# Scope

The abstracts of the lectures and talks will be announced here.

# Lectures

## Guido Burkard

**Quantum computing and cavity QED with spins**

These lectures will cover the basics and highlight several recent advances related to spin-based quantum information processing from a theoretical perspective, including the impact of new material systems, electric spin control, and the coupling of individual spins to the quantized field of a superconducting microwave resonator. We will discuss the importance of spin-charge hybridization for spin qubit control and measurement, and present several different examples. Recently, electric spin control using synthetic spin-orbit coupling due to magnetic field gradients in combination with the exchange coupling has allowed for electrically controlled one- and two-qubit gates for spins in silicon quantum dots. An alternative approach to all-electric control of spin qubits consists in the use of multi-spin qubits consisting of more than one electron, such as the singlet-triplet, exchange-only, resonant-exchange qubits, as well as quadrupolar qubits. The development of superconducting coplanar waveguide resonators has provided new opportunities for coupling spin qubits over long distances which have come into the reach of experimental feasibility with the recent achievement of the strong-coupling regime in spin-cavity quantum electrodynamics (QED).

## Yiwen Chu

# Quantum technologies with defect centers

Defect centers in crystalline materials have unique properties that make them very useful quantum systems. Some of these properties include optical transitions that form a solid-state interface to light and highly coherent spin states, which can be used as qubits for storing and manipulating quantum information. I will give a general overview of the field, along with a more detailed look at the properties of one or two types of color centers as examples. This will then allow us to explore the various quantum technologies enabled by these systems, ranging from long-distance quantum networks to quantum sensors for precision metrology.

## Per Delsing

**Superconducting qubits for quantum information and for physics**

*Lecture I: Superconducting qubits for quantum information*: In the first lecture I will explain how different superconducting qubits operate and in what way they differ from each other. Important properties will be discussed and explained including coupling of qubits to resonators, one and two-qubit gates, decoherence and qubit read-out. We will also discuss scaling of circuits to larger number of qubits and possible problems that a small non-error corrected quantum computer could be used for.

*Lecture II: Superconducting qubits for physics*: Superconducting qubits can also be used as artificial atoms and we will discuss how they can be used to study new physics. In particular I will show how artificial atoms can be used to study vacuum fluctuations by placing the atom in front of a mirror. I will also show how these atoms can be coupled to sound such that they decay by emitting single phonons instead of single photons. Sound coupled atoms can also act as giant atoms, which leads to new physics in terms of large Lamb shifts and non-exponential decay.

## Barbara Kraus

**Entanglement Theory**

In these lectures I will start with bipartite entanglement theory and then discuss some results on multipartite entanglement.

## Sander Otte

**Quantum simulation through atomic assembly**

Scanning tunneling microscopy (STM) at low temperatures allows for atom manipulation: the ability to position individual atoms in a predetermined location. In recent years, this technique is increasingly used to create artificial atomic lattices for the purpose of studying emergent physics. These lattices can be probed and modified locally with atomic precision by means of the STM tip, providing a promising complementary approach for quantum simulation.

In these two lectures I will give an overview of this upcoming experimental field. After covering the basic concepts of STM and its various manipulation and readout possibilities, I will focus on two research avenues: electronic lattices and spin lattices. In the former case, local electronic states on atoms or atomic vacancies are weakly coupled to neighboring sites to form artificial band structures. I will discuss various designs of exotic lattices and their resulting electronic textures. In the latter case, magnetic atoms are used to create model spin chains, providing an excellent test bed for performing experiments on basic spin systems such as the Heisenberg model and its variations.

## Klaus Mølmer

**From quantum optics to bits and pieces**

The analysis of photo detection signals and the quantum properties and evolution of their sources defined the field of quantum optics in the early 1960’es and stimulated the so-called ‘’second quantum revolution’’ in physics. The ability to control single quantum systems and apply special superposition and entangled states of light and matter now extends far beyond the atomic world and many pioneering quantum optics experiments have been successfully implemented with man made solid state systems. In the lectures, I shall discuss three broader aspects of the successful merging of quantum optics and nanotechnology: (i) observed quantum systems and quantum trajectories, (ii) hybrid quantum systems that merge atomic and mesoscopic scale systems, and (iii) input-output-theory for atomic or solid-state quantum networks, connected by optics, microwave or surface acoustic waves.

