# Program

# Program: Abstracts

## Markus Aspelmeyer

**Quantum controlling solid state mechanical systems: how and why?**

We will explore how to use the toolbox of quantum optics to control mechanical motion of solid state systems in the quantum regime. We will review the basic concepts, possible applications, and the potential for testing fundamental physics questions.

## Carlo Beenakker

**How to braid Majoranas**

We will review methods to braid the world lines of non-Abelian anyons (Majorana zero-modes) in topological superconductors. That "Holy Grail" of topological quantum information processing has not yet been reached in the laboratory, but there now exists a variety of platforms in which one can search for the Majorana braiding statistics. After an introduction to the basic concepts of braiding we discuss how one might be able to braid immobile Majorana zero-modes, bound to the end points of a nanowire, by performing the exchange in parameter space, rather than in real space. We explain how Coulomb interaction can be used to both control and read out the braiding operation, even though Majorana zero-modes are charge neutral. We ask whether the fusion rule might provide for an easier pathway towards the demonstration of non-Abelian statistics. We conclude with an approach to braiding in real space, rather than parameter space, using vortices injected into a chiral Majorana edge mode as "flying qubits".

## Juan José García-Ripoll

**Quantum optics and quantum computing with superconducting circuits**

In this series of lectures I will introduce superconducting quantum circuits as a platform both for exploring basic science (quantum optics, photonics, quantum simulation) as well as technological applications (quantum computing). The lectures will mix an introduction to the theory of superconducting circuits, the emergence of photon-like and qubit-like degrees of freedom, the measurement and control of these quantum systems and how they are combined in practical applications.

## Andreas Heinrich

**Quantum-coherent nanoscience and Quantum Spins on Surfaces**

*Quantum-coherent Nanoscience: * For the past three decades, nanoscience has widely affected many parts of physics, chemistry, materials science and engineering. Nanoscience has also led to numerous fundamental discoveries, and has given rise to real-world applications and products. During a similar time period, quantum information science has developed into a cross-disciplinary research field that also spans basic science and real-world applications.\\ Although quantum physics dictates the behavior of all objects that have nanoscale dimensions, the concept of quantum coherence, which is central to quantum information science, has not played much of a role in nanoscience. This changed about 10 to 15 years ago. In this talk I will try to find common concepts and ideas in the application of quantum coherence in nanoscale systems, a research area that we now refer to as quantum-coherent nanoscience [1]. Many degrees of freedom can be controlled in a quantum-coherent manner in nanoscale system, such as charge, spin, mechanical motion and photons. This talk will briefly discuss the state-of-the-art and outstanding challenges and opportunities unlocked by the merging of nanoscience and coherent quantum operations.

*Quantum Spins on Surfaces:* Scanning Tunneling Microscopy (STM) can be combined with electron spin resonance [2]. The major advantage of spin resonance is the fact that the energy resolution is independent of the temperature and thus can be much higher than a Fermi-function limited spectroscopy technique such as STM tunneling. In ESR-STM we apply a microwave-frequency electric field to the STM tunnel junction and convert this AC electric field into a driving field for the ESR. We find an energy resolution, which is about 10,000 times better than low-temperature STM. Two advantages of ESR-STM over ensemble-averaging techniques are first, the obvious fact that individual spin systems are measured and second, that this can be combined with precise atom manipulation to build engineered nanostructures.

We will begin by introducing the basic concepts of STM, which might be new to some members of this community. Then we will focus on one example of ESR-STM. We will utilize the atomic spin of Ti-H molecules which are adsorbed on thin MgO films supported on Ag metal substrates. Ti-H is a beautiful example since it has a spin of S=1/2 in this configuration together with a rich isotope distribution including nuclear spins.

In the second example we will take ESR-STM from the coherent manipulation of a single spin to the resonant control of 2 coupled spins. This important step allows us to perform double resonance experiments (ELDOR), in which we independently and coherently drive both spins in a weakly coupled dimer. A model reveals the complex spin dynamics and allows us to extract the Rabi rate and the spin relaxation time for the remote spin as well as for the one under the tip [under review]. Taken together with the recent advance towards pulsed ESR STM, it might be possible to implement quantum information on coupled spins on surfaces.

ESR-STM is just in its infancy with many groups joining this research effort. I believe that this technique will occupy a bright corner of quantum-coherent nanoscience.

**References**

[1] Andreas J. Heinrich, William D. Oliver, Lieven M. K. Vandersypen, Arzhang Ardavan, Roberta Sessoli, Daniel Loss, Ania Bleszynski Jayich, Joaquin Fernandez-Rossier, Arne Laucht, Andrea Morello, *Quantum-coherent nanoscience*, Nat. Nanotechnol., 16, 1318-1329 (2021).

[2] Susanne Baumann, William Paul, Taeyoung Choi, Christopher P. Lutz, Arzhang Ardavan, Andreas J. Heinrich, *Electron Paramagnetic Resonance of Individual Atoms on a Surface*, Science 350, 417 (2015).

