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Sweet Spot as the Most Precise Position for Atom Qubits in Silicon for Quantum Processor Improvements

A group of researchers from the Center of Excellence for Quantum Computation and Communication Technology (CQC2T) in cooperation with Silicon Quantum Computing (SQC) has invented the quantum of Sweet Spot. The quantum is to position qubit in silicon to improve the atomic based quantum processor.

Sweet Spot is known to have some benefits in the world of quantum computing. One of them is to make the quantum bit, known also as qubit, to place phosphorus atoms in silicon more precisely. This method is pioneered by the Director of CQC2T, Professor Michelle Simmons. Currently, the method is the leading approach in the world in the area of silicon quantum computer development.

It Develops Stronger Interaction

From the result of the research that has been published in Nature Communications, precise placement is proven to be important to develop a stronger interaction among qubits. Not only is it related to the interaction but uniquely, stronger combinations of qubits also happen with Sweet Spot.

In his statement, Professor Sven Rogge who leads the research said that his team has invented the most optimal position to create interactions between qubits that are stronger, faster, and reproducible. The professor also adds that the strong interaction is needed to engineer a kind of multi-qubit processor. In the end, a functional quantum computer can be created as well.

There is also a term namely the gate of two qubits in the research. It refers to the blocks that arrange the center of a quantum computer. It uses an interaction between a pair of qubits to do the quantum operation. For the atomic qubit in silicon, there has been other research to conduct before that. In the previous research, it is suggested that for a certain position in the silicon crystal, the qubit interaction contains oscillation components to slow down the gate operation as well as to make it more difficult to control.

Prof. Rogge stated still in the interview that in almost two decades, the potency of oscillation characteristics from the interactions has been predicted to turn into further challenges for the improvement.

Now, through the new measurement of the qubit interaction, Prof. Rogge’s team has developed a deeper understanding related to the oscillation characteristics. At the same time, it suggests further placement strategies that are more precise. The strategies are to generate a stronger interaction between qubits. Interestingly, the result is what has been believed by many people to be impossible to happen for a long time.

Figuring Out Sweet Spot in the Crystal Asymmetry

Furthermore, the researchers also state that they currently have found out that placing qubits is very important to create stronger and more consistent interactions. This important view has a significant implication for the big-scale processor design.

The main author of the research, Dr. Benoit Voison, states that silicon itself is a kind of anisotropic crystal. It means that the placement direction of the silicon atom significantly influences the interaction between them. 

Dr. Voison said that his team has acknowledged the anisotropic crystal even before. However, further studies have just been done currently. Besides, there is no other team that has been investigated in detail how the crystal is used optimally to reduce the oscillated interaction strength.

The team also has figured out that there is a special corner, which is later called the sweet spot, in a particular area in the silicon crystal. The sweet spot is where the strongest interaction between qubits is done. More importantly, the sweet spot can be achieved by using a lithography technique namely the scanning tunneling microscope (STM.)

STM to Map the Functions of Atomic Waves

By using STM, the team can do some other actions. One of them is mapping the functions of an atomic wave on the 2D image. Besides, it is also to identify the spatial location right in the silicon crystal. Those functions were firstly shown in 2014, in research published in Nature Materials. It was also filed in one of the Nanotechnology papers in 2016.

In the latest research, the team applies the STM technique to observe the details of atom scales from the interaction between atomic qubits that have been combined. According to Dr. Voisin, by using the technique of quantum imagery, his team is able to observe anisotropy in the wave functions as well as the direct interference in the area. It is basically the starting point to understand how the problem happens.

His team understands that in the beginning, they must know the effects of each material separately. It is before seeing the complete imagery to solve the problem. This is how the team found out the sweet spot that is compatible with the atom placement precision offered by the STM lithography technique.

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