TECHNICAL BLOG
A Sky Full of Synthetic Stars: How the Flexibility of Superconducting Qubits Sparks Innovation
Maria Violaris
DEVELOPER ADVOCATE
Maria has a hybrid role at OQC of quantum error correction research towards building a fault-tolerant quantum computer, and technical science communication. She has a PhD in theoretical quantum information from the University of Oxford, alongside which she interned with IBM Quantum making the “Quantum Paradoxes” YouTube series. She has spearheaded multiple new initiatives in the quantum community, including the “Quantum on the Clock” Schools Video Competition; Oxford Quantum Information Society; and quantum computing workshops. She has also written for Physics World magazine; published quantum education research; and hosts a Quantum Foundations Podcast on her YouTube channel, amongst other quantum content.
Have you ever seen a sky full of stars? In Oxford, it is usually hidden by city lights and cloudy weather. And yet, I’ve spent the last few nights watching beautiful sparkling displays light up the sky. I am writing this on the 5th November, “Bonfire Night” in the UK, which we celebrate with fireworks. Or as I prefer to call them, synthetic stars. Now imagine for a moment that we could create synthetic atoms. Like we do with fireworks, we could creatively design these atoms ourselves: constantly iterating to try new ideas and make each one more exciting than the previous version. Oh, wait, that’s exactly what we do at OQC!
The synthetic stars of Oxford’s South Park firework display on Sat 2nd Nov 2024
This blog is the story of synthetic qubits: the fireworks that make the sparks fly in OQC’s quantum computers. You’ll discover why our team gets contagiously excited about superconducting qubits; how these qubits were invented by imitating atoms; and get a behind-the-scenes peek into how today’s scientists are developing tomorrow’s qubits.
If you’re not yet familiar with qubits, quantum computers and superconductors, we’ve got you covered! Check out our Beginners Guides on quantum computing and superconducting qubits.
The first sparks of excitement
When I joined OQC in early October, I had an intro chat with Quantum R&D Lead Brian Vlastakis, during which he expressed how he loves the flexibility of superconducting qubits. Turns out, there’s a whole “periodic table” of different types. But while the real periodic table of elements is fixed and predictable, the species of superconducting qubits are not pre-set by Nature. Instead, new designs are being built by creative scientists and engineers.
Intrigued by Brian’s enthusiasm for the flexibility of these qubits, I did some research into the history of the superconducting qubit. This included watching a great historical overview by Steven Girvin (one of the inventors of the famous transmon qubit!). The key idea that led to superconducting qubits was this: scientists knew atoms had novel quantum properties, and superconductors exhibited similar quantum states on the “macro”-scale, that is much larger than individual atoms. Specifically, nanoscale Josephson Junctions built from superconductors could sustain macroscopic quantum states. Could we use these to create and control an “artificial atom” that exhibits the same novel quantum effects as a real one?
This was precisely what Japanese scientist Yasunobu Nakamura and collaborators implemented in a landmark experiment in 1999—the first spark of superconducting qubits. Excitingly, the quantum operations needed for computation could be performed at record-breaking speeds, unconstrained by the slow interaction time of real atoms. Superconducting qubits could also be conveniently controlled using readily-available microwave technology, which had already been developed for years for existing technologies such as radar and mobile phones.
An explosion of superconducting qubit types
The qubit that Nakamura’s team built is known as a charge qubit, because it encodes information based on superpositions of different amounts of charge. As scientists iterated on qubit designs with various arrangements of Josephson Junctions and other circuit components, soon there was a whole family of superconducting qubits (see Figure 2). These encoded quantum information in other properties, such as current (flux qubits) and phase (phase qubits), all with their own unique features, benefits and drawbacks.
A key property for quantum computation is coherence time, the duration over which a system can maintain a superposition. While real atoms have naturally long coherence times, superconducting experimentalists faced the challenge of engineering their qubits to be more robust to noise.
A breakthrough came in 2007, when scientist Jens Koch and collaborators thought of adding a large shunt capacitor in parallel with the Josephson Junction in a charge qubit. This would make the qubit much less sensitive to electrical charge noise, leading to longer coherence times. Known as the transmon, this qubit is now ubiquitous in superconducting quantum computers.
Figure 2: Examples of different superconducting qubit types. The crossed box is the Josephson Junction. [Image taken from “What are superconducting qubits?” guide]
Dazzling innovations from the Leek Lab
Building on these foundational developments, here’s how OQC joined the superconducting periodic table game. In 2017, Peter Leek’s “Leek Lab” at the University of Oxford demonstrated the coaxmon qubit, which combines the advantages of the transmon with a coaxial architecture for improved connectivity and scalability. Peter, now Chief Scientist at OQC, founded the company to take advantage of the coaxmon architecture for solving the scaling problem of quantum computing, namely the challenge of increasing the number of qubits while maintaining performance and coherence.
While OQC began building and deploying quantum computers, the Leek Lab qubit innovations continued in parallel. James Wills, now a Quantum Engineer at OQC, spent his Oxford DPhil developing a new kind of superconducting qubit, the multi-mode coaxial transmon. As the name suggests, this qubit is capable of operating in multiple quantum modes. The remarkable design can therefore act as two-qubits-in-one, and is also shaped to fit the scalable cylindrical coaxmon architecture.
When he joined OQC in 2022, James realised that this qubit is ideal for implementing hardware-efficient quantum error correction, which is essential to detect and correct errors in quantum computations without measuring the quantum information directly. And so, the qubit was rebranded to the catchier dimon (sometimes qubit names need innovation and iteration too!). After engineering the qubit for the needs of a scalable quantum computer, the team is now running quantum error detection experiments on the dimon, which you can read more about in James’ recent blog.
Surprises for the next show
The process of iterating through qubit designs doesn’t end there. James and the team are using the results of testing 1st-generation dimons to design the 2nd-generation. Meanwhile, other quantum engineers such as Boris Shteynas are doing research into more dramatic qubit innovations (stay tuned for updates in the coming months!).
In future posts, we’ll cover the actual process of designing a superconducting qubit, from idea stage to forming part of a deployed quantum computer. We will also explore the science behind calling superconducting qubits “artificial atoms” — including how we can engineer increasingly complex quantum properties in our synthetic qubit designs.
By the time you read this, I bet the designer of the Oxford South Park firework display that I watched on Saturday is already figuring out how to make next year’s display even more impressive, just as our scientists are busy designing next-gen superconducting hardware. It is not even midnight but I am already excited for 5th November 2025: to experience how next year’s novel synthetic stars and synthetic qubits will electrify our skies and quantum computers.
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