## Lectures

## Program: Lectures

### Jason Alicea

**Topological Qubits**

Topological quantum computation aims to assemble scalable qubits that are intrinsically resilient to errors. The workhorse of a topological quantum computer are emergent particles dubbed “non-Abelian anyons” that are born in certain topological states of matter. I will survey the properties of non-Abelian anyons that underlie their utility for quantum computation, focusing largely on their manifestation in 1D and 2D topological superconductors. I will also discuss candidate experimental platforms, detection and manipulation strategies, topological qubit interrogation protocols, and future challenges and opportunities.

### Daniel Barredo

**Rydberg atom arrays: expanding frontiers in quantum simulation, computation, and metrology**

Rydberg atoms in arrays of optical tweezers open up new horizons for quantum simulation, computation, and metrology. In these lectures, we will present an overview of this architecture. We will explore how individual atoms can be trapped and arranged, creating customizable arrays that serve as a versatile platform for various quantum applications. We will review the unique properties of Rydberg atoms, the mechanism of Rydberg blockade, and the methods to generate entanglement. These lectures will describe ongoing efforts in the community to implement different spin models with hundreds of particles for simulating quantum many-body phenomena. We will gain insight into the role of this platform in quantum computing, highlighting its ability to implement high-fidelity quantum gates and scalable quantum circuits. Furthermore, we will illustrate how entanglement in the out-of-equilibrium dynamics of these systems can be harnessed to generate scalable spin squeezing for metrological applications. Finally, we will provide perspectives on the future developments of Rydberg atoms arrays in the realm of quantum science and technology.

### Harry Buhrman

**Quantum Communication Beyond QKD: Position-Based Cryptography**

On 20 July 1969, millions of people held their breath as they watched, live on television, Neil Armstrong set foot on the Moon. Yet Fox Television has reported that a staggering 20% of Americans have had doubts about the Apollo 11 mission. Could it have been a hoax staged by Hollywood studios here on Earth? Position-based cryptography may offer a solution. This kind of cryptography uses the geographic position of a party as its sole credential. Normally digital keys or biometric features are used.

A central building block in position-based cryptography is that of position verification. The goal is to prove to a set of verifiers that one is at a certain geographical location. Protocols typically assume that messages cannot travel faster than the speed of light. By responding to a verifier in a timely manner one can guarantee that one is within a certain distance of that verifier. It was shown that position-verification protocols only based on this relativistic principle can be broken by attackers who simulate being at the claimed position while physically residing elsewhere in space.

Because of the no-cloning property of quantum information (qubits) it was believed that with the use of quantum messages one could devise protocols that were resistant to such collaborative attacks. Several schemes were proposed that later turned out to be insecure. In 2012 it was shown that also in the quantum case no unconditionally secure scheme is possible. However, many questions concerning the optimality of the attack remain open.

We will review the old results as well as some of the new sometimes very surprising connections with seemingly unconnected research areas such as holography, Anti-de-Sitter/Conformal Field Theory (ADS/CFT) correspondence, and classical primitives like conditional disclosure of secrets (CDS), secure message passing (SMP), and functional analysis. We will also cover some of the recent proposals for implementing position verification protocols that are secure when the attackers have a limited amount of entanglement.

### Klemens Hammerer

**Quantum variational optimization of optical atomic clocks**

Employing entanglement to improve measurements limited by quantum noise is perhaps the first use case of a real quantum advantage. While this promise is already fulfilled in gravitational wave observatories, which nowadays routinely operate with squeezed states of light, other fields in quantum metrology are just beginning to profit from entanglement enhancement. In particular, the use of squeezed and, more generally, entangled states of atoms in frequency metrology and atomic clocks is under intense investigation, with experiments demonstrating significant gains in recent proof-of-principle realizations. In my lectures, I will introduce the general idea and the specific challenges of using entanglement in atomic clocks. I will also discuss the specific needs and requirements of optical atomic clocks, which use optical atomic transitions and are expected to replace the Cesium standard second within the next decade. Since optical clocks are realized with trapped ions or neutral atoms in tweezer arrays or optical lattices, they have much in common with atom-based quantum information processors. This motivates the consideration of frequency metrology protocols from the perspective of quantum variational optimization.

