Innovative chip solves quantum headaches

Innovative chip built by KU physicists solves quantum headaches - University of Copenhagen

The illustration shows the size difference between spin qubits and superconducting qubits. Credit: University of Copenhagen

Quantum physicists at the University of Copenhagen report an international achievement for Denmark in quantum technology. By simultaneously operating multiple spin qubits on the same quantum chip, they overcame an important obstacle on the road to the supercomputer of the future. The result bodes well for the use of semiconductor materials as a platform for solid-state quantum computers.

One of the technical headaches in the global marathon towards a large functional quantum computer is the control of many basic memory devices – qubits – simultaneously. This is because the control of one qubit is typically adversely affected by simultaneous control pulses applied to another qubit. Now a couple of young quantum physicists at the University of Copenhagen’s Niels Bohr Institute work in the group of Assoc. Prof. Ferdinand Kuemmeth, has managed to overcome this obstacle.

Global qubit research is based on various technologies. While Google and IBM have come a long way with quantum processors based on superconducting technology, KU’s research team is focusing on semiconductor qubits – known as spin qubits.

“Broadly speaking, they consist of electron spins trapped in semiconductor nanostructures called quantum dots, so that individual spin states can be controlled and entangled in each other,” explains Federico Fedele.

Spin qubits have the advantage that they maintain their quantum states for a long time. This potentially allows them to perform faster and more error-free calculations than other platform types. And they are so small that far more of them can be squeezed on a chip than with other qubit approaches. The more qubits, the greater the processing power of a computer. The KU team has expanded the latest by manufacturing and operating four qubits in a 2×2 array on a single chip.

Circuitry is the ‘name of the game’

So far, quantum technology’s biggest focus has been on producing better and better qubits. Now it’s about getting them to communicate with each other, explains Anasua Chatterjee:

“Now that we have some pretty good qubits, the name of the game is to connect them in circuits that can operate several qubits, while being complex enough to correct quantum error. So far, research into spin qubits has managed to the point where circuits contain arrays of 2×2 or 3×3 qubits. The problem is that their qubits are only processed one at a time. “

This is where the quantum cycle of young quantum physicists, made of the semiconductor substance gallium arsenide and not larger than the size of a bacterium, makes all the difference:

“The new and really 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, one of Two lead authors of the study, which was recently published in the journal Physical Review X Quantum.

Being able to operate and measure at the same time is crucial for performing quantum calculations. In fact, if you have to measure qubits at the end of a calculation – that is, stop the system to get a result – the fragile quantum states collapse. Thus, it is crucial that the measurement is synchronous so that the quantum states of all qubits are shut down simultaneously. If qubits are measured one by one, the slightest ambient noise can change the quantum information in a system.


The realization of the new circuit is a milestone on the long road to a semiconductor quantum computer.

“To get more powerful quantum processors, we need to not only increase the number of qubits, but also the number of simultaneous operations, which is exactly what we did,” says Professor Kuemmeth, who led the research.

Currently, one of the main challenges is that the chip’s 48 control electrodes must be tuned manually, and kept tuned continuously despite environmental drift, which is a tedious task for a human. That is why his research team is now investigating how optimization algorithms and machine learning can be used to automate tuning. To allow the production of even larger qubit arrays, researchers have begun working with industrial partners to produce the next generation of quantum chips. Overall, the synergistic efforts of computer science, microelectronics engineering and quantum physics can then lead spin-qubits to the next milestones.

About qubits

The brain of the quantum computer that scientists are trying to build will consist of many arrays of qubits that look like bits on smartphone microchips. They will make up the machine’s memory.

The famous difference is that while an ordinary bit can either store data in state 1 or 0, a qubit can reside in both states simultaneously – known as quantum superposition – making quantum computation exponentially more powerful.

About the chip

The four spin qubits in the chip are made of the semiconducting material gallium arsenide. Between the four qubits, there is a larger quantum dot that connects the four qubits to each other and that the researchers can use to tune all the qubits simultaneously.

A three-qubit entangled state has been realized in a fully controllable array of spin-qubits in silicon

More information:
Federico Fedele et al., Simultaneous operations in a Two-Dimensional Array of Singlet-Triplet Qubits, PRX Quantum (2021). DOI: 10.1103 / PRXQuantum.2.040306

Provided by the University of Copenhagen

Citation: Innovative chip solves quantum headaches (2021, October 29) retrieved October 31, 2021 from

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