Support from Institute for Basic Science (IBS-R027-D1) is gratefully acknowledged.

## Jason Petta

**Circuit Quantum Electrodynamics with Semiconductor Quantum Dots**

I will describe the physics of hybrid quantum devices that combine semiconductor qubit and superconducting qubit technologies, with an emphasis on cavity-coupled semiconductor double quantum dots. Topics to be covered include cavity input-output theory, charge-photon coupling, spin-photon coupling, cavity mediated spin-spin coupling, and singlet-triplet readout.

## Peter Rabl

**Ultrastrong light-matter interactions in cavity and circuit QED**

Achieving efficient interactions between light and matter is an important prerequisite for many applications in quantum information processing. Thus reaching the strong coupling regime in cavity QED has been a primary focus in the field of quantum optics for many decades. In solid-state systems, such as nanophotonic and plasmonic cavities, or using microwave photons in circuit QED, the interaction strength can be substantially enhanced and it is now possible to enter the so-called ultrastrong coupling regime, where the coupling becomes comparable to the bare energy of the photon. In this regime, cavity QED becomes non-perturbative and most of our common intuition about light-matter interactions breaks down.

In this short lecture I will give a brief introduction into the physics of ultrastrong coupling cavity and circuit QED. I will start with a step by step derivation of the minimal models for cavity QED in this extreme coupling regime, which already yields many surprises, such as the break-down of gauge invariance. I will then discuss the effective models that describe the non-perturbative ground- and low-energy states in many-body cavity QED and address some of the still prevailing controversies about superradiant phase transitions and related issues in this field.

The lecture will be primarily based on the following publications:

[1] T. Jaako, Z.-L. Xiang, J. J. Garcia-Ripoll, and P. Rabl, Ultrastrong coupling phenomena beyond the Dicke model, Phys. Rev. A 94, 033850 (2016), arXiv:1602.05756

[2] D. De Bernardis, T. Jaako, and P. Rabl, Cavity quantum electrodynamics in the non-perturbative regime, Phys. Rev. A 97, 043820 (2018), arXiv:1712.00015

[3] D. De Bernardis, P. Pilar, T. Jaako, S. De Liberato, and P. Rabl, Breakdown of gauge invariance in ultrastrong-coupling cavity QED, Phys. Rev. A 98, 053819 (2018), arXiv:1805.05339

[4] M. Schuler, D. De Bernardis, A. M. Läuchli, and P. Rabl, The Vacua of Dipolar Cavity Quantum Electrodynamic, SciPost Phys. 9, 066 (2020), arXiv:2004.13738

## Pascale Senellart

**Light matter interaction in semiconductor quantum dots and applications to quantum technologies**

tba

## Jelena Vuckovic

**Scalable semiconductor quantum systems**

Quantum entanglement provides a key resource for all quantum technologies, ranging from quantum computing and quantum error correction to secure quantum communication and quantum metrology. However, record sizes of maximally entangled qubit systems are still quite modest in all platforms, with at most 24 qubits entangled in a GHZ state. On the other hand, the largest quantum network consists of only 3 remotely entangled nodes.

Semiconductor platforms based on color centers (crystal defects with optical transitions that enable spin-to-photon interfaces) would in principle be suitable for implementing scalable quantum systems, based on excellent spin quantum memories with direct photonic interfaces, the possibility to perform high speed and high fidelity quantum gates, combined with expertise in scaling semiconductor circuits. However, there are many challenges that need to be addressed to enable this, including color centers integration into optical structures while preserving their coherence, as well as the inhomogeneity and imperfections of the qubits resulting from the nonuniform solid state environment. We show how a combination of new computational techniques (photonics inverse design, optimized driving of quantum systems), nanofabrication approaches, and fast optical and electrical control offers a unique way to address these challenges and to implement scalable quantum systems in diamond and silicon carbide.

We will also describe photonic inverse design approach which is crucial for implementation of scalable quantum and photonic technologies. Despite a great progress in photonics over the past few decades, we are nowhere near the level of integration and complexity in photonic systems that would be comparable to those of electronic circuits, which prevents use of photonics in many applications. This lag in integration scale is in big part a result of how we traditionally design photonics: by combining building blocks from a limited library of known designs, and by manual tuning a few parameters. Unfortunately, the resulting photonic circuits are very sensitive to errors in manufacturing and to environmental instabilities, bulky, and often inefficient. We show how a departure from this old fashioned approach can lead to optimal photonic designs that are much better than state of the art on many metrics (smaller, more efficient, more robust). This departure is enabled by development of inverse design approach and computer software which designs photonic systems by searching through all possible combinations of realistic parameters and geometries. We also show how this inverse design approach can enable new functionalities for photonics, including compact particle accelerators on chip which are 10 thousand times smaller than traditional accelerators, chip-to-chip on on-chip optical interconnects with error free terabit per second communication rates, and quantum technologies.