### Silvia Kusminskiy

**Cavity Magnonics: Fundamentals and Applications**

Cavity magnonic systems are ideally suited to explore the range of possibilities opened by tailoring the interactions between photons, phonons, and magnons. In these lectures I will go over the basic coupling mechanisms between these excitations for ferromagnetic and antiferromagnetic systems, and cover the concept of dynamical backaction. I will discuss possible applications such as the generation of quantum magnon states, transduction, and thermometry.

## Hannes Pichler

**Chiral Quantum Optics**

Advanced photonic nanostructures enable novel functionalities for spin-photon interfaces. The strong light confinement in these structures can lock the local polarization of the light to its propagation direction, leading to propagation-direction-dependent emission, scattering and absorption of photons by quantum emitters. In these lectures I will develop the theoretical formalism used in quantum optics to describe such chiral light-matter interaction within the framework of the cascaded master equation.

### Seigo Tarucha

**Spin Physics in Quantum Dots and Application to Spin-based Quantum Computing**

Semiconductor quantum dots (QDs) provide a promising platform for application to spin-based quantum computing and study on quantum dynamics of single electron spins. Quantum coherence and entanglement are both fundamental concepts in quantum computing and utilized to construct quantum circuits. The circuit performance depends on spin dephasing and spin-spin interaction. In Part I of my lecture I will explain basics of spin-based quantum computing and underlying physics of single spins and spin entanglement. Control and detection of single spins and entanglement, dephasing of single spins due to interaction of magnetic environment, and quantum non-demolition measurement using two-spin entanglement are the major topics.

Then in Part II, I will address advances in spin-based quantum computing in silicon (Si) QDs. Spin qubits in Si QDs have several advantages, on the grounds of a long intrinsic decoherence time (> msec), a possible high temperature operation (> K), and compatibility in device fabrication with industrial technologies. Motivated by these advantages, enthusiastic effort has recently been made to improve the performance of qubit devices. I will review the progress including implementation of the high-fidelity qubit gates to satisfy the fault-tolerant threshold, quantum error correction and development of multi-qubit devices.

## Program: Talks

### Deung-Jang Choi

**Unveiling Quantum Frontiers: Manipulating atoms and molecules to explore qubit platforms**

In the realm of quantum science and technology, harnessing individual electron spins within solid materials holds immense promise. The longstanding aspiration of constructing a quantum device with precise atomic-level connections has come to fruition in this study. We have achieved the feat of building, manipulating, and observing linked electron-spin qubits on an atom-by-atom basis, all through the ingenious use of a scanning tunneling microscope (STM). We also unravel the emergence of exotic states by exploring the interaction between designed structures on superconducting surfaces. Our results shed light on Majorana bound states and their non-Abelian exchange properties, vital for the development of topological quantum computation. Within the various qubit platforms, we open the door to a realm where quantum capabilities can be harnessed, employing arrays of electron spins painstakingly assembled atom by atom upon a surface.

### Francisco Guinea

**Electronic structure and electronic interactions in moiré systems**

Stacks of two dimensional materials allow for the formation of moiré structures, that is, periodic crystals where the lattice length is mesoscopic, and usually exceeds 10nm. The number of atoms within the unit cell easily exceeds 103. Moiré superlattices can be achieved by tuning the relative angle, microscopic strain, length of the microscopic lattice, of two or more two dimensional layers. They show a variety of interesting phases, from superconductivity to fractional Chern insulators (that is, systems with fractional quasiparticles, akin to the fractional Hall effect, but without the magnetic field).

The lecture will describe different moiré superlattices, and discuss models for their electronic structure, and the role of interactions, geometry, inhomogeneities, …

### Atac Imamoglu

**Electrically-defined quantum dots for excitons in two-dimensional semiconductors**

We experimentally demonstrate a new type of quantum dot where quantum confinement of excitons in two-dimensional semiconductors is achieved exclusively through applied gate voltages and ensures fully electrically tunable confinement length. Our work provides a potential solution to the scalability problem of quantum photonic technologies by establishing a new path for creating arrays of identical solid-state quantum emitters.