonstrated in 2014 with research published in Natural materials and advanced in a 2016 Nature Nanotechnology paper.

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

“Using our quantum state imaging technique, we were able to observe for the first time both the anisotropy in the wave function and the interference effect directly in the plane: this was the starting point for understanding how this problem works. “, says Dr.

“We understood that we first had to calculate the impact of each of these two ingredients separately, before looking at the full picture to solve the problem: this is how we could find this weak point, which is readily compatible with the accuracy of the atomic positioning offered. from our STM lithography technique. “

Building one silicon quantum computer atom per atom

UNSW Scientists at CQC²They are leading the world in the race to build quantum computers based on silicon atoms. Researchers of the CQC²T, and its related marketing company SQC, are the only team in the world that has the ability to see the exact location of their solid-state qubits.

In 2019, the Simmons group reached a major milestone in its precision positioning approach: the team first built the fastest two-qubit gate in silicon by placing two atomic qubits close together, then controllably observing and measuring theirs. spin states in real time. The research was published in Nature.

Now, with the latest advances from the Rogge team, the CQC researchers²T and SQC are positioned to use these interactions in larger scale systems for scalable processors.

“The ability to accurately observe and position atoms in our silicon chips continues to provide a competitive advantage for the fabrication of quantum computers in silicon,” says prof. Simmons.

The combined teams of Simmons, Rogge and Rahman are working with SQC to build the first useful and commercial quantum computer in silicon. Co-located with CQC²On the UNSW campus in Sydney, SQC’s goal is to build the most stable and highest quality quantum processor.