## Steven Simon

**Topological matter and topological quantum computing**

*Lecture 1*: Topologically Ordered Matter and Why You Should be Interested

*Lecture 2*: Quantum Error Correction, Toric Code, and Anyons

## Luis Martín-Moreno

**Nano-optics and ultra-strong light-matter interactions**

*Lecture 1*: Nano-optics in flatland

*Lecture 2*: Ultra-strong coupling regime of light-matter interactions

# Talks

## J. Ignacio Cirac

**Quantum optics in structured reservoirs: from exotic emission to quantum chemistry simulation**

Recent progress in both nano-fabrication and atomic physics allows one to couple atoms (or other emitters) to structured waveguides. In this talk I will report on different opportunities that are opened up by those systems, including the observation of many-photon bound states, the preparation of Fock states, the simulation of long-range spin interacting models, or the observation of exotic features in collective decays. I will also explain how the same phenomena could be observed in a completely different setup consisting of atoms in state-dependent optical lattices. Here the role of the emitters and the photons in the waveguide are played by atoms in different internal states. The special interactions mediated in this setup opens up the possibility of simulating molecules in an analog way.

## Javier Aizpurua

**Quantum nanooptics to address molecular spectroscopy in plasmonics nanocavities**

Surface-enhanced molecular spectroscopy has experienced a dramatic evolution during the last decades, reaching information on molecular excitations at the single molecule level. Extreme plasmonic cavities which can localize light within a few nm3 of effective mode volume, and even around atomic dimensions, have boosted light-matter interactions to the realm of strong coupling, modifying and engineering molecular excited states. In this extreme regime of spectroscopy, theoretical frameworks which can address the quantum dynamics of matter excitations and cavity states are crucial to reveal non-linear processes, light emission statistics, and novel collective effects, among others. During the last years, cavity-Quantum Electrodynamics (QED) methods have been successfully employed to address the interaction between these molecular excitations and optical modes in nanoresonators, allowing for establishing robust frameworks to understand light emission in surface-enhanced fluorescence (SEF) from quantum emitters in plasmonics cavities, as well as to interpret non-linear signals from Surface-Enhanced Raman Scattering (SERS), a fundamental spectroscopic technique which addresses the rich vibrational structure of molecules.

In typical spectroscopy configurations, a plasmonic cavity can enhance the excitations of a molecule located in its proximity, however a number of experiments in molecular spectroscopy reveal unusual dependencies of the spectral signals that go beyond the standard descriptions based on classical field-enhancements and chemical effects. We adopt model hamiltonians within the cavity-QED framework to describe the interaction between plasmons, excitons and vibrations, and trace the quantum dynamics of these excitations. A solution of the master equation for the density matrix of the system, including dissipation terms, reveals a rich variety of non-linear effects, complex dynamics of dark states, or statistics of light emission. The confinement of light to the nanoscale thanks to metallic nanocavities opens the door to access and control complex quantum states in molecules.

**References**

- M.K. Schmidt, R. Esteban, A. Gonzalez-Tudela, G. Giedke, J. Aizpurua, "Quantum Mechanical Description of Raman Scattering from Molecules in Plasmonic Cavities", ACS Nano 10, 6291–6298 (2016).
- F. Benz, M. K. Schmidt, A. Dreismann, R. Chikkaraddy, Y. Zhang, A. Demetriadou, C. Carnegie, H. Ohadi, B. de Nijs, R. Esteban, J. Aizpurua, and J J. Baumberg. "Single-molecule optomechanics in ‘pico-cavities’", Science 354, 726–729 (2016).
- Y. Zhang, Q.-S. Meng, L. Zhang, Y. Luo, Y.-J. Yu, B. Yang, Y. Zhang, R. Esteban, J. Aizpurua,Y. Luo, J.-L. Yang, Z.-C. Dong, J. G. Hou, "Sub-nanometre control of the coherent interaction between a single molecule and a plasmonic nanocavity", Nat. Commun. 8, 15225 (2017).
- T. Neuman and J. Aizpurua, "Origin of the asymmetric light emission from molecular exciton–polaritons", Optica, 5, 1247-1255 (2018).
- J. J. Baumberg, J. Aizpurua, M. H. Mikkelsen, D. R. Smith, "Extreme nanophotonics from ultrathin metallic gaps", Nature Materials, 18, 668–678 (2019).

## Alejandro González-Tudela

**Concluding Discussion: Nanotechnology meets Quantum Information**

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