19 June 2019 Rosalie Cyr
Baptiste Royer successfully defends his thesis

Microwave photons, measurement and quantum computing

Baptiste Royer

Photo : IQ

On June 7, at the Faculty of Science, Sherbrooke physicist Baptiste Royer defended in front of a jury his thesis on microwave photons, measurement and quantum computing. Superconducting circuits constitute a promising architecture for the creation of a universal quantum computer. However, before such a computer can be built, several obstacles must be overcome from both a hardware and software point of view.

During his accelerated transition from master’s to doctoral studies in physics, Baptiste quickly began his research to find a relevant and interesting subject for his thesis. As a first project, he undertook work based on a thesis submitted in 2015 by Kevin Lalumière, Ph. D. in physics, while keeping an open mind and adopting an exploratory research approach. His hard work on that first project unfortunately doesn’t pay off and no interesting conclusions emerge. Consequently, he decides to change his research focus to two-qubit gates. Still using the same basic tools and concepts, he finds that it is possible to apply the method used to create the two-qubit gate in another context, namely the detection of unique microwave photons. The development of this unique photon detector led to a publication in Physical Review Letters, which earned Baptiste the Relève étoile Louis-Berlinguet of the FRQNT in March.

The challenges of single photon detection

Through the research of Baptiste and his collaborators, two methods have been proposed to produce a unique photon detector with high quantum efficiency. The first of these methods, mentioned above, is based on a set of superconducting qubits. Working in collaboration with Arne Grimsmo, a former postdoctoral fellow in Alexandre Blais’ group, Baptiste discovered that the idea behind this project did not work as anticipated because of fundamental limitations in quantum mechanics measurement.

Through determination and perseverance in his search for solutions to this problem, he eventually discovered a new method without those limitations.

The second method proposed by Baptiste and Arne is based on quantum meta-material. Baptiste first proposed a design using this idea, which operated in a rather narrow bandwidth. Then, with Arne’s help, they refined this proposal and designed a more complex detector that would operate in a wider frequency band, allowing for a greater range of possible use cases.

Quantum information and parity measurement

Baptiste quickly learned that he was able to work on multiple projects at the same time, and that may even amplify his productivity. He took every good opportunity that was presented to him. One of them was the idea of developing a parity measurement which came up during a working session with Shruti Puri, a former postdoctoral fellow in Alexandre Blais’ group. This measurement is an essential operation to make the error correction in the quantum computer. This measurement is an essential operation of most error correction schemes for quantum computers. The collaboration between Shruti and Baptiste led to a publication in the journal Science Advances in December 2018, where they present an experimentally compact method for performing a parity measurement. [Quote 2]

The final chapter

As the last chapter of his thesis, Baptiste worked in a theoretical support role for Andreas Wallraff’s group at ETH Zurich. In concrete terms, this role consisted in investigating certain experimental measurements that were less understood and in carrying out numerical simulations in order to identify the source of experimental errors.

This collaboration subsequently led to a publication in the journal Nature. This study demonstrates a quantum state transfer between two superconducting qubits located on two different samples and separated by one meter of cable. Thanks to this device, it is therefore possible to use unique microwave photons to create entanglement at a distance, laying the foundations for a future quantum internet.

Introspection on the doctoral experience

Given the multitude of projects in which he is involved, obtaining his doctorate is just another step in the same direction for Baptiste : “I have so many other interesting projects in progress that it is difficult for me to sort through the projects in which I invest my time. For me, my thesis is not an ending-point, but a continuation of my research efforts. ». In his view, the main challenge of his doctoral experience was to show determination and perseverance at several points during his studies, especially in his research on photon detection.

All the collaborations that have marked Baptiste’s path do not seem to surprise his director Alexandre Blais, for whom : “Baptiste is not only a brilliant researcher with a remarkable intuition for physics, but he is also someone with whom it is really very pleasant to work. »

As for the future, the physicist will continue his postdoctoral studies at Yale University alongside Shruti Puri, in the team of professors Steve Girvin and Liang Jiang. He will therefore have the opportunity to explore all the subjects that fascinate him. We wish him good luck!

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