Installationsföreläsning till Professor - Anton Frisk Kockum

Abstract:

In my research group, we work towards giving humanity the tools to understand quantum systems consisting of hundreds or thousands of atoms. I believe that having these tools will enable breakthroughs in chemistry, materials science, and medicine, with great societal benefits. In this lecture, I will describe how we work towards these goals along two main tracks: supporting the development of large-scale quantum computers and exploring new parameter regimes in the quantum interaction between light and matter. At the end of the talk, you will see how these two tracks start to merge.

As quantum computers now are scaling to sizes of hundreds of qubits and beyond, a multitude of both practical and theoretical challenges arise. On the theory side, the exponential growth of the Hilbert space with qubit number makes it difficult to accurately model and measure properties of the quantum computer. On the experimental side, for solid-state quantum processors operated at millikelvin temperatures, having individual control lines for each qubit go down into the cryostat runs into problems with increasing thermal load and lack of space as the qubit number grows. In the first part of my talk, I will give three examples of how we are addressing these scaling challenges in my research group. First, I will show that reducing the number of control lines for a quantum computer by incorporating switches and time-multiplexed control leads to a surprisingly small overhead in the execution time for quantum algorithms, improving the outlook for scaling up quantum computers. Second, I will demonstrate how we use numerical techniques adapted from many-body quantum physics to calculate properties such as ZZ coupling and localization across quantum processors with several hundred qubits. Third, I will discuss selective quantum state tomography, which enables us to extract partial information of many-qubit states using relatively few measurements.

In the second part of the talk, we turn to quantum optics. When studying the interaction between light and matter at the quantum level, it is common to apply the dipole approximation, where atoms or other quantum emitters are assumed small compared to the wavelength of the light. However, recent experiments with superconducting qubits (artificial atoms), coupled to surface acoustic waves or microwave transmission lines, have shown that it is possible to realize ”giant emitters”, i.e., quantum emitters that couple to light (or sound) at multiple points that are wavelengths apart. Here, I will present an overview of theoretical and experimental work on such giant emitters. In particular, I will explain how the relaxation rate of a single giant emitter can be designed and how multiple giant emitters can interact via a waveguide while remaining protected from relaxation into the waveguide. Finally, I will give an outlook for how these properties of giant emitters may be used for quantum technologies such as quantum simulation, quantum communication, and quantum sensing.