Quantum Chip Bets on Semiconductor Qubits or “Spin Qubits”

December 04, 2021 by Kristijan Nelkovski

Researchers from the University of Copenhagen might have the key to overcoming one of the biggest quantum computer challenges on the road to solid-state quantum computers.

Nearing the limitations of our traditional semiconductor computers, researchers from many companies and universities are working on various materials and ways to process data faster and more efficiently. 

Currently, quantum computing is one of the most promising future technologies that could deliver super-fast processing power for new and emerging industries that are constantly in need of a computational performance boost.


A general overview of quantum computing vs classical computing.

A general overview of quantum computing vs classical computing. Image used courtesy of CB Insights


Quantum computing uses the quantum states of particles (such as spin or charge) to represent quantum bits called qubits. This technology comes in a couple of different forms, each with individual benefits and drawbacks. 

Companies, such as Google and Intel, have already started developing their versions of quantum computers that showcase the technology but aren't nearly as powerful as engineers would like them to have any practical and real-world uses.

The main problem towards achieving this goal, caused by various factors, is the inability to control a large number of qubits efficiently. Generally, for quantum bits to be coherent and controllable, noise and interference from other sources must be at an absolute minimum. 

Creating minimum noise and interference requires excessive signal shielding and near absolute zero cooling. In turn, it requires a large and expensive infrastructure that can handle only a handful of qubits, which is inefficient for scaling and mass-producing the technology at that stage.

Recently, a team of physicists from the Niels Bohr Institute at the University of Copenhagen has developed a potential way to overcome one of these challenges by using semiconductor materials as a platform for a so-called solid-state quantum computer.


University of Copenhagen's Quantum Chip

Research by Ph.D. student Federico Fedele, Professor Anasua Chatterjee, and Professor Ferdinand Kuemmeth's group fabricated a quantum chip based on quantum dot semiconductor qubits that utilize the dot spin property as logic states for their system. 


High-level representation of semiconductor qubits by application and size

High-level representation of semiconductor qubits by application and size. Image used courtesy of Chatterjee et al


Unlike other types of qubits, dot spin qubits have relatively longer coherence times which means that they can maintain their state for a longer time allowing for faster performance and a smaller chance of error during computations and readouts.

The chip features four qubits in a two-by-two array of quantum dots made out of a gallium arsenide semiconductor structure.


University of Copenhagen's two-by-two array of quantum dots.

University of Copenhagen's two-by-two array of quantum dots. Image used courtesy of Fedele et al


Between these qubits is a larger central quantum dot that connects all four qubits allowing for parallel control of every single one of them simultaneously.

This attribute is what makes this research groundbreaking. While other quantum technologies require each qubit to be individually "driven" to control and read its state, they can be operated as one structure by not having to wire every qubit to the system individually. 

This structural change creates a significant advantage in qubit coherence and eliminates a portion of that large wiring infrastructure that other quantum computers exhibit. 


Semiconductor Spin Qubit Challenges

Although this research does resolve one of the biggest underlying issues of current leading quantum computer designs, it still leaves the door open for improvements in scalability both for manufacturing and processing power.

Since this is still a semiconductor technology, the hope is that engineers could adapt current semiconductor techniques to use gallium arsenide for qubit chip manufacturing, leading to a mass fabrication method for a future quantum processor

The second biggest challenge, developing a quantum chip with a more extensive array of simultaneously operated qubits, is a whole research problem of its own. 

On the surface level, some of the issues that semiconductor spin qubits face are fairly similar to those other quantum computing technologies such as ion trap quantum computers or superconductor quantum computers.

Nevertheless, a semiconductor platform is still a very interesting approach since, as with our current traditional semiconductor computers, which operate using billions of transistors on a single chip, quantum semiconductor chips might also feature a similar qubit density and allow engineers to potentially integrate billions of qubits onto a single chip as well.


Semiconductor Qubits = Future of Quantum Computing?

Currently, many scientists and engineers are researching this topic. Still, due to the price of entry and the relatively new technology, quantum computing is far from an industry tool, let alone a household item. 

Useful and manufacturable quantum devices might vary between these technologies and probably won't become a reality until a true quantum computing standard exists.

Semiconductor spin quantum computers are just one of many technologies that, while solving some problems in our current understanding of the technology, might show other problems down the road or not be compatible with solutions for some other quantum computing challenges.

Still, this is a valuable step forward into qubit control that will hopefully inform other engineers into their future research as the current goal of the technology should not be the immediate scaling of manufacturing to deliver a product. Hopefully, opening the gate to more researchers to collectively better understand the scope of quantum's complexity and bring even more solutions to the table to truly develop quantum technologies.