Seminar Schedule

Perfect diamond is transparent for visible light but there are famous diamonds, such as the famous Oppenheim Blue or the Pink Panther worth ten's of millions of dollar, which have intense colour. An important source of colour in diamond are lattice defects which emit and absorb light at optical frequencies and may indeed possess a non-vanishing ground state electronic spin. Unlike atomic physics systems, which are operated under extreme conditions to ensure isolation from their environment, colour centers in diamond are by their very nature in direct contact with the environment that is constituted by uncontrolled phonon, spin and charge degrees of freedom that form part of the host material. As a result they suffer from environmental noise. In order to address this challenge a combination of material science to improve the hardware properties and of quantum control methods to further decouple the solid state qubits from their environment while maintaining desired interactions that are needed to achieve the ultimate goal of practically usable coherent quantum dynamics. In this lecture I will explore the physics of one of these defects, the nitrogen vacancy center, and show how we can manipulate its electronic spin to develop quantum simulators, nanoscale quantum sensors and sources of nuclear hyperpolarisation. Applications of such devices range from sensing in biology to medical imaging.

Gauge theories are the cornerstone of our understanding of fundamental interactions among particles. Their properties are often probed in dynamical experiments, such as those performed at ion colliders and high-intensity laser facilities. Describing the evolution of these strongly coupled systems is a formidable challenge for classical computers, and represents one of the key open quests for quantum simulation approaches to particle physics phenomena. In our work, we show how recent experiments done on Rydberg atom chains naturally realize the real-time dynamics of a lattice gauge theory at system sizes at the boundary of classical computational methods.

Understanding the behavior of an impurity strongly-interacting with a Fermi sea is a long-standing challenge in many-body physics. For a single impurity, a first-order transition is predicted between polaronic quasiparticle and dressed molecular ground states. However, at realistic conditions, the fate of this transition is still unknown. We study imbalanced ultracold Fermi gas with a novel high-sensitivity Raman spectroscopy technique to isolate the quasiparticle contribution and extract physical parameters. We observe a reduction of the quasiparticle weight with increasing interaction and support this finding with a theoretical model of thermally occupied quasiparticles, revealing a smooth transition with a coexistence region.

With quantum computers on the brink of outperforming their classical counterpart, it becomes increasingly important to find ways of verifying that these devices indeed perform as advertised. We approach this task by challenging independent quantum processors with seemingly random sampling problems of different sizes, which are linked through the principles of measurement-based quantum computing. This enables efficient cross-checking of multiple quantum processors, as well as internal consistency checks of individual quantum processors. The protocol is hardware agnostic and requires no classical simulation as we demonstrate using five state-of-the-art quantum processors.

The extraordinary advances in quantum control of matter and light have been transformative for precision measurements enabling probes of the most basic laws of Nature to gain fundamental understanding of the physical Universe. Exceptional versatility, inventiveness, and rapid development of precision experiments supported by continuous technological advances and improved theory give a high chance for paradigm-shifting discovery. The development of atomic clocks with systematic uncertainties in the 10^-18 range enables searches for the variation of fundamental constants, dark matter, and violations of Lorentz invariance. I will give an overview of dark matter searches with clocks including prospects for significantly improved sensitivity with highly charged ions and a nuclear clock. At the end, I will also introduce our new online portal for high-precision atomic data and computations.

Control refers to the ability to steer a dynamical system using external fields; quantum control does so by exploiting quantum coherence, i.e., the wave nature of matter. One way to think of it is in terms of constructive and destructive interference between different quantum pathways, all connecting the same initial and final states. I will illustrate the concept of pathway interference using the photoionization of chiral molecules, i.e., molecules with a left-handed or right-handed nuclear scaffold, as example. The ionizing field may be tailored to minimize or maximize the signature of molecular handedness in the photoelectron spectrum, using interference between pathways probing different intermediate states.
The essential elements of quantum physics, quantum coherence and entanglement, are not only the agents of quantum control, they are also at the core of emerging quantum technologies such as quantum-enhanced sensing or quantum information processing. I will discuss how quantum control allows to identify fundamental performance bounds and derive protocols to reach these performance bounds in realistic models for basic building blocks of quantum-enhanced sensing and quantum information processing.