Quantum physicists at the University of Copenhagen have made a major leap forward in the field of quantum technology, overcoming a major obstacle. The team simultaneously operated multiple spin qubits on the same quantum chip, which will help lead to supercomputers.
One of the major obstacles to achieving a large funcional quantum computer is the control of the many basic memory devices, or qubits, simultaneously. When one qubit is controlled, there is usually a negative effect by simultaneous control pulses applied to another.
The team that overcame this challenge included a pair of quantum physicists at the University of Copenhagen’s Niels Bohr Institute – PhD student and Postdoc Frederico Fedele, and Assistant Professor Anasua Chatterjee. The two were working in the group of Associate Professor Ferdinand Kuemmeth.
The study was published in the journal Physical Review X Quantum.
New Approach With Spin Qubits
While companies like Google and IBM have focused on superconductor technology for quantum processors, the research group is looking more at semiconductor qubits, or spin qubits.
“Broadly speaking, they consist of electron spins trapped in semiconducting nanostructures called quantum dots, such that individual spin states can be controlled and entangled with each other,” says Fedele.
Spin qubits can maintain their quantum states for a long time, which potentially enables them to perform faster and more accurate computations than other platform types. Because of their miniscule size, a lot more of them can fit on a chip compared to other qubit approaches. This is important given the fact that more qubits results in a greater computer processing power.
The research team was able to fabricate and operate four qubits in a 2×2 array on a single chip.
Getting Qubits to Communicate
According to Anasua Chatterjee, one of the most important goals is to get qubits to communicate with each other.
“Now that we have some pretty good qubits, the name of the game is connecting them in circuits which can operate numerous qubits, while also being complex enough to be able to correct quantum calculation errors,” Chatterjee says. “Thus far, research in spin qubits has gotten to the point where circuits contain arrays of 2×2 or 3×3 qubits. The problem is that their qubits are only dealt with one at a time.”
The quantum circuit developed by the team is made from the semiconducting substance gallium arsenide, and it is no larger than the size of a bacterium.
Chaterjee is one of the two lead authors of the study.
“The new and truly significant thing about our chip is that we can simultaneously operate and measure all qubits. This has never been demonstrated before with spin qubits — nor with many other types of qubits,” says Chatterjee.
In order to perform quantum calculations, it is important to operate and measure simultaneously. Qubits are highly sensitive, and when they are measured one by one, even a small ambient noise can alter the quantum information in a system.
“To get more powerful quantum processors, we have to not only increase the number of qubits, but also the number of simultaneous operations, which is exactly what we did,” says Professor Kuemmeth.
Another major challenge is that the chip’s 48 control electrodes must be tuned manually, and they need to continuously be kept tuned. This is a time consuming task for humans, so the team is now looking for a way to use optimization algorithms and machine learning to automate the process.
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