In their attempt to create a quantum computer chip in silicon, a team of researchers from the Centre of Excellence for Quantum Computation and Communication Technology (CQC2T) at UNSW Sydney has demonstrated successfully how to tune the control frequency of a qubit. And the way in which they did that was by engineering its atomic composition.

The work, which was published in Science Advances, involved the researchers building two qubits. One was an engineered molecule made up of two phosphorus atoms and just one electron, while the other consisted of a single phosphorus atom and a single electron. Both were placed in a silicon chip, just 16 nanometers apart. The qubits were exposed to frequencies of approximately 40 GHz from a precisely aligned, microwave antenna that sat above them.





Results from the study showed that when they changed the frequency of the signal that was being used to control the electron spin, the single atom had a completely different control frequency to that of the molecule containing two phosphorus atoms. The researchers turned to experts at Purdue University for a little help in understanding how the positioning of the atoms affected the control frequencies of the electrons.

“Individually addressing each qubit when they are so close is challenging,” remarks UNSW Scientia Professor Michelle Simmons, co-author of the paper and Director of CQC2T. “The research confirms the ability to tune neighboring qubits into resonance without impacting each other.”





By creating engineered phosphorus molecules that have different separations between the atoms makes allowances for groups of qubits with varying control frequencies. “We can tune into this or that molecule – a bit like tuning into different radio stations,” says lead co-author of the study, Sam Hilde, a Research Fellow at UNSW. “It creates a built-in address which will provide significant benefits for building a silicon quantum computer.”

Tuning qubits in such a way are just the beginning of demonstrating the entangled states that are needed for a quantum computer to work as it should and carry out complex calculations. By engineering these qubit’s atoms within a silicon chip, molecules can be developed with different resonance frequencies. Essentially what this means is that the control or manipulation of one, will not affect another and that is something that is a vital component of quantum computing.





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