cQED Celebrates Its Twentieth Anniversary: Commemorating Two Decades Of Discoveries.

In January of this year, twenty years ago, the first articles were published that laid the foundation for circuit quantum electrodynamics, or cQED. Once restricted to the infinitely small, the observation of quantum effects is now being expanded to the macroscopic scale, providing a completely new insight into quantum information. cQED is a technological playground that promises computers that can solve problems that are difficult to solve today, in addition to pushing the boundaries of our basic understanding of quantum mechanics. This fascinating chapter redefines our relationship with the quantum world as it tells the story of an intersection between theory and technical ingenuity.

The artificial atom: a conceptual pivot

In the beginning of the new millennium, researchers were able to separate atoms and induce their interaction with individual photons in confined spaces. These atoms could theoretically be employed as qubits, the fundamental building blocks of quantum computing. Nevertheless, there are significant technological challenges in the way of assembling multiple of these qubits, which are necessary to build a working quantum computer. A team of scientists realized in 2004 that it was possible to create superconducting electrical circuits that would behave similarly to actual atoms interacting with light at the quantum level. Alexandre Blais, Institut quantique’s Scientific Director and first author of the paper notes, “It’s something that had never been said so clearly before – we realized that we had found the right language to describe these systems.” In the article they published on the subject, this approach makes it possible to control, interact and measure superconducting qubits acting as artificial atoms. This is the beginning of cQED.

Harmony between theory and practice: unparalleled precision in forecasting

This theoretical framework created a strong link between experiment and theory, leading to extraordinarily precise predictions. “For the first time in the field, it worked: the correspondence between theoretical calculations and experiment was incredible,” he recalls. More than a theoretical proposition, it was the outline of a real quantum computer architecture, lining up all the components needed to build it.

The transmon: a catalyst for progress

An additional significant turning point occurred in 2007 with the arrival of the transmon. This kind of superconducting qubit gets around a lot of obstacles and makes it easier to integrate more qubits, which encourages the application of cQED. Rapid advancements in cQED have drawn the interest and funding of major players in the technology industry, including Google and IBM, which have taken notice of and modified these systems. Google even claims to have achieved quantum supremacy – that point at which quantum computers will be able to solve problems impossible for classical computers to accomplish. “When I make predictions about how the field will develop, I’m always wrong, but always in the same way: it goes faster than I think every time,” comments Prof. Blais.

Perspectives

These fundamental propositions, which originated 20 years ago, now have unforeseen applications, notably in the search for dark matter – the hypothetical elusive substance that explains many astrophysical observations. “When we developed these ideas, we weren’t thinking about detecting dark matter! We were trying to find the best way to measure and control a qubit. That’s what fundamental research is all about – surprises. And I know there will be more, because there’s an amazing community working on this now. This community is going in directions we didn’t suspect at first. I think it’s going to stay really exciting for a long time to come,” concludes Alexandre Blais enthusiastically.